% -*- latex -*-
\chapter{Strings and characters}

Scsh provides a set of procedures for processing strings and characters.
The procedures provided match regular expressions, search strings,
parse file-names, and manipulate sets of characters.

Also see chapters \ref{chapt:sre}, \ref{chapt:rdelim} and \ref{chapt:fr-awk}
on regular-expressions, record I/O, field parsing, and the awk loop.
The procedures documented there allow you to search and pattern-match strings,
read character-delimited records from ports, 
use regular expressions to split the records into fields 
(for example, splitting a string at every occurrence of colon or white-space),
and loop over streams of these records in a convenient way.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{String manipulation}
\label{sec:stringmanip}

Strings are the basic communication medium for {\Unix} processes, so a
shell language must have reasonable facilities for manipulating them.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Manipulating file-names}
\label{sec:filenames}

These procedures do not access the file-system at all; they merely operate
on file-name strings. Much of this structure is patterned after the gnu emacs
design. Perhaps a more sophisticated system would be better, something
like the pathname abstractions of {\CommonLisp} or MIT Scheme. However,
being {\Unix}-specific, we can be a little less general. 

\subsubsection{Terminology}
These procedures carefully adhere to the {\Posix} standard for file-name
resolution, which occasionally entails some slightly odd things.
This section will describe these rules, and give some basic terminology.

A \emph{file-name} is either the file-system root (``/''), 
or a series of slash-terminated directory components, followed by
a a file component.
Root is the only file-name that may end in slash.
Some examples:
\begin{center}
\begin{tabular}{lll}
  File name            & Dir components        & File component \\\hline
  \ex{src/des/main.c}  & \ex{("src" "des")}    & \ex{"main.c"} \\
  \ex{/src/des/main.c} & \ex{("" "src" "des")} & \ex{"main.c"} \\
  \ex{main.c}          & \ex{()}               & \ex{"main.c"} \\
\end{tabular}
\end{center}

Note that the relative filename \ex{src/des/main.c} and the absolute filename
\ex{/src/des/main.c} are distinguished by the presence of the root component
\ex{""} in the absolute path.

Multiple embedded slashes within a path have the same meaning as 
a single slash.
More than two leading slashes at the beginning of a path have the same
meaning as a single leading slash---they indicate that the file-name
is an absolute one, with the path leading from root.
However, {\Posix} permits the OS to give special meaning to 
\emph{two} leading slashes.
For this reason, the routines in this section do not simplify two leading
slashes to a single slash.

A file-name in \emph{directory form} is either a file-name terminated by 
a slash, \eg, ``\ex{/src/des/}'', or the empty string, ``''.
The empty string corresponds to the current working directory, 
whose file-name is dot (``\ex{.}'').
Working backwards from the append-a-slash rule, 
we extend the syntax of {\Posix} file-names to define the empty string
to be a file-name form of the root directory ``\ex{/}''.
(However, ``\ex{/}'' is also acceptable as a file-name form for root.)
So the empty string has two interpretations: 
as a file-name form, it is the file-system root;
as a directory form, it is the current working directory.
Slash is also an ambiguous form: \ex{/} is both a directory-form and
a file-name form.

The directory form of a file-name is very rarely used.
Almost all of the procedures in scsh name directories by giving
their file-name form (without the trailing slash), not their directory form.
So, you say ``\ex{/usr/include}'', and ``\ex{.}'', not
``\ex{/usr/include/}'' and ``''.
The sole exceptions are
\ex{file-name-as-directory} and \ex{directory-as-file-name},
whose jobs are to convert back-and-forth between these forms,
and \ex{file-name-directory}, whose job it is to split out the
directory portion of a file-name.
However, most procedures that expect a directory argument will coerce
a file-name in directory form to file-name form if it does not have
a trailing slash.
Bear in mind that the ambiguous case, empty string, will be
interpreted in file-name form, \ie, as root.



