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<title>SRFI 1: List Library</title>
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<!--========================================================================-->
<h1>Title</h1>
<div class=title-text>
List Library
</div>
<!--========================================================================-->
<h1>Author</H1>
<address>
<a href="http://www.ai.mit.edu/~shivers/">Olin Shivers</A> /
<a href="mailto:shivers@ai.mit.edu">shivers@ai.mit.edu</A>
</address>
<!--========================================================================-->
<h1>Table of contents</H1>
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<ul id=toc-table>
<li><a href="#Abstract">Abstract</a>
<li><a href="#Introduction">Introduction</a>
<li><a href="#ProcedureIndex">Procedure index</a>
<li><a href="#GeneralDiscussion">General discussion</a>
<ul>
<li><a href="#LinearUpdateProcedures">"Linear update" procedures</a>
<li><a href="#ImproperLists">Improper lists</a>
<li><a href="#Errors">Errors</a>
<li><a href="#NotIncludedInThisLibrary">Not included in this library</a>
</ul>
<li><a href="#TheProcedures">The procedures</a>
<ul>
<li><a href="#Constructors">Constructors</a>
<li><a href="#Predicates">Predicates</a>
<li><a href="#Selectors">Selectors</a>
<li><a href="#Miscellaneous">Miscellaneous: length, append, reverse, zip &amp; count</a>
<li><a href="#FoldUnfoldMap">Fold, unfold, and map</a>
<li><a href="#FilteringPartitioning">Filtering &amp; partitioning</a>
<li><a href="#Searching">Searching</a>
<li><a href="#Deletion">Deletion</a>
<li><a href="#AssociationLists">Association lists</a>
<li><a href="#SetOperationsOnLists">Set operations on lists</a>
<li><a href="#PrimitiveSideEffects">Primitive side-effects</a>
</ul>
<li><a href="#Acknowledgements">Acknowledgements</a>
<li><a href="#ReferencesLinks">References &amp; links</a>
<li><a href="#Copyright">Copyright</a>
</ul>
<!--========================================================================-->
<h1><a name="Abstract">Abstract</a></H1>
<p>
<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> Scheme has an impoverished set of list-processing utilities, which is a
problem for authors of portable code. This <abbr title="Scheme Request for Implementation">SRFI</abbr> proposes a coherent and
comprehensive set of list-processing procedures; it is accompanied by a
reference implementation of the spec. The reference implementation is
<ul>
<li>portable
<li>efficient
<li>completely open, public-domain source
</ul>
<strong><em>Note: This is a working draft, and tends to lag the plain-text version in terms of actual content.
See <a href="ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/srfi-1.txt">
ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/srfi-1.txt</a>
for the latest copy of the plain-text draft.
</em></strong>
<!--========================================================================-->
<h1><a name="Introduction">Introduction</a></h1>
<p>
The set of basic list and pair operations provided by R4RS/<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> Scheme is far
from satisfactory. Because this set is so small and basic, most
implementations provide additional utilities, such as a list-filtering
function, or a "left fold" operator, and so forth. But, of course, this
introduces incompatibilities -- different Scheme implementations provide
different sets of procedures.
<p>
I have designed a full-featured library of procedures for list processing.
While putting this library together, I checked as many Schemes as I could get
my hands on. (I have a fair amount of experience with several of these
already.) I missed Chez -- no on-line manual that I can find -- but I hit most
of the other big, full-featured Schemes. The complete list of list-processing
systems I checked is:
<div class=indent>
<abbr title="Revised^4 Report on Scheme">R4RS</abbr>/<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> Scheme, MIT Scheme, Gambit, RScheme, MzScheme, slib,
<a href="#CommonLisp">Common Lisp</a>, Bigloo, guile, T, APL and the SML standard basis
</div>
<p>
As a result, the library I am proposing is fairly rich.
<p>
Following this initial design phase, this library went through several
months of discussion on the SRFI mailing lists, and was altered in light
of the ideas and suggestions put forth during this discussion.
<p>
In parallel with designing this API, I have also written a reference
implementation. I have placed this source on the Net with an unencumbered,
"open" copyright. A few notes about the reference implementation:
<ul>
<li>Although I got procedure names and specs from many Schemes, I wrote this
code myself. Thus, there are <em>no</em> entanglements.
Any Scheme implementor
can pick this library up with no worries about copyright problems -- both
commercial and non-commercial systems.
<li>The code is written for portability and should be trivial to port to
any Scheme. It has only four deviations from <abbr title="Revised^4 Report on Scheme">R4RS</abbr>, clearly discussed
in the comments: <ul>
<li>Use of an <code>error</code> procedure;
<li>Use of the <abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> <code>values</code> and a simple <code>receive</code> macro for producing
and consuming multiple return values;
<li>Use of simple <code>:optional</code> and <code>let-optionals</code> macros for optional
argument parsing and defaulting;
<li>Use of a simple <code>check-arg</code> procedure for argument checking.
</ul>
<li>It is written for clarity and well-commented. The current source is
706 lines of source code and 818 lines of comments and white space.
<li>It is written for efficiency. Fast paths are provided for common
cases. Side-effecting procedures such as <code>filter!</code> avoid unnecessary,
redundant <code>set-cdr!</code>s which would thrash a generational GC's write barrier
and the store buffers of fast processors. Functions reuse longest common
tails from input parameters to construct their results where
possible. Constant-space iterations are used in preference to recursions;
local recursions are used in preference to consing temporary intermediate
data structures.
<p>
This is not to say that the implementation can't be tuned up for
a specific Scheme implementation. There are notes in comments addressing
ways implementors can tune the reference implementation for performance.
</ul>
<p>
In short, I've written the reference implementation to make it as painless
as possible for an implementor -- or a regular programmer -- to adopt this
library and get good results with it.
<!--========================================================================-->
<h1><a name="ProcedureIndex">Procedure Index</a></h1>
<p>
Here is a short list of the procedures provided by the list-lib package.
<a href="#R5RS">R5RS</a></abbr> procedures are shown in
<span class=r5rs-proc>bold</span class=r5rs-proc>;
extended <a href="#R5RS">R5RS</a></abbr>
procedures, in <span class=r5rs-procx>bold italic</span>.
<div class=indent>
<dl>
<dt class=proc-index> Constructors
<dd class=proc-index>
<pre class=proc-index>
<span class=r5rs-proc><a href="#cons">cons</a> <a href="#list">list</a></span>
<a href="#xcons">xcons</a> <a href="#cons*">cons*</a> <a href="#make-list">make-list</a> <a href="#list-tabulate">list-tabulate</a>
<a href="#list-copy">list-copy</a> <a href="#circular-list">circular-list</a> <a href="#iota">iota</a>
</pre>
<dt class=proc-index> Predicates
<dd class=proc-index>
<pre class=proc-index>
<span class=r5rs-proc><a href="#pair-p">pair?</a> <a href="#null-p">null?</a></span>
<a href="#proper-list-p">proper-list?</a> <a href="#circular-list-p">circular-list?</a> <a href="#dotted-list-p">dotted-list?</a>
<a href="#not-pair-p">not-pair?</a> <a href="#null-list-p">null-list?</a>
<a href="#list=">list=</a>
</pre>
<dt class=proc-index> Selectors
<dd class=proc-index>
<pre class=proc-index>
<span class=r5rs-proc><a href="#car">car</a> <a href="#cdr">cdr</a> ... <a href="#cddadr">cddadr</a> <a href="#cddddr">cddddr</a> <a href="#list-ref">list-ref</a></span>
<a href="#first">first</a> <a href="#second">second</a> <a href="#third">third</a> <a href="#fourth">fourth</a> <a href="#fifth">fifth</a> <a href="#sixth">sixth</a> <a href="#seventh">seventh</a> <a href="#eighth">eighth</a> <a href="#ninth">ninth</a> <a href="#tenth">tenth</a>
<a href="#car+cdr">car+cdr</a>
<a href="#take">take</a> <a href="#drop">drop</a>
<a href="#take-right">take-right</a> <a href="#drop-right">drop-right</a>
<a href="#take!">take!</a> <a href="#drop-right!">drop-right!</a>
<a href="#last">last</a> <a href="#last-pair">last-pair</a>
</pre>
<dt class=proc-index> Miscellaneous: length, append, reverse, zip &amp; count
<dd class=proc-index>
<pre class=proc-index>
<span class=r5rs-proc><a href="#length">length</a></span> <a href="#length+">length+</a>
<span class=r5rs-proc><a href="#append">append</a> <a href="#reverse">reverse</a></span>
<a href="#append!">append!</a> <a href="#reverse!">reverse!</a>
<a href="#append-reverse">append-reverse</a> <a href="#append-reverse!">append-reverse!</a>
<a href="#zip">zip</a> <a href="#unzip1">unzip1</a> <a href="#unzip2">unzip2</a> <a href="#unzip3">unzip3</a> <a href="#unzip4">unzip4</a> <a href="#unzip5">unzip5</a>
<a href="#count">count</a>
</pre>
<dt class=proc-index> Fold, unfold &amp; map
<dd class=proc-index>
<pre class=proc-index>
<span class=r5rs-procx><a href="#map">map</a> <a href="#for-each">for-each</a></span>
<a href="#fold">fold</a> <a href="#unfold">unfold</a> <a href="#pair-fold">pair-fold</a> <a href="#reduce">reduce</a>
<a href="#fold-right">fold-right</a> <a href="#unfold-right">unfold-right</a> <a href="#pair-fold-right">pair-fold-right</a> <a href="#reduce-right">reduce-right</a>
<a href="#append-map">append-map</a> <a href="#append-map!">append-map!</a>
<a href="#map!">map!</a> <a href="#pair-for-each">pair-for-each</a> <a href="#filter-map">filter-map</a> <a href="#map-in-order">map-in-order</a>
</pre>
<dt class=proc-index> Filtering &amp; partitioning
<dd class=proc-index>
<pre class=proc-index>
<a href="#filter">filter</a> <a href="#partition">partition</a> <a href="#remove">remove</a>
<a href="#filter!">filter!</a> <a href="#partition!">partition!</a> <a href="#remove!">remove!</a>
</pre>
<dt class=proc-index> Searching
<dd class=proc-index>
<pre class=proc-index>
<span class=r5rs-procx><a href="#member">member</a></span> <span class=r5rs-proc><a href="#memq">memq</a> <a href="#memv">memv</a></span>
<a href="#find">find</a> <a href="#find-tail">find-tail</a>
<a href="#any">any</a> <a href="#every">every</a>
<a href="#list-index">list-index</a>
</pre>
<dt class=proc-index> Deleting
<dd class=proc-index>
<pre class=proc-index>
<a href="#delete">delete</a> <a href="#delete-duplicates">delete-duplicates</a>
<a href="#delete!">delete!</a> <a href="#delete-duplicates!">delete-duplicates!</a>
</pre>
<dt class=proc-index> Association lists
<dd class=proc-index>
<pre class=proc-index>
<span class=r5rs-procx><a href="#assoc">assoc</a></span> <span class=r5rs-proc><a href="#assq">assq</a> <a href="#assv">assv</a></span>
<a href="#alist-cons">alist-cons</a> <a href="#alist-copy">alist-copy</a>
<a href="#alist-delete">alist-delete</a> <a href="#alist-delete!">alist-delete!</a>
</pre>
<dt class=proc-index> Set operations on lists
<dd class=proc-index>
<pre class=proc-index>
<a href="#lset<=">lset&lt;=</a> <a href="#lset=">lset=</a> <a href="#lset-adjoin">lset-adjoin</a>
<a href="#lset-union">lset-union</a> <a href="#lset-union!">lset-union!</a>
<a href="#lset-intersection">lset-intersection</a> <a href="#lset-intersection!">lset-intersection!</a>
<a href="#lset-difference">lset-difference</a> <a href="#lset-difference!">lset-difference!</a>
<a href="#lset-xor">lset-xor</a> <a href="#lset-xor!">lset-xor!</a>
<a href="#lset-diff+intersection">lset-diff+intersection</a> <a href="#lset-diff+intersection!">lset-diff+intersection!</a>
</pre>
<dt class=proc-index> Primitive side-effects
<dd class=proc-index>
<pre class=proc-index>
<span class=r5rs-proc><a href="#set-car!">set-car!</a> <a href="#set-cdr!">set-cdr!</a></span>
</pre>
</dl>
</div>
<p>
Four <abbr title="Revised^4 Report on Scheme">R4RS</abbr>/<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> list-processing procedures are extended by this library in
backwards-compatible ways:
<div class=indent>
<table cellspacing=0>
<tr valign=baseline><td><code>map for-each</code>
<td>(Extended to take lists of unequal length)
<tr valign=baseline><td><code>member assoc</code>
<td>(Extended to take an optional comparison procedure.)
</table>
</div>
<p>
The following <abbr title="Revised^4 Report on Scheme">R4RS</abbr>/<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> list- and pair-processing procedures are also part of
list-lib's exports, as defined by the <abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>:
<div class=indent>
<pre>
cons pair? null?
car cdr ... cdddar cddddr
set-car! set-cdr!
list append reverse
length list-ref
memq memv assq assv
</pre>
</div>
<p>
The remaining two <abbr title="Revised^4 Report on Scheme">R4RS</abbr>/<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> list-processing
procedures are <em>not</em> part of
this library:
<div class=indent>
<table cellspacing=0>
<tr><td><code>list-tail</code>
<td>(renamed <code>drop</code>)
<tr valign=baseline><td><code>list?</code>
<td>(see <code>proper-list?</code>,
<code>circular-list?</code> and
<code>dotted-list?</code>)
</table>
</div>
<!--========================================================================-->
<h1><a name="GeneralDiscussion">General discussion</a></h1>
<p>
A set of general criteria guided the design of this library.
<p>
I don't require "destructive" (what I call "linear update") procedures to
alter and recycle cons cells from the argument lists. They are allowed to, but
not required to. (And the reference implementations I have written <em>do</em>
recycle the argument lists.)
<p>
List-filtering procedures such as <code>filter</code> or <code>delete</code> do not disorder
lists. Elements appear in the answer list in the same order as they appear in
the argument list. This constrains implementation, but seems like a desirable
feature, since in many uses of lists, order matters. (In particular,
disordering an alist is definitely a bad idea.)
<p>
Contrariwise, although the reference implementations of the list-filtering
procedures share longest common tails between argument and answer lists,
it not is part of the spec.
<p>
Because lists are an inherently sequential data structure (unlike, say,
vectors), list-inspection functions such as <code>find</code>, <code>find-tail</code>, <code>for-each</code>, <code>any</code>
and <code>every</code> commit to a left-to-right traversal order of their argument list.