\subsubsection{Procedures}

\defun  {file-name-directory?}     {fname} \boolean
\defunx {file-name-non-directory?} {fname} \boolean
\begin{desc}
These predicates return true if the string is in directory form, or
file-name form (see the above discussion of these two forms).
Note that they both return true on the ambiguous case of empty string,
which is both a directory (current working directory), and a file name 
(the file-system root).
\begin{center}
\begin{tabular}{lll}
File name & \ex{\ldots-directory?} & \ex{\ldots-non-directory?} \\
\hline
\ex{"src/des"}	& \ex{\sharpf} & \ex{\sharpt} \\
\ex{"src/des/"} & \ex{\sharpt} & \ex{\sharpf} \\
\ex{"/"}	& \ex{\sharpt} & \ex{\sharpf} \\
\ex{"."}	& \ex{\sharpf} & \ex{\sharpt} \\
\ex{""}		& \ex{\sharpt} & \ex{\sharpt}
\end{tabular}
\end{center}
\end{desc}

\begin{defundesc} {file-name-as-directory} {fname} \str
    Convert a file-name to directory form.
    Basically, add a trailing slash if needed:
        \begin{exampletable}
        \ex{(file-name-as-directory "src/des")}  & \ex{"src/des/"} \\
        \ex{(file-name-as-directory "src/des/")} & \ex{"src/des/"} \\[2ex]
        %
        \header{\ex{.}, \ex{/}, and \ex{""} are special:}
        \ex{(file-name-as-directory ".")} & \ex{""}  \\
        \ex{(file-name-as-directory "/")} & \ex{"/"} \\
        \ex{(file-name-as-directory "")}  & \ex{"/"}
        \end{exampletable}
\end{defundesc}

\begin{defundesc} {directory-as-file-name} {fname} \str
    Convert a directory to a simple file-name.
    Basically, kill a trailing slash if one is present:
    \begin{exampletable}
    \ex{(directory-as-file-name "foo/bar/")} & \ex{"foo/bar"} \\[2ex]
    %
    \header{\ex{/} and \ex{""} are special:}
    \ex{(directory-as-file-name "/")} & \ex{"/"} \\
    \ex{(directory-as-file-name "")}  & \ex{"."} (\ie, the cwd) \\
    \end{exampletable}
\end{defundesc}

\begin{defundesc} {file-name-absolute?} {fname} \boolean
    Does \var{fname} begin with a root or \ex{\~} component?
    (Recognising \ex{\~} as a home-directory specification
    is an extension of {\Posix} rules.)
%
    \begin{exampletable}
    \ex{(file-name-absolute? "/usr/shivers")}        & {\sharpt}  \\
    \ex{(file-name-absolute? "src/des")}             & {\sharpf}  \\
    \ex{(file-name-absolute? "\~/src/des")}          & {\sharpt}  \\[2ex]
    %
    \header{Non-obvious case:}
    \ex{(file-name-absolute? "")}                    & {\sharpt} (\ie, root)
    \end{exampletable}
\end{defundesc}


\begin{defundesc} {file-name-directory} {fname} {{\str} or false}
    Return the directory component of \var{fname} in directory form.
    If the file-name is already in directory form, return it as-is.
%
    \begin{exampletable}
    \ex{(file-name-directory "/usr/bdc")}   & \ex{"/usr/"}  \\
    {\ex{(file-name-directory "/usr/bdc/")}} &
              {\ex{"/usr/bdc/"}} \\
    \ex{(file-name-directory "bdc/.login")} & \ex{"bdc/"}       \\
    \ex{(file-name-directory "main.c")}      & \ex{""} \\[2ex]
    %
    \header{Root has no directory component:}
    \ex{(file-name-directory "/")}           & \ex{""} \\
    \ex{(file-name-directory "")}            & \ex{""}
    \end{exampletable}
\end{defundesc}


\begin{defundesc} {file-name-nondirectory} {fname} \str
    Return non-directory component of fname.
%
    \begin{exampletable}
    {\ex{(file-name-nondirectory "/usr/ian")}} &
              {\ex{"ian"}} \\
    \ex{(file-name-nondirectory "/usr/ian/")}  & \ex{""}       \\
    {\ex{(file-name-nondirectory "ian/.login")}} &
              {\ex{".login"}} \\
    \ex{(file-name-nondirectory "main.c")}         & \ex{"main.c"} \\
    \ex{(file-name-nondirectory "")}               & \ex{""} \\
    \ex{(file-name-nondirectory "/")}              & \ex{"/"}
    \end{exampletable}
\end{defundesc}