<p>
However, constructor functions, such as <code><code>list-tabulate</code></code> and the mapping
procedures (<code>append-map</code>, <code>append-map!</code>, <code>map!</code>, <code>pair-for-each</code>, <code>filter-map</code>,
<code>map-in-order</code>), do <em>not</em> specify the dynamic order in which their procedural
argument is applied to its various values.
<p>
Predicates return useful true values wherever possible. Thus <code>any</code> must return
the true value produced by its predicate, and <code>every</code> returns the final true
value produced by applying its predicate argument to the last element of its
argument list.
<p>
Functionality is provided both in pure and linear-update (potentially
destructive) forms wherever this makes sense.
<p>
No special status accorded Scheme's built-in equality functions.
Any functionality provided in terms of <code>eq?</code>, <code>eqv?</code>, <code>equal?</code> is also
available using a client-provided equality function.
<p>
Proper design counts for more than backwards compatibility, but I have tried,
<em>ceteris paribus</em>,
to be as backwards-compatible as possible with existing
list-processing libraries, in order to facilitate porting old code to run as a
client of the procedures in this library. Name choices and semantics are, for
the most part, in agreement with existing practice in many current Scheme
systems. I have indicated some incompatibilities in the following text.
<p>
These procedures are <em>not</em> "sequence generic" -- <em>i.e.</em>, procedures that
operate on either vectors and lists. They are list-specific. I prefer to
keep the library simple and focussed.
<p>
I have named these procedures without a qualifying initial "list-" lexeme,
which is in keeping with the existing set of list-processing utilities in
Scheme.
I follow the general Scheme convention (vector-length, string-ref) of
placing the type-name before the action when naming procedures -- so
we have <code>list-copy</code> and <code>pair-for-each</code> rather than the perhaps
more fluid, but less consistent, <code>copy-list</code> or <code>for-each-pair</code>.
<p>
I have generally followed a regular and consistent naming scheme, composing
procedure names from a set of basic lexemes.
<!--========================================================================-->
<h2><a name="LinearUpdateProcedures">"Linear update" procedures</a></h2>
<p>
Many procedures in this library have "pure" and "linear update" variants. A
"pure" procedure has no side-effects, and in particular does not alter its
arguments in any way. A "linear update" procedure is allowed -- but <em>not</em>
required -- to side-effect its arguments in order to construct its
result. "Linear update" procedures are typically given names ending with an
exclamation point. So, for example, <code>(append! list1 list2)</code> is allowed to
construct its result by simply using <code>set-cdr!</code> to set the cdr of the last pair
of <var>list<sub>1</sub></var> to point to <var>list<sub>2</sub></var>, and then returning <var>list<sub>1</sub></var> (unless <var>list<sub>1</sub></var> is the
empty list, in which case it would simply return <var>list<sub>2</sub></var>). However, <code>append!</code> may
also elect to perform a pure append operation -- this is a legal definition
of <code>append!</code>:
<pre class=code-example>
(define append! append)
</pre>
<p>
This is why we do not call these procedures "destructive" -- because they
aren't <em>required</em> to be destructive. They are <em>potentially</em> destructive.
<p>
What this means is that you may only apply linear-update procedures to
values that you know are "dead" -- values that will never be used again
in your program. This must be so, since you can't rely on the value passed
to a linear-update procedure after that procedure has been called. It
might be unchanged; it might be altered.
<p>
The "linear" in "linear update" doesn't mean "linear time" or "linear space"
or any sort of multiple-of-n kind of meaning. It's a fancy term that
type theorists and pure functional programmers use to describe
systems where you are only allowed to have exactly one reference to each
variable. This provides a guarantee that the value bound to a variable is
bound to no other variable. So when you <em>use</em> a variable in a variable
reference, you "use it up." Knowing that no one else has a pointer to that
value means the a system primitive is free to side-effect its arguments to
produce what is, observationally, a pure-functional result.
<p>
In the context of this library, "linear update" means you, the programmer,
know there are <em>no other</em> live references to the value passed to the
procedure -- after passing the value to one of these procedures, the
value of the old pointer is indeterminate. Basically, you are licensing
the Scheme implementation to alter the data structure if it feels like
it -- you have declared you don't care either way.
<p>
You get no help from Scheme in checking that the values you claim are "linear"
really are. So you better get it right. Or play it safe and use the non-!
procedures -- it doesn't do any good to compute quickly if you get the wrong
answer.
<p>
Why go to all this trouble to define the notion of "linear update" and use it
in a procedure spec, instead of the more common notion of a "destructive"
operation? First, note that destructive list-processing procedures are almost
always used in a linear-update fashion. This is in part required by the
special case of operating upon the empty list, which can't be side-effected.
This means that destructive operators are not pure side-effects -- they have
to return a result. Second, note that code written using linear-update
operators can be trivially ported to a pure, functional subset of Scheme by
simply providing pure implementations of the linear-update operators. Finally,
requiring destructive side-effects ruins opportunities to parallelise these
operations -- and the places where one has taken the trouble to spell out
destructive operations are usually exactly the code one would want a
parallelising compiler to parallelise: the efficiency-critical kernels of the
algorithm. Linear-update operations are easily parallelised. Going with a
linear-update spec doesn't close off these valuable alternative implementation
techniques. This list library is intended as a set of low-level, basic
operators, so we don't want to exclude these possible implementations.
<p>
The linear-update procedures in this library are
<div class=indent><code>
take! drop-right!
append! reverse! append-reverse!
append-map! map!
filter! partition! remove!
delete! alist-delete! delete-duplicates!
lset-adjoin! lset-union! lset-intersection!
lset-difference! lset-xor! lset-diff+intersection!
</code></div>
<!--========================================================================-->
<h2><a name="ImproperLists">Improper Lists</a></h2>
<p>
Scheme does not properly have a list type, just as C does not have a string
type. Rather, Scheme has a binary-tuple type, from which one can build binary
trees. There is an <em>interpretation</em> of Scheme values that allows one to
treat these trees as lists. Further complications ensue from the fact that
Scheme allows side-effects to these tuples, raising the possibility of lists
of unbounded length, and trees of unbounded depth (that is, circular data
structures).
<p>
However, there is a simple view of the world of Scheme values that considers
every value to be a list of some sort. that is, every value is either
<ul>
<li>a "proper list" -- a finite, nil-terminated list, such as:<br>
<code>(a b c)</code><br>
<code>()</code><br>
<code>(32)</code><br>
<li>a "dotted list" -- a finite, non-nil terminated list, such as:<br>
<code>(a b c . d)</code><br>
<code>(x . y)</code><br>
<code>42</code><br>
<code>george</code><br>
<li>or a "circular list" -- an infinite, unterminated list.
</ul>
<p>
Note that the zero-length dotted lists are simply all the non-null, non-pair
values.
<p>
This view is captured by the predicates <code>proper-list?</code>, <code>dotted-list?</code>, and
<code>circular-list?</code>. List-lib users should note that dotted lists are not commonly
used, and are considered by many Scheme programmers to be an ugly artifact of
Scheme's lack of a true list type. However, dotted lists do play a noticeable
role in the <em>syntax</em> of Scheme, in the "rest" parameters used by n-ary
lambdas: <code>(lambda (x y . rest) ...)</code>.
<p>
Dotted lists are <em>not</em> fully supported by list-lib. Most procedures are
defined only on proper lists -- that is, finite, nil-terminated lists. The
procedures that will also handle circular or dotted lists are specifically
marked. While this design decision restricts the domain of possible arguments
one can pass to these procedures, it has the benefit of allowing the
procedures to catch the error cases where programmers inadvertently pass
scalar values to a list procedure by accident,
<em>e.g.</em>, by switching the arguments to a procedure call.
<!--========================================================================-->
<h2><a name="Errors">Errors</a></h2>
<p>
Note that statements of the form "it is an error" merely mean "don't
do that." They are not a guarantee that a conforming implementation will
"catch" such improper use by, for example, raising some kind of exception.
Regrettably, <abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> Scheme requires no firmer guarantee even for basic operators such
as <code>car</code> and <code>cdr</code>, so there's little point in requiring these procedures to do
more. Here is the relevant section of the <abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>:
<blockquote>
<p>
When speaking of an error situation, this report uses the phrase "an
error is signalled" to indicate that implementations must detect and
report the error. If such wording does not appear in the discussion
of an error, then implementations are not required to detect or
report the error, though they are encouraged to do so. An error
situation that implementations are not required to detect is usually
referred to simply as "an error."
<p>
For example, it is an error for a procedure to be passed an argument
that the procedure is not explicitly specified to handle, even though
such domain errors are seldom mentioned in this report.
Implementations may extend a procedure's domain of definition to
include such arguments.
</blockquote>
<!--========================================================================-->
<h2><a name="NotIncludedInThisLibrary">Not included in this library</a></h2>
<p>
The following items are not in this library:
<ul>
<li>Sort routines
<li>Destructuring/pattern-matching macro
<li>Tree-processing routines
</ul>
<p>
They should have their own <abbr title="Scheme Request for Implementation">SRFI</abbr> specs.
<p>
<!--========================================================================-->
<h1><a name="TheProcedures">The procedures</a></h1>
<p>
In a Scheme system that has a module or package system, these procedures
should be contained in a module named "list-lib".
The templates given below obey the following conventions for procedure formals:
<table>
<tr valign=baseline><th align=left> <var>list</var>
<td> A proper (finite, nil-terminated) list
<tr valign=baseline><th align=left> <var>clist</var>
<td> A proper or circular list
<tr valign=baseline><th align=left> <var>flist</var>
<td> A finite (proper or dotted) list
<tr valign=baseline><th align=left> <var>pair</var>
<td> A pair
<tr valign=baseline>
<th align=left> <var>x</var>, <var>y</var>, <var>d</var>, <var>a</var>
<td> Any value
<tr valign=baseline><th align=left> <var>object</var>, <var>value</var>
<td> Any value
<tr valign=baseline><th align=left> <var>n</var>, <var>i</var>
<td> A natural number (an integer &gt;= 0)
<tr valign=baseline><th align=left> <var>proc</var>
<td> A procedure
<tr valign=baseline><th align=left> <var>pred</var>
<td> A procedure whose return value is treated as a boolean
<tr valign=baseline><th align=left> <var>=</var>
<td> A boolean procedure taking two arguments
</table>
<p>
It is an error to pass a circular or dotted list to a procedure not
defined to accept such an argument.
<!--========================================================================-->
<h2><a name="Constructors">Constructors</a></h2>
<p>
<dl>
<!--
==== cons
============================================================================-->
<dt class=proc-def>
<a name="cons"></a>
<code class=proc-def>cons</code> <var>a d -&gt; pair</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
The primitive constructor. Returns a newly allocated pair whose car is
<var>a</var> and whose cdr is <var>d</var>.
The pair is guaranteed to be different (in the sense of <code>eqv?</code>)
from every existing object.
<pre class=code-example>
(cons 'a '()) => (a)
(cons '(a) '(b c d)) => ((a) b c d)
(cons "a" '(b c)) => ("a" b c)
(cons 'a 3) => (a . 3)
(cons '(a b) 'c) => ((a b) . c)
</pre>
<!--
==== list
============================================================================-->
<dt class=proc-def>
<a name="list"></a>
<code class=proc-def>list</code> <var>object ... -&gt; list</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
Returns a newly allocated list of its arguments.
<pre class=code-example>
(list 'a (+ 3 4) 'c) => (a 7 c)
(list) => ()
</pre>
<!--
==== xcons
============================================================================-->
<dt class=proc-def>
<a name="xcons"></a>
<code class=proc-def>xcons</code> <var>d a -&gt; pair</var>
<dd class=proc-def>
<pre>
(lambda (d a) (cons a d))
</pre>
Of utility only as a value to be conveniently passed to higher-order
procedures.
<pre class=code-example>
(xcons '(b c) 'a) =&gt; (a b c)
</pre>
The name stands for "eXchanged CONS."
<!--
==== cons*
============================================================================-->
<a name="cons*"></a>
<dt class=proc-def><code class=proc-def>cons*</code><var> elt<sub>1</sub> elt<sub>2</sub> ... -&gt; object</var>
<dd class=proc-def>
Like <code>list</code>,
but the last argument provides the tail of the constructed list,
returning
<div class=indent><code>
(cons <var>elt<sub>1</sub></var> (cons <var>elt<sub>2</sub></var> (cons ... <var>elt<sub>n</sub></var>)))
</code></div>
This function is called <code>list*</code> in <a href="#CommonLisp">Common Lisp</a> and about
half of the Schemes that provide it,
and <code>cons*</code> in the other half.
<pre class=code-example>
(cons* 1 2 3 4) =&gt; (1 2 3 . 4)
(cons* 1) =&gt; 1
</pre>
<!--
==== make-list
============================================================================-->
<a name="make-list"></a>
<dt class=proc-def> <code class=proc-def>make-list</code> <var>n [fill] -&gt; list</var>
<dd class=proc-def>
Returns an <var>n</var>-element list,
whose elements are all the value <var>fill</var>.
If the <var>fill</var> argument is not given, the elements of the list may
be arbitrary values.
<pre class=code-example>
(make-list 4 'c) =&gt; (c c c c)
</pre>
<!--
==== list-tabulate
============================================================================-->
<a name="list-tabulate"></a>
<dt class=proc-def><code class=proc-def>list-tabulate</code><var> n init-proc -&gt; list</var>
<dd class=proc-def>
Returns an <var>n</var>-element list. Element <var>i</var> of the list, where 0 &lt;= <var>i</var> &lt; <var>n</var>,
is produced by <code>(<var>init-proc</var> <var>i</var>)</code>. No guarantee is made about the dynamic
order in which <var>init-proc</var> is applied to these indices.
<pre class=code-example>
(list-tabulate 4 values) =&gt; (0 1 2 3)
</pre>
<!--
==== list-copy
============================================================================-->
<a name="list-copy"></a>
<dt class=proc-def><code class=proc-def>list-copy</code><var> flist -&gt; flist</var>
<dd class=proc-def>
Copies the spine of the argument.
<!--
==== circular-list
============================================================================-->
<a name="circular-list"></a>
<dt class=proc-def><code class=proc-def>circular-list</code><var> elt<sub>1</sub> elt<sub>2</sub> ... -&gt; list</var>
<dd class=proc-def>
Constructs a circular list of the elements.
<pre class=code-example>
(circular-list 'z 'q) =&gt; (z q z q z q ...)