\begin{defundesc} {split-file-name} {fname} {{\str} list}
    Split a file-name into its components.
%
    \begin{exampletable}
    \splitline{\ex{(split-file-name "src/des/main.c")}}
              {\ex{("src" "des" "main.c")}}      \\[1.5ex]
    %
    \splitline{\ex{(split-file-name "/src/des/main.c")}}
              {\ex{("" "src" "des" "main.c")}}   \\[1.5ex]
    %
    \splitline{\ex{(split-file-name "main.c")}} {\ex{("main.c")}} \\[1.5ex]
    %
    \splitline{\ex{(split-file-name "/")}} {\ex{("")}}
    \end{exampletable}
\end{defundesc}


\begin{defundesc} {path-list->file-name} {path-list [dir]} \str
    Inverse of \ex{split-file-name}.
\begin{code}
(path-list->file-name '("src" "des" "main.c")) 
    {\evalto} "src/des/main.c"
(path-list->file-name '("" "src" "des" "main.c"))
    {\evalto} "/src/des/main.c"
\cb
{\rm{}Optional \var{dir} arg anchors relative path-lists:}
(path-list->file-name '("src" "des" "main.c")
                      "/usr/shivers")
    {\evalto} "/usr/shivers/src/des/main.c"\end{code}
%
    The optional \var{dir} argument is usefully \ex{(cwd)}.
\end{defundesc}


\begin{defundesc} {file-name-extension} {fname} \str
    Return the file-name's extension.
%
    \begin{exampletable}
    \ex{(file-name-extension "main.c")}          & \ex{".c"}    \\
    \ex{(file-name-extension "main.c.old")}      & \ex{".old"}  \\
    \ex{(file-name-extension "/usr/shivers")}    & \ex{""}
    \end{exampletable}
%
    \begin{exampletable}
    \header{Weird cases:}
    \ex{(file-name-extension "foo.")}            & \ex{"."}     \\
    \ex{(file-name-extension "foo..")}           & \ex{"."}
    \end{exampletable}
%
    \begin{exampletable}
    \header{Dot files are not extensions:}
    \ex{(file-name-extension "/usr/shivers/.login")} & \ex{""}
    \end{exampletable}
\end{defundesc}


\begin{defundesc} {file-name-sans-extension} {fname} \str
    Return everything but the extension.
%
    \begin{exampletable}
    \ex{(file-name-sans-extension "main.c")}            & \ex{"main"}   \\
    \ex{(file-name-sans-extension "main.c.old")}        & \ex{"main.c""} \\
    \splitline{\ex{(file-name-sans-extension "/usr/shivers")}}
              {\ex{"/usr/shivers"}}
    \end{exampletable}
%
    \begin{exampletable}
    \header{Weird cases:}
    \ex{(file-name-sans-extension "foo.")}      & \ex{"foo"}    \\
    \ex{(file-name-sans-extension "foo..")}     & \ex{"foo."}   \\[2ex]
    %
    \header{Dot files are not extensions:}
    \splitline{\ex{(file-name-sans-extension "/usr/shivers/.login")}}
        {\ex{"/usr/shivers/.login}}
    \end{exampletable}

    Note that appending the results of \ex{file-name-extension} and 
    {\ttt file\=name\=sans\=extension} in all cases produces the original file-name.
\end{defundesc}


\begin{defundesc} {parse-file-name} {fname} {[dir name extension]}
    Let $f$ be \ex{(file-name-nondirectory \var{fname})}.
    This function returns the three values:
    \begin{itemize}
    \item \ex{(file-name-directory \var{fname})}
    \item \ex{(file-name-sans-extension \var{f}))}
    \item \ex{(file-name-extension \var{f}\/)}
    \end{itemize}
    The inverse of \ex{parse-file-name}, in all cases, is \ex{string-append}.
    The boundary case of \ex{/} was chosen to preserve this inverse.
\end{defundesc}

\begin{defundesc} {replace-extension} {fname ext} \str
    This procedure replaces \var{fname}'s extension with \var{ext}.
    It is exactly equivalent to 
    \codex{(string-append (file-name-sans-extension \var{fname}) \var{ext})}
\end{defundesc}

\defun{simplify-file-name}{fname}\str
\begin{desc}
    Removes leading and internal occurrences of dot. 
    A trailing dot is left alone, as the parent could be a symlink.
    Removes internal and trailing double-slashes.
    A leading double-slash is left alone, in accordance with {\Posix}.
    However, triple and more leading slashes are reduced to a single slash,
    in accordance with {\Posix}.
    Double-dots (parent directory) are left alone, in case they come after
    symlinks or appear in a \ex{/../\var{machine}/\ldots} ``super-root'' form
    (which {\Posix} permits).
\end{desc}