</pre>
<!--
==== iota
============================================================================-->
<a name="iota"></a>
<dt class=proc-def><code class=proc-def>iota</code><var> count [start step] -&gt; list</var>
<dd class=proc-def>
Returns a list containing the elements
<pre class=code-example>
(<var>start</var> <var>start</var>+<var>step</var> ... <var>start</var>+(<var>count</var>-1)*<var>step</var>)
</pre>
The <var>start</var> and <var>step</var> parameters default to 0 and 1, respectively.
This procedure takes its name from the APL primitive.
<pre class=code-example>
(iota 5) =&gt; (0 1 2 3 4)
(iota 5 0 -0.1) =&gt; (0 -0.1 -0.2 -0.3 -0.4)
</pre>
</dl>
<!--========================================================================-->
<h2><a name="Predicates">Predicates</a></h2>
<p>
Note: the predicates <code>proper-list?</code>, <code>circular-list?</code>, and <code>dotted-list?</code>
partition the entire universe of Scheme values.
<dl>
<!--
==== proper-list?
============================================================================-->
<dt class=proc-def>
<code class=proc-def>proper-list?</code><var> x -&gt; boolean</var>
<a name="proper-list-p"></a>
<dd class=proc-def>
Returns true iff <var>x</var> is a proper list -- a finite, nil-terminated list.
<p>
More carefully: The empty list is a proper list. A pair whose cdr is a
proper list is also a proper list:
<pre>
&lt;proper-list&gt; ::= () (Empty proper list)
| (cons &lt;x&gt; &lt;proper-list&gt;) (Proper-list pair)
</pre>
Note that this definition rules out circular lists. This
function is required to detect this case and return false.
<p>
Nil-terminated lists are called "proper" lists by <abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> and <a href="#CommonLisp">Common Lisp</a>.
The opposite of proper is improper.
<p>
<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr> binds this function to the variable <code>list?</code>.
<p>
<pre>
(not (proper-list? <var>x</var>)) = (or (dotted-list? <var>x</var>) (circular-list? <var>x</var>))
</pre>
<!--
==== circular-list?
============================================================================-->
<a name="circular-list-p"></a>
<dt class=proc-def><code class=proc-def>circular-list?</code><var> x -&gt; boolean</var>
<dd class=proc-def>
True if <var>x</var> is a circular list. A circular list is a value such that
for every <var>n</var> &gt;= 0, cdr<sup><var>n</var></sup>(<var>x</var>) is a pair.
<p>
Terminology: The opposite of circular is finite.
<pre>
(not (circular-list? <var>x</var>)) = (or (proper-list? <var>x</var>) (dotted-list? <var>x</var>))
</pre>
<!--
==== dotted-list?
============================================================================-->
<a name="dotted-list-p"></a>
<dt class=proc-def><code class=proc-def>dotted-list?</code><var> x -&gt; boolean</var>
<dd class=proc-def>
True if <var>x</var> is a finite, non-nil-terminated list. That is, there exists
an <var>n</var> &gt;= 0 such that cdr<sup><var>n</var></sup>(<var>x</var>) is neither a pair nor ().
This includes
non-pair, non-() values (<em>e.g.</em> symbols, numbers),
which are considered to be dotted lists of length 0.
<pre>
(not (dotted-list? <var>x</var>)) = (or (proper-list? <var>x</var>) (circular-list? <var>x</var>))
</pre>
<!--
==== pair?
============================================================================-->
<a name="pair-p"></a>
<dt class=proc-def><code class=proc-def>pair?</code><var> object -&gt; boolean</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
Returns #t if <var>object</var> is a pair; otherwise, #f.
<pre class=code-example>
(pair? '(a . b)) => #t
(pair? '(a b c)) => #t
(pair? '()) => #f
(pair? '#(a b)) => #f
(pair? 7) => #f
(pair? 'a) => #f
</pre>
<!--
==== null?
============================================================================-->
<a name="null-p"></a>
<dt class=proc-def><code class=proc-def>null?</code><var> object -&gt; boolean</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
Returns #t if <var>object</var> is the empty list; otherwise, #f.
<!--
==== null-list?
============================================================================-->
<a name="null-list-p"></a>
<dt class=proc-def><code class=proc-def>null-list?</code><var> list -&gt; boolean</var>
<dd class=proc-def>
<var>List</var> is a proper or circular list. This procedure returns true if
the argument is the empty list (), and false otherwise. It is an
error to pass this procedure a value which is not a proper or
circular list.
This procedure is recommended as the termination condition for
list-processing procedures that are not defined on dotted lists.
<!--
==== not-pair?
============================================================================-->
<dt class=proc-def>
<a name="not-pair-p"></a>
<code class=proc-def>not-pair?</code><var> x -&gt; boolean</var>
<dd class=proc-def>
<pre>(lambda (x) (not (pair? x)))</pre>
Provided as a procedure as it can be useful as the termination condition
for list-processing procedures that wish to handle all finite lists,
both proper and dotted.
<!--
==== list=
============================================================================-->
<dt class=proc-def>
<a name="list="></a>
<code class=proc-def>list=</code><var> elt= list<sub>1</sub> ... -&gt; boolean</var>
<dd class=proc-def>
Determines list equality, given an element-equality procedure.
Proper list <var>A</var> equals proper list <var>B</var>
if they are of the same length,
and their corresponding elements are equal,
as determined by <var>elt=</var>.
If the element-comparison procedure's first argument is
from <var>list<sub>i</sub></var>,
then its second argument is from <var>list<sub>i+1</sub></var>,
<em>i.e.</em> it is always called as
<code>(<var>elt=</var> <var>a</var> <var>b</var>)</code>
for <var>a</var> an element of list <var>A</var>,
and <var>b</var> an element of list <var>B</var>.
<p>
In the <var>n</var>-ary case,
every <var>list<sub>i</sub></var> is compared to
<var>list<sub>i+1</sub></var>
(as opposed, for example, to comparing
<var>list<sub>1</sub></var> to every <var>list<sub>i</sub></var>,
for <var>i</var>>1).
If there are no list arguments at all,
<code>list=</code> simply returns true.
<p>
It is an error to apply <code>list=</code> to anything except proper lists.
While
implementations may choose to extend it to circular lists, note that it
cannot reasonably be extended to dotted lists, as it provides no way to
specify an equality procedure for comparing the list terminators.
<p>
Note that the dynamic order in which the <var>elt=</var> procedure is
applied to pairs of elements is not specified.
For example, if <code>list=</code> is applied
to three lists, <var>A</var>, <var>B</var>, and <var>C</var>,
it may first completely compare <var>A</var> to <var>B</var>,
then compare <var>B</var> to <var>C</var>,
or it may compare the first elements of <var>A</var> and <var>B</var>,
then the first elements of <var>B</var> and <var>C</var>,
then the second elements of <var>A</var> and <var>B</var>, and so forth.
<p>
The equality procedure must be consistent with <code>eq?</code>.
That is, it must be the case that
<div class=indent>
<code>(eq? <var>x</var> <var>y</var>)</code> => <code>(<var>elt=</var> <var>x</var> <var>y</var>)</code>.
</div>
Note that this implies that two lists which are <code>eq?</code>
are always <var>list=</var>, as well; implementations may exploit this
fact to "short-cut" the element-by-element comparisons.
<pre class=code-example>
(list= eq?) => #t ; Trivial cases
(list= eq? '(a)) => #t
</pre>
</dl>
<!--========================================================================-->
<h2><a name="Selectors">Selectors</a></h2>
<dl>
<!--
==== car cdr
============================================================================-->
<a name="car"></a>
<a name="cdr"></a>
<dt class=proc-def1><code class=proc-def>car</code><var> pair -&gt; value</var>
<dt class=proc-defn><code class=proc-def>cdr</code><var> pair -&gt; value</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
These functions return the contents of the car and cdr field of their
argument, respectively.
Note that it is an error to apply them to the empty list.
<pre class=code-example>
(car '(a b c)) => a (cdr '(a b c)) => (b c)
(car '((a) b c d)) => (a) (cdr '((a) b c d)) => (b c d)
(car '(1 . 2)) => 1 (cdr '(1 . 2)) => 2
(car '()) => *error* (cdr '()) => *error*
</pre>
<!--
==== caar cadr ... cdddar cddddr
============================================================================-->
<a name="caar"></a>
<a name="cadr"></a>
<a name="cdddar"></a>
<a name="cddddr"></a>
<dt class=proc-def1><code class=proc-def>caar</code><var> pair -&gt; value</var>
<dt class=proc-defi><code class=proc-def>cadr</code><var> pair -&gt; value</var>
<dt class=proc-defi><code class=proc-def>:</code>
<dt class=proc-defi><code class=proc-def>cdddar</code><var> pair -&gt; value</var>
<dt class=proc-defn><code class=proc-def>cddddr</code><var> pair -&gt; value</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
These procedures are compositions of <code>car</code> and <code>cdr</code>,
where for example <code>caddr</code> could be defined by
<pre class=code-example>
(define caddr (lambda (x) (car (cdr (cdr x))))).
</pre>
Arbitrary compositions, up to four deep, are provided. There are
twenty-eight of these procedures in all.
<!--
==== list-ref
============================================================================-->
<a name="list-ref"></a>
<dt class=proc-def><code class=proc-def>list-ref</code><var> clist i -&gt; value</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
Returns the <var>i</var><sup>th</sup> element of <var>clist</var>.
(This is the same as the car of
<code>(drop <var>clist</var> <var>i</var>)</code>.)
It is an error if <var>i</var> >= <var>n</var>,
where <var>n</var> is the length of <var>clist</var>.
<pre class=code-example>
(list-ref '(a b c d) 2) => c
</pre>
<!--
==== tenth
==== ninth
==== eighth
==== seventh
==== sixth
==== fifth
==== fourth
==== third
==== second
==== first
============================================================================-->
<dt class=proc-def1>
<a name="first"></a>
<code class=proc-def>first&nbsp;&nbsp;&nbsp;</code><var>pair -&gt; object </var>
<dt class=proc-defi>
<a name="second"></a>
<code class=proc-def>second&nbsp;&nbsp;</code><var>pair -&gt; object </var>
<dt class=proc-defi>
<a name="third"></a>
<code class=proc-def>third&nbsp;&nbsp;&nbsp;</code><var>pair -&gt; object </var>
<dt class=proc-defi>
<a name="fourth"></a>
<code class=proc-def>fourth&nbsp;&nbsp;</code><var>pair -&gt; object </var>
<dt class=proc-defi>
<a name="fifth"></a>
<code class=proc-def>fifth&nbsp;&nbsp;&nbsp;</code><var>pair -&gt; object </var>
<dt class=proc-defi>
<a name="sixth"></a>
<code class=proc-def>sixth&nbsp;&nbsp;&nbsp;</code><var>pair -&gt; object </var>
<dt class=proc-defi>
<a name="seventh"></a>
<code class=proc-def>seventh&nbsp;</code><var>pair -&gt; object </var>
<dt class=proc-defi>
<a name="eighth"></a>
<code class=proc-def>eighth&nbsp;&nbsp;</code><var>pair -&gt; object </var>
<dt class=proc-defi>
<a name="ninth"></a>
<code class=proc-def>ninth&nbsp;&nbsp;&nbsp;</code><var>pair -&gt; object </var>
<dt class=proc-defn>
<a name="tenth"></a>
<code class=proc-def>tenth&nbsp;&nbsp;&nbsp;</code><var>pair -&gt; object </var>
<dd class=proc-def>
Synonyms for <code>car</code>, <code>cadr</code>, <code>caddr</code>, ...
<pre class=code-example>
(third '(a b c d e)) =&gt; c
</pre>
<!--
==== car+cdr
============================================================================-->
<dt class=proc-def>
<a name="car+cdr"></a>
<code class=proc-def>car+cdr</code><var> pair -&gt; [x y]</var>
<dd class=proc-def>
The fundamental pair deconstructor:
<pre class=code-example>
(lambda (p) (values (car p) (cdr p)))
</pre>
This can, of course, be implemented more efficiently by a compiler.
<!--
==== drop
==== take
============================================================================-->
<dt class=proc-def1>
<a name="take"></a>
<code class=proc-def>take</code><var> x i -&gt; list</var>
<dt class=proc-defn>
<a name="drop"></a>
<code class=proc-def>drop</code><var> x i -&gt; object</var>
<dd class=proc-def>
<code>take</code> returns the first <var>i</var> elements of list <var>x</var>.<br>
<code>drop</code> returns all but the first <var>i</var> elements of list <var>x</var>.
<pre class=code-example>
(take '(a b c d e) 2) =&gt; (a b)
(drop '(a b c d e) 2) =&gt; (c d e)
</pre>
<var>x</var> may be any value -- a proper, circular, or dotted list:
<pre class=code-example>
(take '(1 2 3 . d) 2) =&gt; (1 2)
(drop '(1 2 3 . d) 2) =&gt; (3 . d)
(take '(1 2 3 . d) 3) =&gt; (1 2 3)
(drop '(1 2 3 . d) 3) =&gt; d
</pre>
For a legal <var>i</var>, <code>take</code> and <code>drop</code> partition the list in a manner which
can be inverted with <code>append</code>:
<pre class=code-example>
(append (take <var>x</var> <var>i</var>) (drop <var>x</var> <var>i</var>)) = <var>x</var>
</pre>
<code>drop</code> is exactly equivalent to performing <var>i</var> cdr operations on <var>x</var>;
the returned value shares a common tail with <var>x</var>.
If the argument is a list of non-zero length, <code>take</code> is guaranteed to
return a freshly-allocated list, even in the case where the entire
list is taken, <em>e.g.</em> <code>(take lis (length lis))</code>.
<!--
==== drop-right
==== take-right
============================================================================-->
<dt class=proc-def1>
<a name="take-right"></a>
<code class=proc-def>take-right</code><var> flist i -&gt; object</var>
<dt class=proc-defn>
<a name="drop-right"></a>
<code class=proc-def>drop-right</code><var> flist i -&gt; list</var>
<dd class=proc-def>
<code>take-right</code> returns the last <var>i</var> elements of <var>flist</var>.<br>
<code>drop-right</code> returns all but the last <var>i</var> elements of <var>flist</var>.
<pre class=code-example>
(take-right '(a b c d e) 2) =&gt; (d e)
(drop-right '(a b c d e) 2) =&gt; (a b c)
</pre>
The returned list may share a common tail with the argument list.
<p>
<var>flist</var> may be any finite list, either proper or dotted:
<pre class=code-example>
(take-right '(1 2 3 . d) 2) =&gt; (2 3 . d)
(drop-right '(1 2 3 . d) 2) =&gt; (1)
(take-right '(1 2 3 . d) 0) =&gt; d
(drop-right '(1 2 3 . d) 0) =&gt; (1 2 3)
</pre>
For a legal <var>i</var>, <code>take-right</code> and <code>drop-right</code> partition the list in a manner
which can be inverted with <code>append</code>:
<pre class=code-example>
(append (take <var>flist</var> <var>i</var>) (drop <var>flist</var> <var>i</var>)) = <var>flist</var>
</pre>
<code>take-right</code>'s return value is guaranteed to share a common tail with <var>flist</var>.
If the argument is a list of non-zero length, <code>drop-right</code> is guaranteed to
return a freshly-allocated list, even in the case where nothing is
dropped, <em>e.g.</em> <code>(drop-right lis 0)</code>.
<!--
==== drop-right!
==== take!
============================================================================-->
<dt class=proc-def1>
<a name="take!"></a>
<code class=proc-def>take!</code><var> x i -&gt; list</var>
<dt class=proc-defn>
<a name="drop-right!"></a>
<code class=proc-def>drop-right!</code><var> flist i -&gt; list</var>
<dd class=proc-def>
<code>take!</code> and <code>drop-right!</code> are "linear-update" variants of <code>take</code> and
<code>drop-right</code>: the procedure is allowed, but not required, to alter the
argument list to produce the result.
<p>
If <var>x</var> is circular, <code>take!</code> may return a shorter-than-expected list:
<pre class=code-example>
(take! (circular-list 1 3 5) 8) =&gt; (1 3)
(take! (circular-list 1 3 5) 8) =&gt; (1 3 5 1 3 5 1 3)
</pre>
<!--
==== last-pair
==== last
============================================================================-->
<dt class=proc-def1>
<a name="last"></a>
<code class=proc-def>last</code><var> pair -&gt; object</var>
<dt class=proc-defn>
<a name="last-pair"></a>
<code class=proc-def>last-pair</code><var> pair -&gt; pair</var>
<dd class=proc-def>
<code>last</code> returns the last element of the non-empty,
finite list <var>pair</var>.
<code>last-pair</code> returns the last pair in the non-empty,
finite list <var>pair</var>.
<pre class=code-example>
(last '(a b c)) =&gt; c
(last-pair '(a b c)) =&gt; (c)
</pre>
</dl>
<!--========================================================================-->
<h2><a name="Miscellaneous">Miscellaneous: length, append, reverse, zip &amp; count</a></h2>
<dl>
<!--
==== length+
==== length
============================================================================-->
<dt class=proc-def1>
<a name="length"></a>
<code class=proc-def>length&nbsp;&nbsp;</code><var>list -&gt; integer</var>
<dt class=proc-defn>
<a name="length+"></a>
<code class=proc-def>length+&nbsp;</code><var>clist -&gt; integer or #f</var>
<dd class=proc-def>
Both <code>length</code> and <code>length+</code> return the length of the argument.
It is an error to pass a value to <code>length</code> which is not a proper
list (finite and nil-terminated). In particular, this means an
implementation may diverge or signal an error when <code>length</code> is
applied to a circular list.
<p>
<code>length+</code>, on the other hand, returns <code>#F</code> when applied to a circular
list.
<p>
The length of a proper list is a non-negative integer <var>n</var> such that <code>cdr</code>
applied <var>n</var> times to the list produces the empty list.
<!--
==== append append!
============================================================================-->
<dt class=proc-def1>
<a name="append"></a>
<code class=proc-def>append&nbsp;</code><var> list<sub>1</sub> ... -&gt; list</var>
<dt class=proc-defn>
<a name="append!"></a>
<code class=proc-def>append!</code><var> list<sub>1</sub> ... -&gt; list</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
<code>append</code> returns a list consisting of the elements
of <var>list<sub>1</sub></var>
followed by the elements of the other list parameters.
<pre class=code-example>
(append '(x) '(y)) => (x y)
(append '(a) '(b c d)) => (a b c d)
(append '(a (b)) '((c))) => (a (b) (c))
</pre>
The resulting list is always newly allocated, except that it
shares structure with the final <var>list<sub>i</sub></var> argument.
This last argument may be any value at all;
an improper list results if it is not
a proper list. All other arguments must be proper lists.
<pre class=code-example>
(append '(a b) '(c . d)) => (a b c . d)
(append '() 'a) => a
(append '(x y)) => (x y)
(append) => ()
</pre>
<code>append!</code> is the "linear-update" variant of <code>append</code>
-- it is allowed, but not required, to alter cons cells in the argument
lists to construct the result list.
The last argument is never altered; the result
list shares structure with this parameter.
<!--
==== reverse reverse!
============================================================================-->
<dt class=proc-def1>
<a name="reverse"></a>
<code class=proc-def>reverse&nbsp;</code><var> list -&gt; list</var>
<dt class=proc-defn>
<a name="reverse!"></a>
<code class=proc-def>reverse!</code><var> list -&gt; list</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
<code>reverse</code> returns a newly allocated list consisting of
the elements of <var>list</var> in reverse order.
<pre class=code-example>
(reverse '(a b c)) => (c b a)
(reverse '(a (b c) d (e (f))))
=> ((e (f)) d (b c) a)
</pre>
<code>reverse!</code> is the linear-update variant of <code>reverse</code>.
It is permitted, but not required, to alter the argument's cons cells
to produce the reversed list.
<!--
==== append-reverse!
==== append-reverse
============================================================================-->
<dt class=proc-def1>
<a name="append-reverse"></a>
<code class=proc-def>append-reverse&nbsp;&nbsp;</code><var>rev-head tail -&gt; list</var>
<dt class=proc-defn>
<a name="append-reverse!"></a>
<code class=proc-def>append-reverse!&nbsp;</code><var>rev-head tail -&gt; list</var>
<dd class=proc-def>
<code>append-reverse</code> returns
<code>(append (reverse <var>rev-head</var>) <var>tail</var>)</code>.
It is provided because it is a common operation -- a common
list-processing style calls for this exact operation to transfer values
accumulated in reverse order onto the front of another list, and because
the implementation is significantly more efficient than the simple
composition it replaces. (But note that this pattern of iterative
computation followed by a reverse can frequently be rewritten as a
recursion, dispensing with the <code>reverse</code> and <code>append-reverse</code> steps, and
shifting temporary, intermediate storage from the heap to the stack,
which is typically a win for reasons of cache locality and eager storage
reclamation.)
<p>
<code>append-reverse!</code> is just the linear-update variant -- it is allowed, but
not required, to alter <var>rev-head</var>'s cons cells to construct the result.
<!--
==== zip
============================================================================-->
<a name="zip"></a>
<dt class=proc-def><code class=proc-def>zip</code> <var>clist<sub>1</sub> clist<sub>2</sub> ... -&gt; list</var>
<dd class=proc-def>
<pre>(lambda lists (apply map list lists))
</pre>
If <code>zip</code> is passed <var>n</var> lists, it returns a list as long as the shortest
of these lists, each element of which is an <var>n</var>-element list comprised
of the corresponding elements from the parameter lists.
<pre class=code-example>
(zip '(one two three)
'(1 2 3)
'(odd even odd even odd even odd even))
=&gt; ((one 1 odd) (two 2 even) (three 3 odd))
(zip '(1 2 3)) =&gt; ((1) (2) (3))
</pre>
At least one of the argument lists must be finite:
<pre class=code-example>
(zip '(3 1 4 1) (circular-list #f #t))
=> ((3 #f) (1 #t) (4 #f) (1 #t))
</pre>
<!--
==== unzip5
==== unzip4
==== unzip3
==== unzip2
==== unzip1
============================================================================-->
<a name="unzip1"></a>
<dt class=proc-def1> <code class=proc-def>unzip1</code><var> list -&gt; list</var>
<a name="unzip2"></a>
<dt class=proc-defi> <code class=proc-def>unzip2</code><var> list -&gt; [list list]</var>
<a name="unzip3"></a>
<dt class=proc-defi> <code class=proc-def>unzip3</code><var> list -&gt; [list list list]</var>
<a name="unzip4"></a>
<dt class=proc-defi> <code class=proc-def>unzip4</code><var> list -&gt; [list list list list]</var>
<a name="unzip5"></a>
<dt class=proc-defn> <code class=proc-def>unzip5</code><var> list -&gt; [list list list list list]</var>
<dd class=proc-def>
<code>unzip1</code> takes a list of lists,
where every list must contain at least one element,
and returns a list containing the initial element of each such list.
That is, it returns <code>(map car lists)</code>.
<code>unzip2</code> takes a list of lists, where every list must contain at least
two elements, and returns two values: a list of the first elements,
and a list of the second elements. <code>unzip3</code> does the same for the first
three elements of the lists, and so forth.
<pre class=code-example>
(unzip2 '((1 one) (2 two) (3 three))) =&gt;
(1 2 3)
(one two three)
</pre>
<!--
==== count
============================================================================-->
<dt class=proc-def>
<a name="count"></a>
<code class=proc-def>count</code><var> pred clist<sub>1</sub> clist<sub>2</sub> -&gt; integer</var>
<dd class=proc-def>
<var>pred</var> is a procedure taking as many arguments as there
are lists and returning a single value. It is applied
element-wise to the elements of the <var>list</var>s, and a count is
tallied of the number of elements that produce a true value. This count
is returned. <code>count</code> is "iterative" in that it is guaranteed
to apply <var>pred</var> to the <var>list</var> elements in a
left-to-right order.
The counting stops when the shortest list expires.
<pre class=code-example>
(count even? '(3 1 4 1 5 9 2 5 6)) => 3
(count < '(1 2 4 8) '(2 4 6 8 10 12 14 16)) => 3
</pre>
At least one of the argument lists must be finite:
<pre class=code-example>
(count < '(3 1 4 1) (circular-list 1 10)) => 2
</pre>
</dl>
<!--========================================================================-->
<h2><a name="FoldUnfoldMap">Fold, unfold &amp; map</a></h2>
<dl>
<!--
==== fold
============================================================================-->
<dt class=proc-def>
<a name="fold"></a>
<code class=proc-def>fold</code><var> kons knil clist<sub>1</sub> clist<sub>2</sub> ... -&gt; value</var>
<dd class=proc-def>
The fundamental list iterator.
<p>
First, consider the single list-parameter case. If <var>clist<sub>1</sub></var> = (<var>e<sub>1</sub></var> <var>e<sub>2</sub></var> ... <var>e<sub>n</sub></var>),
then this procedure returns
<div class=indent>
<code>(<var>kons</var> <var>e<sub>n</sub></var> ... (<var>kons</var> <var>e<sub>2</sub></var> (<var>kons</var> <var>e<sub>1</sub></var> <var>knil</var>)) ... )</code>
</div>
That is, it obeys the (tail) recursion
<pre class=code-example>
(fold <var>kons</var> <var>knil</var> <var>lis</var>) = (fold <var>kons</var> (<var>kons</var> (car <var>lis</var>) <var>knil</var>) (cdr <var>lis</var>))
(fold <var>kons</var> <var>knil</var> '()) = <var>knil</var>
</pre>
Examples:
<pre class=code-example>
(fold + 0 lis) ; Add up the elements of LIS.
(fold cons '() lis) ; Reverse LIS.
(fold cons tail rev-head) ; See APPEND-REVERSE.
;; How many symbols in LIS?
(fold (lambda (x count) (if (symbol? x) (+ count 1) count))
0
lis)
;; Length of the longest string in LIS:
(fold (lambda (s max-len) (max max-len (string-length s)))
0
lis)
</pre>
If <var>n</var> list arguments are provided, then the <var>kons</var> function must take
<var>n</var>+1 parameters: one element from each list, and the "seed" or fold
state, which is initially <var>knil</var>. The fold operation terminates when
the shortest list runs out of values:
<pre class=code-example>
(fold cons* '() '(a b c) '(1 2 3 4 5)) =&gt; (c 3 b 2 a 1)
</pre>
At least one of the list arguments must be finite.
<!--
==== fold-right
============================================================================-->
<dt class=proc-def>
<a name="fold-right"></a>
<code class=proc-def>fold-right</code><var> kons knil clist<sub>1</sub> clist<sub>2</sub> ... -&gt; value</var>
<dd class=proc-def>
The fundamental list recursion operator.
<p>
First, consider the single list-parameter case. If <var>clist<sub>1</sub></var> = <code>(<var>e<sub>1</sub></var> <var>e<sub>2</sub></var> ... <var>e<sub>n</sub></var>)</code>,
then this procedure returns
<div class=indent><code>
(<var>kons</var> <var>e<sub>1</sub></var> (<var>kons</var> <var>e<sub>2</sub></var> ... (<var>kons</var> <var>e<sub>n</sub></var> <var>knil</var>)))
</code></div>
That is, it obeys the recursion
<pre class=code-example>
(fold-right <var>kons</var> <var>knil</var> <var>lis</var>) = (<var>kons</var> (car <var>lis</var>) (fold-right <var>kons</var> <var>knil</var> (cdr <var>lis</var>)))
(fold-right <var>kons</var> <var>knil</var> '()) = <var>knil</var>
</pre>
Examples:
<pre class=code-example>
(fold-right cons '() lis) ; Copy LIS.
;; Filter the even numbers out of LIS.
(fold-right (lambda (x l) (if (even? x) (cons x l) l)) '() lis))
</pre>
If <var>n</var> list arguments are provided, then the <var>kons</var> function must take
<var>n</var>+1 parameters: one element from each list, and the "seed" or fold
state, which is initially <var>knil</var>. The fold operation terminates when
the shortest list runs out of values:
<pre class=code-example>
(fold-right cons* '() '(a b c) '(1 2 3 4 5)) =&gt; (a 1 b 2 c 3)
</pre>
At least one of the list arguments must be finite.
<!--
==== pair-fold
============================================================================-->
<dt class=proc-def>
<a name="pair-fold"></a>
<code class=proc-def>pair-fold</code><var> kons knil clist<sub>1</sub> clist<sub>2</sub> ... -&gt; value</var>
<dd class=proc-def>
Analogous to <code>fold</code>, but <var>kons</var> is applied to successive sublists of the
lists, rather than successive elements -- that is, <var>kons</var> is applied to the
pairs making up the lists, giving this (tail) recursion:
<pre class=code-example>
(pair-fold <var>kons</var> <var>knil</var> <var>lis</var>) = (let ((tail (cdr <var>lis</var>)))
(pair-fold <var>kons</var> (<var>kons</var> <var>lis</var> <var>knil</var>) tail))
(pair-fold <var>kons</var> <var>knil</var> <code>'()</code>) = <var>knil</var>
</pre>
For finite lists, the <var>kons</var> function may reliably apply
<code>set-cdr!</code> to the pairs it is given
without altering the sequence of execution.
<p>
Example:
<pre class=code-example>
;;; Destructively reverse a list.