\defun{resolve-file-name}{fname [dir]}\str
\begin{desc}
    \begin{itemize}
    \item Do \ex{\~} expansion.
    \item If \var{dir} is given,
      convert a relative file-name to an absolute file-name,
      relative to directory \var{dir}.
    \end{itemize}
\end{desc}

\begin{defundesc} {expand-file-name} {fname [dir]} \str
Resolve and simplify the file-name.
\end{defundesc}
    
\begin{defundesc} {absolute-file-name} {fname [dir]} \str
Convert file-name \var{fname} into an absolute file name,
relative to directory \var{dir}, which defaults to the current
working directory. The file name is simplified before being
returned.

This procedure does not treat a leading tilde character specially.
\end{defundesc}

\begin{defundesc}  {home-dir} {[user]} \str
    \ex{home-dir} returns \var{user}'s home directory. 
        \var{User} defaults to the current user.

    \begin{exampletable}
    \ex{(home-dir)}          & \ex{"/user1/lecturer/shivers"}  \\
    \ex{(home-dir "ctkwan")} & \ex{"/user0/research/ctkwan"}
    \end{exampletable}
\end{defundesc}

\begin{defundesc} {home-file} {[user] fname} \str
    Returns file-name \var{fname} relative to \var{user}'s home directory;
    \var{user} defaults to the current user.
%    
    \begin{exampletable}
    \ex{(home-file "man")}            & \ex{"/usr/shivers/man"}        \\
    \ex{(home-file "fcmlau" "man")}   & \ex{"/usr/fcmlau/man"}
    \end{exampletable}
\end{defundesc}

The general \ex{substitute-env-vars} string procedure, 
defined in the previous section,
is also frequently useful for expanding file-names.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Other string manipulation facilities}

\defun  {index}  {string char [start]} {{\fixnum} or false}
\defunx {rindex} {string char [start]} {{\fixnum} or false}
\begin{desc}
    These procedures search through \var{string} looking for an occurrence
    of character \var{char}. \ex{index} searches left-to-right; \ex{rindex}
    searches right-to-left.

    \ex{index} returns the smallest index $i$ of \var{string} greater
    than or equal to \var{start} such that $\var{string}[i] = \var{char}$.
    The default for \var{start} is zero. If there is no such match,
    \ex{index} returns false.

    \ex{rindex} returns the largest index $i$ of \var{string} less than
    \var{start} such that $\var{string}[i] = \var{char}$.
    The default for \var{start} is \ex{(string-length \var{string})}.
    If there is no such match, \ex{rindex} returns false.
\end{desc}

I should probably snarf all the MIT Scheme string functions, and stick them
in a package.  {\Unix} programs need to mung character strings a lot.

MIT string match commands: 
\begin{tightcode}
[sub]string-match-{forward,backward}[-ci]
[sub]string-{prefix,suffix}[-ci]?
[sub]string-find-{next,previous}-char[-ci]
[sub]string-find-{next,previous}-char-in-set
[sub]string-replace[!]
\ldots\etc\end{tightcode}
These are not currently provided.

\begin{defundesc} {substitute-env-vars} {fname} \str
    Replace occurrences of environment variables with their values.
    An environment variable is denoted by a dollar sign followed by
    alphanumeric chars and underscores, or is surrounded by braces.

    \begin{exampletable}
    \splitline{\ex{(substitute-env-vars "\$USER/.login")}} 
              {\ex{"shivers/.login"}}  \\
    \cd{(substitute-env-vars "$\{USER\}_log")}  & \cd{"shivers_log"}
    \end{exampletable}
\end{defundesc}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{ASCII encoding}

\defun  {char->ascii}{\character}    \integer
\defunx {ascii->char}{\integer} \character
\begin{desc}
  These are identical to \ex{char->integer} and \ex{integer->char} except that
  they use the {\Ascii} encoding.
\end{desc}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Character sets}
\label{sec:char-sets}