(pair-fold (lambda (pair tail) (set-cdr! pair tail) pair) '() lis))
</pre>
At least one of the list arguments must be finite.
<!--
==== pair-fold-right
============================================================================-->
<dt class=proc-def>
<a name="pair-fold-right"></a>
<code class=proc-def>pair-fold-right</code><var> kons knil clist<sub>1</sub> clist<sub>2</sub> ... -&gt; value</var>
<dd class=proc-def>
Holds the same relationship with <code>fold-right</code> that <code>pair-fold</code> holds with <code>fold</code>.
Obeys the recursion
<pre class=code-example>
(pair-fold-right <var>kons</var> <var>knil</var> <var>lis</var>) =
(<var>kons</var> <var>lis</var> (pair-fold-right <var>kons</var> <var>knil</var> (cdr <var>lis</var>)))
(pair-fold-right <var>kons</var> <var>knil</var> <code>'()</code>) = <var>knil</var>
</pre>
Example:
<pre class=code-example>
(pair-fold-right cons '() '(a b c)) =&gt; ((a b c) (b c) (c))
</pre>
At least one of the list arguments must be finite.
<!--
==== reduce
============================================================================-->
<dt class=proc-def>
<a name="reduce"></a>
<code class=proc-def>reduce</code><var> f ridentity list -&gt; value</var>
<dd class=proc-def>
<code>reduce</code> is a variant of <code>fold</code>.
<p>
<var>ridentity</var> should be a "right identity" of the procedure <var>f</var> -- that is,
for any value <var>x</var> acceptable to <var>f</var>,
<pre class=code-example>
(<var>f</var> <var>x</var> <var>ridentity</var>) = <var>x</var>
</pre>
<code>reduce</code> has the following definition:
<div class=indent>
If <var>list</var> = (), return <var>ridentity</var>;<br>
Otherwise, return <code>(fold <var>f</var> (car <var>list</var>) (cdr <var>list</var>))</code>.
</div>
...in other words, we compute
<code>(fold <var>f</var> <var>ridentity</var> <var>list</var>)</code>.
<p>
Note that <var>ridentity</var> is used <em>only</em> in the empty-list case.
You typically use <code>reduce</code> when applying <var>f</var> is expensive and you'd
like to avoid the extra application incurred when <code>fold</code> applies
<var>f</var> to the head of <var>list</var> and the identity value,
redundantly producing the same value passed in to <var>f</var>.
For example, if <var>f</var> involves searching a file directory or
performing a database query, this can be significant.
In general, however, <code>fold</code> is useful in many contexts where <code>reduce</code> is not
(consider the examples given in the <code>fold</code> definition -- only one of the
five folds uses a function with a right identity.
The other four may not be performed with <code>reduce</code>).
<p>
Note: MIT Scheme and Haskell flip F's arg order for their <code>reduce</code> and
<code>fold</code> functions.
<!--
==== reduce-right
============================================================================-->
<dt class=proc-def>
<a name="reduce-right"></a>
<code class=proc-def>reduce-right</code><var> f ridentity list -&gt; value</var>
<dd class=proc-def>
<code>reduce-right</code> is the fold-right variant of <code>reduce</code>.
It obeys the following definition:
<pre class=code-example>
(reduce-right <var>f</var> <var>ridentity</var> '()) = <var>ridentity</var>
(reduce-right <var>f</var> <var>ridentity</var> '(<var>e<sub>1</sub></var>)) = (<var>f</var> <var>e<sub>1</sub></var> <var>ridentity</var>) = <var>e<sub>1</sub></var>
(reduce-right <var>f</var> <var>ridentity</var> '(<var>e<sub>1</sub></var> <var>e<sub>2</sub></var> ...)) =
(<var>f</var> <var>e<sub>1</sub></var> (reduce <var>f</var> <var>ridentity</var> (<var>e<sub>2</sub></var> ...)))
</pre>
...in other words, we compute
<code>(fold-right <var>f</var> <var>ridentity</var> <var>list</var>)</code>.
<!--
==== unfold
============================================================================-->
<dt class=proc-def>
<a name="unfold"></a>
<code class=proc-def>unfold</code><var> p f g seed [tail] -&gt; list</var>
<dd class=proc-def>
<code>unfold</code> constructs a list with the following loop:
<pre class=code-example>
(let lp ((seed seed) (lis tail))
(if (p seed) lis
(lp (g seed)
(cons (f seed) lis))))
</pre>
<dl>
<dt> <var>p</var> <dd> Determines when to stop unfolding.
<dt> <var>f</var> <dd> Maps each seed value to the corresponding list element.
<dt> <var>g</var> <dd> Maps each seed value to next seed value.
<dt> <var>seed</var> <dd> The "state" value for the unfold.
<dt> <var>tail</var> <dd> list terminator; defaults to <code>'()</code>.
</dl>
<p>
<code>unfold</code> is the fundamental iterative list constructor,
just as <code>fold</code> is the
fundamental iterative list consumer.
While <code>unfold</code> may seem a bit abstract
to novice functional programmers, it can be used in a number of ways:
<pre class=code-example>
;; List of squares: 1^2 ... 10^2
(unfold zero?
(lambda (x) (* x x))
(lambda (x) (- x 1))
10)
;; Reverse a proper list.
(unfold null-list? car cdr lis)
;; Read current input port into a list of values.
(unfold eof-object? values (lambda (x) (read)) (read))
;; (append-reverse rev-head tail)
(unfold null-list? car cdr rev-head tail)
</pre>
Interested functional programmers may enjoy noting that
<code>fold</code> and <code>unfold</code>
are in some sense inverses.
That is, given operations <var>knull?</var>, <var>kar</var>,
<var>kdr</var>, <var>kons</var>, and <var>knil</var> satisfying
<div class=indent>
<code>(<var>kons</var> (<var>kar</var> <var>x</var>) (<var>kdr</var> <var>x</var>))</code> = <code>x</code>
and
<code>(<var>knull?</var> <var>knil</var>)</code> = <code>#t</code>
</div>
then
<div class=indent>
<code>(fold <var>kons</var> <var>knil</var> (unfold <var>knull?</var> <var>kar</var> <var>kdr</var> <var>x</var>))</code> = <var>x</var>
</div>
and
<div class=indent>
<code>(unfold <var>knull?</var> <var>kar</var> <var>kdr</var> (fold <var>kons</var> <var>knil</var> <var>x</var>))</code> = <var>x</var>.
</div>
This combinator presumably has some pretentious mathematical name;
interested readers are invited to communicate it to the author.
<!--
==== unfold-right
============================================================================-->
<dt class=proc-def>
<a name="unfold-right"></a>
<code class=proc-def>unfold-right</code><var> p f g seed [tail-gen] -&gt; list</var>
<dd class=proc-def>
<code>unfold</code> is best described by its basic recursion:
<pre class=code-example>
(unfold-right <var>p</var> <var>f</var> <var>g</var> <var>seed</var>) =
(if (<var>p</var> <var>seed</var>) (<var>tail-gen</var> <var>seed</var>)
(cons (<var>f</var> <var>seed</var>)
(unfold-right <var>p</var> <var>f</var> <var>g</var> (<var>g</var> <var>seed</var>))))
</pre>
<dl>
<dt> <var>p</var> <dd> Determines when to stop unfolding.
<dt> <var>f</var> <dd> Maps each seed value to the corresponding list element.
<dt> <var>g</var> <dd> Maps each seed value to next seed value.
<dt> <var>seed</var> <dd> The "state" value for the unfold.
<dt> <var>tail-gen</var> <dd> Creates the tail of the list;
defaults to <code>(lambda (x) '())</code>
</dl>
<p>
<code>unfold-right</code> is the fundamental recursive list constructor,
just as <code>fold-right</code> is
the fundamental recursive list consumer.
While <code>unfold-right</code> may seem a bit abstract
to novice functional programmers, it can be used in a number of ways:
<pre class=code-example>
;; List of squares: 1^2 ... 10^2
(unfold-right (lambda (x) (&gt; x 10))
(lambda (x) (* x x))
(lambda (x) (+ x 1))
1)
(unfold-right null-list? car cdr lis) ; Copy a proper list.
;; Read current input port into a list of values.
(unfold-right eof-object? values (lambda (x) (read)) (read))
;; Copy a possibly non-proper list:
(unfold-right not-pair? car cdr lis
values)
;; Append HEAD onto TAIL:
(unfold-right null-list? car cdr head
(lambda (x) tail))
</pre>
Interested functional programmers may enjoy noting that
<code>fold-right</code> and <code>unfold-right</code>
are in some sense inverses.
That is, given operations <var>knull?</var>, <var>kar</var>,
<var>kdr</var>, <var>kons</var>, and <var>knil</var> satisfying
<div class=indent>
<code>(<var>kons</var> (<var>kar</var> <var>x</var>) (<var>kdr</var> <var>x</var>))</code> = <code>x</code>
and
<code>(<var>knull?</var> <var>knil</var>)</code> = <code>#t</code>
</div>
then
<div class=indent>
<code>(fold-right <var>kons</var> <var>knil</var> (unfold-right <var>knull?</var> <var>kar</var> <var>kdr</var> <var>x</var>))</code> = <var>x</var>
</div>
and
<div class=indent>
<code>(unfold-right <var>knull?</var> <var>kar</var> <var>kdr</var> (fold-right <var>kons</var> <var>knil</var> <var>x</var>))</code> = <var>x</var>.
</div>
This combinator sometimes is called an "anamorphism;" when an
explicit <var>tail-gen</var> procedure is supplied, it is called an
"apomorphism."
<!--
==== map
============================================================================-->
<dt class=proc-def>
<a name="map"></a>
<code class=proc-def>map</code><var> proc clist<sub>1</sub> clist<sub>2</sub> ... -&gt; list</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>+]
<var>proc</var> is a procedure taking as many arguments
as there are list arguments and returning a single value.
<code>map</code> applies <var>proc</var> element-wise to the elements
of the lists and returns a list of the results,
in order.
The dynamic order in which <var>proc</var>
is applied to the elements of the lists is unspecified.
<pre class=code-example>
(map cadr '((a b) (d e) (g h))) => (b e h)
(map (lambda (n) (expt n n))
'(1 2 3 4 5))
=> (1 4 27 256 3125)
(map + '(1 2 3) '(4 5 6)) => (5 7 9)
(let ((count 0))
(map (lambda (ignored)
(set! count (+ count 1))
count)
'(a b))) => (1 2) <em>or</em> (2 1)
</pre>
This procedure is extended from its
<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>
specification to allow the arguments to be of unequal length;
it terminates when the shortest list runs out.
<p>
At least one of the argument lists must be finite:
<pre class=code-example>
(map + '(3 1 4 1) (circular-list 1 0)) => (4 1 5 1)
</pre>
<!--
==== for-each
============================================================================-->
<dt class=proc-def>
<a name="for-each"></a>
<code class=proc-def>for-each</code><var> proc clist<sub>1</sub> clist<sub>2</sub> ... -&gt; unspecified</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>+]
The arguments to <code>for-each</code> are like the arguments to
<code>map</code>, but
<code>for-each</code> calls <var>proc</var> for its side effects rather
than for its values.
Unlike <code>map</code>, <code>for-each</code> is guaranteed to call
<var>proc</var> on the elements of the lists in order from the first
element(s) to the last,
and the value returned by <code>for-each</code> is unspecified.
<pre class=code-example>
(let ((v (make-vector 5)))
(for-each (lambda (i)
(vector-set! v i (* i i)))
'(0 1 2 3 4))
v) => #(0 1 4 9 16)
</pre>
This procedure is extended from its
<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>
specification to allow the arguments to be of unequal length;
it terminates when the shortest list runs out.
<p>
At least one of the argument lists must be finite.
<!--
==== append-map!
==== append-map
============================================================================-->
<dt class=proc-def1>
<a name="append-map"></a>
<code class=proc-def>append-map&nbsp;&nbsp;</code><var>f clist<sub>1</sub> clist<sub>2</sub> ... -&gt; value</var>
<dt class=proc-defn>
<a name="append-map!"></a>
<code class=proc-def>append-map!&nbsp;</code><var>f clist<sub>1</sub> clist<sub>2</sub> ... -&gt; value</var>
<dd class=proc-def>
Equivalent to
<div class=indent><code>
(apply append (map <var>f</var> <var>clist<sub>1</sub></var> <var>clist<sub>2</sub></var> ...))
</code></div>
and
<div class=indent><code>
(apply append! (map <var>f</var> <var>clist<sub>1</sub></var> <var>clist<sub>2</sub></var> ...))
</code></div>
Map <var>f</var> over the elements of the lists, just as in the <code>map</code> function.
However, the results of the applications are appended together to
make the final result. <code>append-map</code> uses <code>append</code> to append the results
together; <code>append-map!</code> uses <code>append!</code>.
<p>
The dynamic order in which the various applications of <var>f</var> are made is
not specified.
<p>
Example:
<pre class=code-example>
(append-map! (lambda (x) (list x (- x))) '(1 3 8))
=&gt; (1 -1 3 -3 8 -8)
</pre>
At least one of the list arguments must be finite.
<!--
==== map!
============================================================================-->
<dt class=proc-def>
<a name="map!"></a>
<code class=proc-def>map!</code><var> f list<sub>1</sub> clist<sub>2</sub> ... -&gt; list</var>
<dd class=proc-def>
Linear-update variant of <code>map</code> -- <code>map!</code> is allowed, but not required, to
alter the cons cells of <var>list<sub>1</sub></var> to construct the result list.
<p>
The dynamic order in which the various applications of <var>f</var> are made is
not specified.
In the n-ary case, <var>clist<sub>2</sub></var>, <var>clist<sub>3</sub></var>, ... must have at least as many
elements as <var>list<sub>1</sub></var>.
<!--
==== map-in-order
============================================================================-->
<dt class=proc-def>
<a name="map-in-order"></a>
<code class=proc-def>map-in-order </code><var>f</var> <var>clist<sub>1</sub> clist<sub>2</sub> ... -&gt; list</var>
<dd class=proc-def>
A variant of the <code>map</code> procedure that guarantees to apply <var>f</var> across
the elements of the <var>list<sub>i</sub></var> arguments in a left-to-right order. This
is useful for mapping procedures that both have side effects and
return useful values.
<p>
At least one of the list arguments must be finite.
<!--
==== pair-for-each
============================================================================-->
<dt class=proc-def>
<a name="pair-for-each"></a>
<code class=proc-def>pair-for-each </code><var>f clist<sub>1</sub> clist<sub>2</sub> ... -&gt; unspecific</var>
<dd class=proc-def>
Like <code>for-each</code>, but <var>f</var> is applied to successive sublists of the argument
lists. That is, <var>f</var> is applied to the cons cells of the lists, rather
than the lists' elements. These applications occur in left-to-right
order.
<p>
The <var>f</var> procedure may reliably apply <code>set-cdr!</code> to the pairs it is given
without altering the sequence of execution.
<pre class=code-example>
(pair-for-each (lambda (pair) (display pair) (newline)) '(a b c)) ==&gt;
(a b c)
(b c)
(c)
</pre>
At least one of the list arguments must be finite.
<!--
==== filter-map
============================================================================-->
<dt class=proc-def>
<a name="filter-map"></a>
<code class=proc-def>filter-map</code><var> f clist<sub>1</sub> clist<sub>2</sub> ... -&gt; list</var>
<dd class=proc-def>
Like <code>map</code>, but only true values are saved.
<pre class=code-example>
(filter-map (lambda (x) (and (number? x) (* x x))) '(a 1 b 3 c 7))
=&gt; (1 9 49)
</pre>
The dynamic order in which the various applications of <var>f</var> are made is
not specified.
<p>
At least one of the list arguments must be finite.
</dl>
<!--========================================================================-->
<h2><a name="FilteringPartitioning">Filtering &amp; partitioning</a></h2>
<dl>
<!--
==== filter
============================================================================-->
<dt class=proc-def>
<a name="filter"></a>
<code class=proc-def>filter</code><var> pred list -&gt; list</var>
<dd class=proc-def>
Return all the elements of <var>list</var> that satisfy predicate <var>pred</var>.
The list is not disordered -- elements that appear in the result list
occur in the same order as they occur in the argument list.
The returned list may share a common tail with the argument list.
The dynamic order in which the various applications of <var>pred</var> are made is
not specified.
<pre class=code-example>
(filter even? '(0 7 8 8 43 -4)) =&gt; (0 8 8 -4)
</pre>
<!--
==== partition
============================================================================-->
<dt class=proc-def>
<a name="partition"></a>
<code class=proc-def>partition</code><var> pred list -&gt; [list list]</var>
<dd class=proc-def>
Partitions the elements of <var>list</var> with predicate <var>pred</var>, and returns two
values: the list of in-elements and the list of out-elements.
The list is not disordered -- elements occur in the result lists
in the same order as they occur in the argument list.
The dynamic order in which the various applications of <var>pred</var> are made is
not specified. One of the returned lists may share a common tail with the
argument list.
<pre class=code-example>
(partition symbol? '(one 2 3 four five 6)) =&gt;
(one four five)
(2 3 6)
</pre>
<!--
==== remove
============================================================================-->
<dt class=proc-def>
<a name="remove"></a>
<code class=proc-def>remove</code><var> pred list -&gt; list</var>
<dd class=proc-def>
Returns <var>list</var> without the elements that satisfy predicate <var>pred</var>:
<pre class=code-example>
(lambda (pred list) (filter (lambda (x) (not (pred x))) list))
</pre>
The list is not disordered -- elements that appear in the result list
occur in the same order as they occur in the argument list.
The returned list may share a common tail with the argument list.
The dynamic order in which the various applications of <var>pred</var> are made is
not specified.
<pre class=code-example>
(remove even? '(0 7 8 8 43 -4)) =&gt; (7 43)
</pre>
<!--
==== remove!
==== partition!
==== filter!
============================================================================-->
<dt class=proc-def1>
<a name="filter!"></a>
<code class=proc-def>filter!&nbsp;&nbsp;&nbsp;&nbsp;</code><var>pred list -&gt; list</var>
<dt class=proc-defi>
<a name="partition!"></a>
<code class=proc-def>partition!&nbsp;</code><var>pred list -&gt; [list list]</var>
<dt class=proc-defn>
<a name="remove!"></a>
<code class=proc-def>remove!&nbsp;&nbsp;&nbsp;&nbsp;</code><var>pred list -&gt; list</var>
<dd class=proc-def>
Linear-update variants of <code>filter</code>, <code>partition</code> and <code>remove</code>.
These procedures are allowed, but not required, to alter the cons cells
in the argument list to construct the result lists.
</dl>
<!--========================================================================-->
<h2><a name="Searching">Searching</a></h2>
<p>
The following procedures all search lists for a leftmost element satisfying
some criteria. This means they do not always examine the entire list; thus,
there is no efficient way for them to reliably detect and signal an error when
passed a dotted or circular list. Here are the general rules describing how
these procedures work when applied to different kinds of lists:
<dl>
<dt> Proper lists:
<dd> The standard, canonical behavior happens in this case.
<dt> Dotted lists:
<dd> It is an error to pass these procedures a dotted list
that does not contain an element satisfying the search
criteria. That is, it is an error if the procedure has
to search all the way to the end of the dotted list.
However, this <abbr title="Scheme Request for Implementation">SRFI</abbr> does <em>not</em> specify anything at all
about the behavior of these procedures when passed a
dotted list containing an element satisfying the search
criteria. It may finish successfully, signal an error,
or perform some third action. Different implementations
may provide different functionality in this case; code
which is compliant with this <abbr title="Scheme Request for Implementation">SRFI</abbr> may not rely on any
particular behavior. Future <abbr title="Scheme Request for Implementation">SRFI</abbr>'s may refine SRFI-1
to define specific behavior in this case.
<p>
In brief, SRFI-1 compliant code may not pass a dotted
list argument to these procedures.
<dt> Circular lists:
<dd> It is an error to pass these procedures a circular list
that does not contain an element satisfying the search
criteria. Note that the procedure is not required to
detect this case; it may simply diverge. It is, however,
acceptable to search a circular list <em>if the search is
successful</em> -- that is, if the list contains an element
satisfying the search criteria.
</dl>
<p>
Here are some examples, using the <code>find</code> and <code>any</code> procedures as canonical
representatives:
<pre class=code-example>
;; Proper list -- success
(find even? '(1 2 3)) =&gt; 2
(any even? '(1 2 3)) =&gt; #t
;; proper list -- failure
(find even? '(1 7 3)) =&gt; #f
(any even? '(1 7 3)) =&gt; #f
;; Failure is error on a dotted list.
(find even? '(1 3 . x)) =&gt; error
(any even? '(1 3 . x)) =&gt; error
;; The dotted list contains an element satisfying the search.
;; This case is not specified -- it could be success, an error,
;; or some third possibility.
(find even? '(1 2 . x)) =&gt; error/undefined
(any even? '(1 2 . x)) =&gt; error/undefined ; success, error or other.
;; circular list -- success
(find even? (circular-list 1 6 3)) =&gt; 6
(any even? (circular-list 1 6 3)) =&gt; #t
;; circular list -- failure is error. Procedure may diverge.
(find even? (circular-list 1 3)) =&gt; error
(any even? (circular-list 1 3)) =&gt; error
</pre>
<dl>
<!--
==== find
============================================================================-->
<dt class=proc-def>
<a name="find"></a>
<code class=proc-def>find</code><var> pred clist -&gt; value</var>
<dd class=proc-def>
Return the first element of <var>clist</var> that satisfies predicate <var>pred</var>;
false if no element does.
<pre class=code-example>
(find even? '(3 1 4 1 5 9)) =&gt; 4
</pre>
Note that <code>find</code> has an ambiguity in its lookup semantics -- if <code>find</code>
returns <code>#f</code>, you cannot tell (in general) if it found a <code>#f</code> element
that satisfied <var>pred</var>, or if it did not find any element at all. In
many situations, this ambiguity cannot arise -- either the list being
searched is known not to contain any <code>#f</code> elements, or the list is
guaranteed to have an element satisfying <var>pred</var>. However, in cases
where this ambiguity can arise, you should use <code>find-tail</code> instead of
<code>find</code> -- <code>find-tail</code> has no such ambiguity:
<pre class=code-example>
(cond ((find-tail pred lis) =&gt; (lambda (pair) ...)) ; Handle (CAR PAIR)
(else ...)) ; Search failed.
</pre>
<!--
==== find-tail
============================================================================-->
<dt class=proc-def>
<a name="find-tail"></a>
<code class=proc-def>find-tail</code><var> pred clist -&gt; pair or false</var>
<dd class=proc-def>
Return the first pair of <var>clist</var> whose car satisfies <var>pred</var>. If no pair does,
return false.
<p>
<code>find-tail</code> can be viewed as a general-predicate variant of the <code>member</code>
function.
<p>
Examples:
<pre class=code-example>
(find-tail even? '(3 1 37 -8 -5 0 0)) =&gt; (-8 -5 0 0)
(find-tail even? '(3 1 37 -5)) =&gt; #f
;; MEMBER X LIS:
(find-tail (lambda (elt) (equal? x elt)) lis)
</pre>
In the circular-list case, this procedure "rotates" the list.
<!--
==== any
============================================================================-->
<dt class=proc-def>
<a name="any"></a>
<code class=proc-def>any</code><var> pred clist<sub>1</sub> clist<sub>2</sub> ... -&gt; value</var>
<dd class=proc-def>
Applies the predicate across the lists, returning true if the predicate
returns true on any application.
<p>
If there are <var>n</var> list arguments <var>clist<sub>1</sub></var> ... <var>clist<sub>n</sub></var>, then <var>pred</var> must be a
procedure taking <var>n</var> arguments and returning a boolean result.
<p>
<code>any</code> applies <var>pred</var> to the first elements of the <var>clist<sub>i</sub></var> parameters.
If this application returns a true value, <code>any</code> immediately returns
that value. Otherwise, it iterates, applying <var>pred</var> to the second
elements of the <var>clist<sub>i</sub></var> parameters, then the third, and so forth.
The iteration stops when a true value is produced or one of the lists runs
out of values; in
the latter case, <code>any</code> returns <code>#f</code>.
The application of <var>pred</var> to the last element of the
lists is a tail call.
<p>
Note the difference between <code>find</code> and <code>any</code> -- <code>find</code> returns the element
that satisfied the predicate; <code>any</code> returns the true value that the
predicate produced.
<p>
Like <code>every</code>, <code>any</code>'s name does not end with a question mark -- this is to
indicate that it does not return a simple boolean (<code>#t</code> or <code>#f</code>), but a
general value.
<pre class=code-example>
(any integer? '(a 3 b 2.7)) =&gt; #t
(any integer? '(a 3.1 b 2.7)) =&gt; #f
(any &lt; '(3 1 4 1 5)
'(2 7 1 8 2)) =&gt; #t
</pre>
<!--
==== every
============================================================================-->
<dt class=proc-def>
<a name="every"></a>
<code class=proc-def>every</code><var> pred clist<sub>1</sub> clist<sub>2</sub> ... -&gt; value</var>
<dd class=proc-def>
Applies the predicate across the lists, returning true if the predicate
returns true on every application.
<p>
If there are <var>n</var> list arguments <var>clist<sub>1</sub></var> ... <var>clist<sub>n</sub></var>, then <var>pred</var> must be a
procedure taking <var>n</var> arguments and returning a boolean result.
<p>
<code>every</code> applies <var>pred</var> to the first elements of the <var>clist<sub>i</sub></var> parameters.
If this application returns false, <code>every</code> immediately returns false.
Otherwise, it iterates, applying <var>pred</var> to the second elements of the
<var>clist<sub>i</sub></var> parameters, then the third, and so forth. The iteration stops
when a false value is produced or one of the lists runs out of values.
In the latter case, <code>every</code> returns
the true value produced by its final application of <var>pred</var>.
The application of <var>pred</var> to the last element of the lists
is a tail call.
<p>
If one of the <var>clist<sub>i</sub></var> has no elements, <code>every</code> simply returns <code>#t</code>.
<p>
Like <code>any</code>, <code>every</code>'s name does not end with a question mark -- this is to
indicate that it does not return a simple boolean (<code>#t</code> or <code>#f</code>), but a
general value.
<!--
==== list-index
============================================================================-->
<dt class=proc-def>
<a name="list-index"></a>
<code class=proc-def>list-index</code><var> pred clist<sub>1</sub> clist<sub>2</sub> ... -&gt; integer or false</var>
<dd class=proc-def>
Return the index of the leftmost element that satisfies <var>pred</var>.
<p>
If there are <var>n</var> list arguments <var>clist<sub>1</sub></var> ... <var>clist<sub>n</sub></var>, then <var>pred</var> must be a
function taking <var>n</var> arguments and returning a boolean result.
<p>
<code>list-index</code> applies <var>pred</var> to the first elements of the <var>clist<sub>i</sub></var> parameters.
If this application returns true, <code>list-index</code> immediately returns zero.
Otherwise, it iterates, applying <var>pred</var> to the second elements of the
<var>clist<sub>i</sub></var> parameters, then the third, and so forth. When it finds a tuple of
list elements that cause <var>pred</var> to return true, it stops and returns the
zero-based index of that position in the lists.
<p>
The iteration stops when one of the lists runs out of values; in this
case, <code>list-index</code> returns <code>#f</code>.
<pre class=code-example>
(list-index even? '(3 1 4 1 5 9)) =&gt; 2
(list-index &lt; '(3 1 4 1 5 9 2 5 6) '(2 7 1 8 2)) =&gt; 1
(list-index = '(3 1 4 1 5 9 2 5 6) '(2 7 1 8 2)) =&gt; #f
</pre>
<!--
==== member memq memv
============================================================================-->
<dt class=proc-def1>
<a name="member"></a>
<code class=proc-def>member</code><var> x list [=] -&gt; list</var>
<dt class=proc-defi>
<a name="memq"></a>
<code class=proc-def>memq</code><var> x list -&gt; list</var>
<dt class=proc-defn>
<a name="memv"></a>
<code class=proc-def>memv</code><var> x list -&gt; list</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>+]
These procedures return the first sublist of <var>list</var> whose car is
<var>x</var>, where the sublists of <var>list</var> are the
non-empty lists returned by
<code>(drop <var>list</var> <var>i</var>)</code>
for <var>i</var> less than the length of <var>list</var>.
If <var>x</var> does
not occur in <var>list</var>, then <code>#f</code> is returned.
<code>memq</code> uses <code>eq?</code> to compare <var>x</var>
with the elements of <var>list</var>,
while <code>memv</code> uses <code>eqv?</code>, and
<code>member</code> uses <code>equal?</code>.
<pre class=code-example>
(memq 'a '(a b c)) => (a b c)
(memq 'b '(a b c)) => (b c)
(memq 'a '(b c d)) => #f
(memq (list 'a) '(b (a) c)) => #f
(member (list 'a)
'(b (a) c)) => ((a) c)
(memq 101 '(100 101 102)) => *unspecified*
(memv 101 '(100 101 102)) => (101 102)
</pre>
<code>member</code> is extended from its
<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>
definition to allow the client to pass in
an optional equality procedure <var>=</var> used to compare keys.
<p>
The comparison procedure is used to compare the elements <var>e<sub>i</sub></var> of <var>list</var>
to the key <var>x</var> in this way:
<div class=indent><code>
(= <var>x</var> <var>e<sub>i</sub></var>) ; list is (E1 ... En)
</code></div>
That is, the first argument is always <var>x</var>, and the second argument is
one of the list elements. Thus one can reliably find the first element
of <var>list</var> that is greater than five with
<code>(member 5 <var>list</var> &lt;)</code>
<p>
Note that fully general list searching may be performed with
the <code>find-tail</code> and <code>find</code> procedures, <em>e.g.</em>
<pre class=code-example>
(find-tail even? list) ; Find the first elt with an even key.
</pre>
</dl>
<!--========================================================================-->
<h2><a name="Deletion">Deletion</a></h2>
<p>
<dl>
<!--
==== delete!
==== delete
============================================================================-->
<dt class=proc-def1>
<a name="delete"></a>
<code class=proc-def>delete&nbsp;&nbsp;</code><var>x list [=] -&gt; list</var>
<dt class=proc-defn>
<a name="delete!"></a>
<code class=proc-def>delete!&nbsp;</code><var>x list [=] -&gt; list</var>
<dd class=proc-def>
<code>delete</code> uses the comparison procedure =, which defaults to <code>equal?</code>, to find
all elements of <var>list</var> that are equal to <var>x</var>, and deletes them from <var>list</var>. The
dynamic order in which the various applications of <var>=</var> are made is not
specified.
<p>
The list is not disordered -- elements that appear in the result list
occur in the same order as they occur in the argument list.
The result may share a common tail with the argument list.
<p>
Note that fully general element deletion can be performed with the <code>remove</code>
and <code>remove!</code> procedures, <em>e.g.</em>:
<pre class=code-example>
;; Delete all the even elements from LIS:
(remove even? lis)
</pre>
The comparison procedure is used in this way:
<code>(= <var>x</var> <var>e<sub>i</sub></var>)</code>.
That is, <var>x</var> is always the first argument,
and a list element is always the
second argument. The comparison procedure will be used to compare each
element of <var>list</var> exactly once; the order in which it is applied to the
various <var>e<sub>i</sub></var> is not specified. Thus, one can reliably remove all the
numbers greater than five from a list with
<code>(delete 5 list &lt;)</code>
<p>
<code>delete!</code> is the linear-update variant of <code>delete</code>.
It is allowed, but not required, to alter the cons cells in
its argument list to construct the result.
<!--
==== delete-duplicates!
==== delete-duplicates
============================================================================-->
<dt class=proc-def1>
<a name="delete-duplicates"></a>
<code class=proc-def>delete-duplicates&nbsp;&nbsp;</code><var>list [=] -&gt; list</var>
<dt class=proc-defn>
<a name="delete-duplicates!"></a>
<code class=proc-def>delete-duplicates!&nbsp;</code><var>list [=] -&gt; list</var>
<dd class=proc-def>
<code>delete-duplicates</code> removes duplicate elements from the
list argument.
If there are multiple equal elements in the argument list, the result list
only contains the first or leftmost of these elements in the result.
The order of these surviving elements is the same as in the original
list -- <code>delete-duplicates</code> does not disorder the list (hence it is useful
for "cleaning up" association lists).
<p>
The <var>=</var> parameter is used to compare the elements of the list; it defaults
to <code>equal?</code>. If <var>x</var> comes before <var>y</var> in <var>list</var>, then the comparison is performed
<code>(= <var>x</var> <var>y</var>)</code>.
The comparison procedure will be used to compare each pair of elements in
<var>list</var> no more than once;
the order in which it is applied to the various pairs is not specified.
<p>
Implementations of <code>delete-duplicates</code>
are allowed to share common tails
between argument and result lists -- for example, if the list argument
contains only unique elements, it may simply return exactly
this list.
<p>
Be aware that, in general, <code>delete-duplicates</code>
runs in time O(n<sup>2</sup>) for <var>n</var>-element lists.
Uniquifying long lists can be accomplished in O(n lg n) time by sorting
the list to bring equal elements together, then using a linear-time
algorithm to remove equal elements. Alternatively, one can use algorithms
based on element-marking, with linear-time results.
<p>
<code>delete-duplicates!</code> is the linear-update variant of <code>delete-duplicates</code>; it
is allowed, but not required, to alter the cons cells in its argument
list to construct the result.
<pre class=code-example>
(delete-duplicates '(a b a c a b c z)) =&gt; (a b c z)
;; Clean up an alist:
(delete-duplicates '((a . 3) (b . 7) (a . 9) (c . 1))
(lambda (x y) (eq? (car x) (car y))))
=&gt; ((a . 3) (b . 7) (c . 1))
</pre>
</dl>
<!--========================================================================-->
<h2><a name="AssociationLists">Association lists</a></h2>
<p>
An "association list" (or "alist") is a list of pairs. The car of each pair
contains a key value, and the cdr contains the associated data value. They can
be used to construct simple look-up tables in Scheme. Note that association
lists are probably inappropriate for performance-critical use on large data;
in these cases, hash tables or some other alternative should be employed.
<dl>
<!--
==== assoc assq assv
============================================================================-->
<dt class=proc-def1>
<a name="assoc"></a>
<code class=proc-def>assoc</code><var> key alist [=] -&gt; pair or #f</var>
<dt class=proc-defi>
<a name="assq"></a>
<code class=proc-def>assq</code><var> key alist -&gt; pair or #f</var>
<dt class=proc-defn>
<a name="assv"></a>
<code class=proc-def>assv</code><var> key alist -&gt; pair or #f</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>+]
<var>alist</var> must be an association list -- a list of pairs.
These procedures
find the first pair in <var>alist</var> whose car field is <var>key</var>,
and returns that pair.
If no pair in <var>alist</var> has <var>key</var> as its car,
then <code>#f</code> is returned.
<code>assq</code> uses <code>eq?</code> to compare <var>key</var>
with the car fields of the pairs in <var>alist</var>,
while <code>assv</code> uses <code>eqv?</code>
and <code>assoc</code> uses <code>equal?</code>.
<pre class=code-example>
(define e '((a 1) (b 2) (c 3)))
(assq 'a e) => (a 1)
(assq 'b e) => (b 2)
(assq 'd e) => #f
(assq (list 'a) '(((a)) ((b)) ((c)))) => #f
(assoc (list 'a) '(((a)) ((b)) ((c)))) => ((a))
(assq 5 '((2 3) (5 7) (11 13))) => *unspecified*
(assv 5 '((2 3) (5 7) (11 13))) => (5 7)
</pre>
<code>assoc</code> is extended from its
<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>
definition to allow the client to pass in
an optional equality procedure <var>=</var> used to compare keys.
<p>
The comparison procedure is used to compare the elements <var>e<sub>i</sub></var> of <var>list</var>
to the <var>key</var> parameter in this way:
<div class=indent><code>
(= <var>key</var> (car <var>e<sub>i</sub></var>)) ; list is (E1 ... En)
</code></div>
That is, the first argument is always <var>key</var>,
and the second argument is one of the list elements.
Thus one can reliably find the first entry
of <var>alist</var> whose key is greater than five with
<code>(assoc 5 <var>alist</var> &lt;)</code>
<p>
Note that fully general alist searching may be performed with
the <code>find-tail</code> and <code>find</code> procedures, <em>e.g.</em>
<pre class=code-example>
;; Look up the first association in <var>alist</var> with an even key:
(find (lambda (a) (even? (car a))) alist)
</pre>
<!--
==== alist-cons
============================================================================-->
<dt class=proc-def>
<a name="alist-cons"></a>
<code class=proc-def>alist-cons</code><var> key datum alist -&gt; alist</var>
<dd class=proc-def>
<pre>
(lambda (key datum alist) (cons (cons key datum) alist))
</pre>
Cons a new alist entry mapping <var>key</var> -&gt; <var>datum</var> onto <var>alist</var>.
<!--
==== alist-copy
============================================================================-->
<dt class=proc-def>
<a name="alist-copy"></a>
<code class=proc-def>alist-copy</code><var> alist -&gt; alist</var>
<dd class=proc-def>
Make a fresh copy of <var>alist</var>. This means copying each pair that
forms an association as well as the spine of the list, <em>i.e.</em>
<pre>
(lambda (a) (map (lambda (elt) (cons (car elt) (cdr elt))) a))
</pre>
<!--
==== alist-delete!
==== alist-delete
============================================================================-->
<dt class=proc-def1>
<a name="alist-delete"></a>
<code class=proc-def>alist-delete&nbsp;&nbsp;</code><var>key alist [=] -&gt; alist</var>
<dt class=proc-defn>
<a name="alist-delete!"></a>
<code class=proc-def>alist-delete!&nbsp;</code><var>key alist [=] -&gt; alist</var>
<dd class=proc-def>
<code>alist-delete</code> deletes all associations from <var>alist</var> with the given <var>key</var>,
using key-comparison procedure =, which defaults to <code>equal?</code>.
The dynamic order in which the various applications of <var>=</var> are made is not
specified.
<p>
Return values may share common tails with the <var>alist</var> argument.
The alist is not disordered -- elements that appear in the result alist
occur in the same order as they occur in the argument alist.
<p>
The comparison procedure is used to compare the element keys <var>k<sub>i</sub></var> of <var>alist</var>'s
entries to the <var>key</var> parameter in this way:
<code>(= <var>key</var> <var>k<sub>i</sub></var>)</code>.
Thus, one can reliably remove all entries of <var>alist</var> whose key is greater
than five with
<code>(alist-delete 5 <var>alist</var> &lt;)</code>
<p>
<code>alist-delete!</code> is the linear-update variant of <code>alist-delete</code>.
It is allowed, but not required,
to alter cons cells from the <var>alist</var> parameter to construct the result.
</dl>
<!--========================================================================-->
<h2><a name="SetOperationsOnLists">Set operations on lists</a></h2>
<p>
These procedures implement operations on sets represented as lists of elements.
They all take an <var>=</var> argument used to compare elements of lists.
This equality procedure is required to be consistent with <code>eq?</code>.
That is, it must be the case that
<div class=indent>
<code>(eq? <var>x</var> <var>y</var>)</code> => <code>(<var>=</var> <var>x</var> <var>y</var>)</code>.
</div>
Note that this implies, in turn, that two lists that are <code>eq?</code> are
also set-equal by any legal comparison procedure. This allows for
constant-time determination of set operations on <code>eq?</code> lists.
<p>
Be aware that these procedures typically run in time
O(<var>n</var> * <var>m</var>)
for <var>n</var>- and <var>m</var>-element list arguments.
Performance-critical applications
operating upon large sets will probably wish to use other data
structures and algorithms.
<dl>
<!--
==== lset<=
============================================================================-->
<dt class=proc-def>
<a name="lset<="></a>
<code class=proc-def>lset&lt;=</code><var> = list<sub>1</sub> ... -&gt; boolean</var>
<dd class=proc-def>
Returns true iff every <var>list<sub>i</sub></var> is a subset of <var>list<sub>i+1</sub></var>, using <var>=</var> for
the element-equality procedure.
List <var>A</var> is a subset of list <var>B</var> if every
element in <var>A</var> is equal to some element of <var>B</var>.
When performing an element comparison,
the <var>=</var> procedure's first argument is an element
of <var>A</var>; its second, an element of <var>B</var>.
<pre class=code-example>
(lset&lt;= eq? '(a) '(a b a) '(a b c c)) =&gt; #t
(lset<= eq?) => #t ; Trivial cases
(lset<= eq? '(a)) => #t
</pre>
<!--
==== lset=
============================================================================-->
<dt class=proc-def>
<a name="lset="></a>
<code class=proc-def>lset=</code><var> = list<sub>1</sub> list<sub>2</sub> ... -&gt; boolean</var>
<dd class=proc-def>
Returns true iff every <var>list<sub>i</sub></var> is set-equal to <var>list<sub>i+1</sub></var>, using <var>=</var> for
the element-equality procedure. "Set-equal" simply means that
<var>list<sub>i</sub></var> is a subset of <var>list<sub>i+1</sub></var>, and <var>list<sub>i+1</sub></var> is a subset of <var>list<sub>i</sub></var>.
The <var>=</var> procedure's first argument is an element of <var>list<sub>i</sub></var>; its second is an element of
<var>list<sub>i+1</sub></var>.
<pre class=code-example>
(lset= eq? '(b e a) '(a e b) '(e e b a)) =&gt; #t
(lset= eq?) => #t ; Trivial cases
(lset= eq? '(a)) => #t
</pre>
<!--
==== lset-adjoin
============================================================================-->
<dt class=proc-def>
<a name="lset-adjoin"></a>
<code class=proc-def>lset-adjoin</code><var> = list elt<sub>1</sub> ... -&gt; list</var>
<dd class=proc-def>
Adds the <var>elt<sub>i</sub></var> elements not already in the list parameter to the
result list. The result shares a common tail with the list parameter.
The new elements are added to the front of the list, but no guarantees
are made about their order. The <var>=</var> parameter is an equality procedure
used to determine if an <var>elt<sub>i</sub></var> is already a member of <var>list</var>. Its first
argument is an element of <var>list</var>; its second is one of the <var>elt<sub>i</sub></var>.
<p>
The list parameter is always a suffix of the result -- even if the list
parameter contains repeated elements, these are not reduced.
<pre class=code-example>
(lset-adjoin eq? '(a b c d c e) 'a 'e 'i 'o 'u) =&gt; (u o i a b c d c e)
</pre>
<!--
==== lset-union
============================================================================-->
<dt class=proc-def>
<a name="lset-union"></a>
<code class=proc-def>lset-union</code><var> = list<sub>1</sub> ... -&gt; list</var>
<dd class=proc-def>
Returns the union of the lists, using <var>=</var> for the element-equality
procedure.
<p>
The union of lists <var>A</var> and <var>B</var> is constructed as follows:
<ul>
<li> If <var>A</var> is the empty list,
the answer is <var>B</var> (or a copy of <var>B</var>).
<li> Otherwise, the result is initialised to be list <var>A</var>
(or a copy of <var>A</var>).
<li> Proceed through the elements of list <var>B</var>
in a left-to-right order.
If <var>b</var> is such an element of <var>B</var>,
compare every element <var>r</var> of the current result list
to <var>b</var>:
<code>(= <var>r</var> <var>b</var>)</code>.
If all comparisons fail,
<var>b</var> is consed onto the front of the result.
</ul>
However, there is no guarantee that = will be applied to every pair
of arguments from <var>A</var> and <var>B</var>.
In particular, if <var>A</var> is <code>eq</code>? to <var>B</var>,
the operation may immediately terminate.
<p>
In the n-ary case, the two-argument list-union operation is simply
folded across the argument lists.
<pre class=code-example>
(lset-union eq? '(a b c d e) '(a e i o u)) =>
(u o i a b c d e)
;; Repeated elements in LIST1 are preserved.
(lset-union eq? '(a a c) '(x a x)) => (x a a c)
;; Trivial cases
(lset-union eq?) => ()
(lset-union eq? '(a b c)) => (a b c)
</pre>
<!--
==== lset-intersection
============================================================================-->
<dt class=proc-def>
<a name="lset-intersection"></a>
<code class=proc-def>lset-intersection</code><var> = list<sub>1</sub> list<sub>2</sub> ... -&gt; list</var>
<dd class=proc-def>
Returns the intersection of the lists,
using <var>=</var> for the element-equality procedure.
<p>
The intersection of lists <var>A</var> and <var>B</var>
is comprised of every element of <var>A</var> that is <var>=</var>
to some element of <var>B</var>:
<code>(<var>=</var> <var>a</var> <var>b</var>)</code>,
for <var>a</var> in <var>A</var>, and <var>b</var> in <var>B</var>.
Note this implies that an element which appears in <var>B</var>
and multiple times in list <var>A</var>
will also appear multiple times in the result.
<p>
The order in which elements appear in the result is the same as
they appear in <var>list<sub>1</sub></var> --
that is, <code>lset-intersection</code> essentially filters
<var>list<sub>1</sub></var>,
without disarranging element order.
The result may
share a common tail with <var>list<sub>1</sub></var>.
<p>
In the n-ary case, the two-argument list-intersection operation is simply
folded across the argument lists. However, the dynamic order in which the
applications of <var>=</var> are made is not specified.
The procedure may check an
element of <var>list<sub>1</sub></var> for membership
in every other list before proceeding to
consider the next element of <var>list<sub>1</sub></var>,
or it may completely intersect <var>list<sub>1</sub></var>
and <var>list<sub>2</sub></var>
before proceeding to <var>list<sub>3</sub></var>,
or it may go about its work in some third order.
<pre class=code-example>
(lset-intersection eq? '(a b c d e) '(a e i o u)) =&gt; (a e)
;; Repeated elements in LIST1 are preserved.
(lset-intersection eq? '(a x y a) '(x a x z)) => '(a x a)
(lset-intersection eq? '(a b c)) => (a b c) ; Trivial case
</pre>
<!--
==== lset-difference
============================================================================-->
<dt class=proc-def>
<a name="lset-difference"></a>
<code class=proc-def>lset-difference</code><var> = list<sub>1</sub> list<sub>2</sub> ... -&gt; list</var>
<dd class=proc-def>
Returns the difference of the lists, using <var>=</var> for the element-equality
procedure -- all the elements of <var>list<sub>1</sub></var> that are not
<var>=</var> to any element from one of the
other <var>list<sub>i</sub></var> parameters.
<p>
The <var>=</var> procedure's first argument is
always an element of <var>list<sub>1</sub></var>;
its second is an element of one of the other <var>list<sub>i</sub></var>.
Elements that are repeated multiple times in the
<var>list<sub>1</sub></var> parameter
will occur multiple times in the result.
The order in which elements appear in the result is the same as
they appear in <var>list<sub>1</sub></var> --
that is, <code>lset-difference</code> essentially
filters <var>list<sub>1</sub></var>, without disarranging element order.
The result may share a common tail with <var>list<sub>1</sub></var>.
The dynamic order in which the applications of <var>=</var> are made is not
specified.
The procedure may check an element of <var>list<sub>1</sub></var>
for membership in every other list before proceeding to consider the
next element of <var>list<sub>1</sub></var>,
or it may completely compute the difference of
<var>list<sub>1</sub></var> and <var>list<sub>2</sub></var> before
proceeding to <var>list<sub>3</sub></var>,
or it may go about its work in some third order.
<pre class=code-example>
(lset-difference eq? '(a b c d e) '(a e i o u)) =&gt; (b c d)
(lset-difference eq? '(a b c)) => (a b c) ; Trivial case
</pre>
<!--
==== lset-xor
============================================================================-->
<dt class=proc-def>
<a name="lset-xor"></a>
<code class=proc-def>lset-xor</code><var> = list<sub>1</sub> ... -&gt; list</var>
<dd class=proc-def>
Returns the exclusive-or of the sets,
using <var>=</var> for the element-equality procedure.
If there are exactly two lists, this is all the elements
that appear in exactly one of the two lists. The operation is associative,
and thus extends to the n-ary case -- the elements that appear in an
odd number of the lists. The result may share a common tail with any of
the <var>list<sub>i</sub></var> parameters.
<p>
More precisely, for two lists <var>A</var> and <var>B</var>,
<var>A</var> xor <var>B</var> is a list of
<ul>
<li> every element <var>a</var> of <var>A</var>
such that there is no element <var>b</var> of <var>B</var>
such that <code>(<var>=</var> <var>a</var> <var>b</var>)</code>, and
<li> every element <var>b</var> of <var>B</var>
such that there is no element <var>a</var> of <var>A</var>
such that <code>(<var>=</var> <var>b</var> <var>a</var>)</code>.
</ul>
However, an implementation is allowed to assume that <var>=</var> is
symmetric -- that is, that
<div class=indent>
<code>(<var>=</var> <var>a</var> <var>b</var>)</code> =&gt;
<code>(<var>=</var> <var>b</var> <var>a</var>)</code>.
</div>
This means, for example, that if a comparison
<code>(<var>=</var> <var>a</var> <var>b</var>)</code> produces
true for some <var>a</var> in <var>A</var>
and <var>b</var> in <var>B</var>,
both <var>a</var> and <var>b</var> may be removed from
inclusion in the result.
<p>
In the n-ary case, the binary-xor operation is simply folded across
the lists.
<pre class=code-example>
(lset-xor eq? '(a b c d e) '(a e i o u)) =&gt; (d c b i o u)
;; Trivial cases.
(lset-xor eq?) => ()
(lset-xor eq? '(a b c d e)) => (a b c d e)
</pre>
<!--
==== lset-diff+intersection
============================================================================-->
<dt class=proc-def>
<a name="lset-diff+intersection"></a>
<code class=proc-def>lset-diff+intersection</code><var> = list<sub>1</sub> list<sub>2</sub> ... -&gt; [list list]</var>
<dd class=proc-def>
Returns two values -- the difference and the intersection of the lists.
Is equivalent to
<pre class=code-example>
(values (lset-difference <var>=</var> <var>list<sub>1</sub></var> <var>list<sub>2</sub></var> ...)
(lset-intersection <var>=</var> <var>list<sub>1</sub></var>
(lset-union <var>=</var> <var>list<sub>2</sub></var> ...)))
</pre>
but can be implemented more efficiently.
<p>
The <var>=</var> procedure's first argument is an element of <var>list<sub>1</sub></var>; its second
is an element of one of the other <var>list<sub>i</sub></var>.
<p>
Either of the answer lists may share a
common tail with <var>list<sub>1</sub></var>.
This operation essentially partitions <var>list<sub>1</sub></var>.
<!--
==== lset-diff+intersection!
==== lset-xor!
==== lset-difference!
==== lset-intersection!
==== lset-union!
============================================================================-->
<dt class=proc-def1>
<a name="lset-union!"></a>
<code class=proc-def>lset-union!&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><var>= list<sub>1</sub> ... -&gt; list</var>
<dt class=proc-defi>
<a name="lset-intersection!"></a>
<code class=proc-def>lset-intersection!&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><var>= list<sub>1</sub> list<sub>2</sub> ... -&gt; list</var>
<dt class=proc-defi>
<a name="lset-difference!"></a>
<code class=proc-def>lset-difference!&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><var>= list<sub>1</sub> list<sub>2</sub> ... -&gt; list</var>
<dt class=proc-defi>
<a name="lset-xor!"></a>
<code class=proc-def>lset-xor!&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><var>= list<sub>1</sub> ... -&gt; list</var>
<dt class=proc-defn>
<a name="lset-diff+intersection!"></a>
<code class=proc-def>lset-diff+intersection!&nbsp;</code><var>= list<sub>1</sub> list<sub>2</sub> ... -&gt; [list list]</var>
<dd class=proc-def>
These are linear-update variants. They are allowed, but not required,
to use the cons cells in their first list parameter to construct their
answer. <code>lset-union!</code> is permitted to recycle cons cells from <em>any</em>
of its list arguments.
</dl>
<!--========================================================================-->
<h2><a name="PrimitiveSideEffects">Primitive side-effects</a></h2>
<p>
These two procedures are the primitive,
<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>
side-effect operations on pairs.
<dl>
<!--
==== set-car!
============================================================================-->
<dt class=proc-def1>
<a name="set-car!"></a>
<code class=proc-def>set-car!</code><var> pair object -&gt; unspecified</var>
<dt class=proc-defn>
<a name="set-cdr!"></a>
<code class=proc-def>set-cdr!</code><var> pair object -&gt; unspecified</var>
<dd class=proc-def>
[<abbr title="Revised^5 Report on Scheme"><a href="#R5RS">R5RS</a></abbr>]
These procedures store <var>object</var> in the car and cdr field
of <var>pair</var>, respectively.
The value returned is unspecified.
<pre class=code-example>
(define (f) (list 'not-a-constant-list))
(define (g) '(constant-list))
(set-car! (f) 3) => *unspecified*
(set-car! (g) 3) => *error*
</pre>
</dl>
<!--========================================================================-->
<h1><a name="Acknowledgements">Acknowledgements</a></h1>
<p>
The design of this library benefited greatly from the feedback provided during
the <abbr title="Scheme Request for Implementation">SRFI</abbr> discussion phase. Among those contributing thoughtful commentary and
suggestions, both on the mailing list and by private discussion, were Mike
Ashley, Darius Bacon, Alan Bawden, Phil Bewig, Jim Blandy, Dan Bornstein, Per
Bothner, Anthony Carrico, Doug Currie, Kent Dybvig, Sergei Egorov, Doug Evans,
Marc Feeley, Matthias Felleisen, Will Fitzgerald, Matthew Flatt, Dan Friedman,
Lars Thomas Hansen, Brian Harvey, Erik Hilsdale, Wolfgang Hukriede, Richard
Kelsey, Donovan Kolbly, Shriram Krishnamurthi, Dave Mason, Jussi Piitulainen,
David Pokorny, Duncan Smith, Mike Sperber, Maciej Stachowiak, Harvey J. Stein,
John David Stone, and Joerg F. Wittenberger. I am grateful to them for their
assistance.
<p>
I am also grateful the authors, implementors and documentors of all the systems
mentioned in the introduction. Aubrey Jaffer and Kent Pitman should be noted
for their work in producing Web-accessible versions of the R5RS and
<a href="#CommonLisp">Common Lisp</a> spec, which was a tremendous aid.
<p>
This is not to imply that these individuals necessarily endorse the final
results, of course.
<!--========================================================================-->
<h1><a name="ReferencesLinks">References &amp; links</a></h1>
<p>
<dl>
<dt class=biblio>This document, in HTML:
<dd><a href="http://srfi.schemers.org/srfi-1/srfi-1.html">
http://srfi.schemers.org/srfi-1/srfi-1.html</a>
<br><a class=draft
href="ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/srfi-1.html">
ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/srfi-1.html (draft)</a>
<dt class=biblio>This document, in simple text format:
<dd><a href="http://srfi.schemers.org/srfi-1/srfi-1.txt">
http://srfi.schemers.org/srfi-1/srfi-1.txt</a>
<br><a class=draft
href="ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/srfi-1.txt">
ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/srfi-1.txt (draft)</a>
<dt class=biblio>Source code for the reference implementation:
<dd><a HREF="http://srfi.schemers.org/srfi-1/srfi-1-reference.scm">
http://srfi.schemers.org/srfi-1/srfi-1-reference.scm</a>
<br><a class=draft
href="ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/srfi-1-reference.scm">
ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/srfi-1-reference.scm (draft)</a>
<dt class=biblio>Archive of SRFI-1 discussion-list email:
<dd><a href="http://srfi.schemers.org/srfi-1/mail-archive/maillist.html">
http://srfi.schemers.org/srfi-1/mail-archive/maillist.html</a>
<dt class=biblio>SRFI web site:
<dd><a href="http://srfi.schemers.org/">
http://srfi.schemers.org/</a>
</dl>
<p>
<dl>
<dt class=biblio><strong><a name="CommonLisp">[CommonLisp]</a></strong></dt>
<dd><em>Common Lisp: the Language</em><br>
Guy L. Steele Jr. (editor).<br>
Digital Press, Maynard, Mass., second edition 1990.<br>
Available at <a href="http://www.elwood.com/alu/table/references.htm#cltl2">
http://www.elwood.com/alu/table/references.htm#cltl2</a>.
<p>
The Common Lisp "HyperSpec," produced by Kent Pitman, is essentially
the ANSI spec for Common Lisp:
<a href="http://www.harlequin.com/education/books/HyperSpec/">
http://www.harlequin.com/education/books/HyperSpec/</a>.
<dt class=biblio><strong><a name="R5RS">[R5RS]</a></strong></dt>
<dd>Revised<sup>5</sup> report on the algorithmic language Scheme.<br>
R. Kelsey, W. Clinger, J. Rees (editors). <br>
Higher-Order and Symbolic Computation, Vol. 11, No. 1, September, 1998. <br>
and ACM SIGPLAN Notices, Vol. 33, No. 9, October, 1998. <br>
Available at <a href="http://www.schemers.org/Documents/Standards/">
http://www.schemers.org/Documents/Standards/</a>.
</dl>
<!--========================================================================-->
<h1><a name="Copyright">Copyright</a></h1>
<p>
Certain portions of this document -- the specific, marked segments of text
describing the R5RS procedures -- were adapted with permission from the R5RS
report.
<p>
All other text is copyright (C) Olin Shivers (1998, 1999).
All Rights Reserved.
<p>
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Scheme Request For
Implementation process or editors, except as needed for the purpose of
developing <abbr title="Scheme Request for Implementation">SRFI</abbr>s in which case the procedures for copyrights defined
in the <abbr title="Scheme Request for Implementation">SRFI</abbr> process must be followed, or as required to translate it
into languages other than English.
<p>
The limited permissions granted above are perpetual and will not be
revoked by the authors or their successors or assigns.
<p>
This document and the information contained herein is provided on an
"AS IS" basis and THE AUTHOR AND THE <abbr title="Scheme Request for Implementation">SRFI</abbr> EDITORS DISCLAIM ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY
RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
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