Scsh provides a \ex{char-set} type for expressing sets of characters.
These sets are used by some of the delimited-input procedures 
(section~\ref{sec:field-reader}).
Scsh's character set package was adapted and extended from
Project Mac's MIT Scheme package.
Note that the character type used in the current implementation corresponds
to the ASCII character set---but you would be wise not to build this
assumption into your code if you can help it.\footnote{
	Actually, it's slightly uglier than that, albeit somewhat more
	useful. The current character type corresponds to an eight-bit
	superset of ASCII. The \ex{ascii->char} and \ex{char->ascii}
 	functions will preserve this eighth bit. However, none of the
	the high 128 characters appear in any of the standard character
	sets defined in section~\ref{sec:std-csets}, except for 
	\ex{char-set:full}. If someone would email the authors a listing
	of the full Latin-1 definition,	we'll be happy to upgrade these
	sets' definitions to make them Latin-1 compliant.}

\defun{char-set?}{x}\boolean
\begin{desc}
Is the object \var{x} a character set?
\end{desc}

\defun{char-set=}{\vari{cs}1 \vari{cs}2\ldots}\boolean
\begin{desc}
Are the character sets equal?
\end{desc}

\defun{char-set<=}{\vari{cs}1 \vari{cs}2\ldots}\boolean
\begin{desc}
Returns true if every character set \vari{cs}{i} is 
a subset of character set \vari{cs}{i+1}.
\end{desc}

\defun{char-set-fold}{kons knil cs}\object
\begin{desc}
This is the fundamental iterator for character sets.
Applies the function \var{kons} across the character set \var{cs} using
initial state value \var{knil}. 
That is, if \var{cs} is the empty set, the procedure returns \var{knil}.
Otherwise, some element \var{c} of \var{cs} is chosen; let \var{cs'} be
the remaining, unchosen characters.
The procedure returns 
\begin{tightcode}
(char-set-fold \var{kons} (\var{kons} \var{c} \var{knil}) \var{cs'})\end{tightcode}
For example, we could define \ex{char-set-members} (see below)
as
\begin{tightcode}
(lambda (cs) (char-set-fold cons '() cs))\end{tightcode}

\remark{This procedure was formerly named \texttt{\indx{reduce-char-set}}.
        The old binding is still provided, but is deprecated and will
	probably vanish in a future release.}
\end{desc}

\defun{char-set-for-each}{p cs}{\undefined}
\begin{desc}
Apply procedure \var{p} to each character in the character set \var{cs}.
Note that the order in which \var{p} is applied to the characters in the
set is not specified, and may even change from application to application.
\end{desc}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Creating character sets}

\defun{char-set}{\vari{char}1\ldots}{char-set}
\begin{desc}
Return a character set containing the given characters.
\end{desc}

\defun{chars->char-set}{chars}{char-set}
\begin{desc}
Return a character set containing the characters in the list \var{chars}.
\end{desc}

\defun{string->char-set}{s}{char-set}
\begin{desc}
Return a character set containing the characters in the string \var{s}.
\end{desc}

\defun{predicate->char-set}{pred}{char-set}
\begin{desc}
Returns a character set containing every character \var{c} such that
\ex{(\var{pred} \var{c})} returns true.
\end{desc}

\defun{ascii-range->char-set}{lower upper}{char-set}
\begin{desc}
Returns a character set containing every character whose {\Ascii}
code lies in the half-open range $[\var{lower},\var{upper})$.
\end{desc}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Querying character sets}
\defun {char-set-members}{char-set}{character-list}
\begin{desc}
This procedure returns a list of the members of \var{char-set}.
\end{desc}

\defunx{char-set-contains?}{char-set char}\boolean
\begin{desc}
This procedure tests \var{char} for membership in set \var{char-set}.
\remark{Previous releases of scsh called this procedure \ex{char-set-member?},
reversing the order of the arguments.
This made sense, but was unfortunately the reverse order in which the
arguments appear in MIT Scheme.
A reasonable argument order was not backwards-compatible with MIT Scheme;
on the other hand, the MIT Scheme argument order was counter-intuitive
and at odds with common mathematical notation and the \ex{member} family
of R4RS procedures.

We sought to escape the dilemma by shifting to a new name.}
\end{desc}

\defun{char-set-size}{cs}\integer
\begin{desc}
Returns the number of elements in character set \var{cs}.
\end{desc}

\defun{char-set-every?}{pred cs}\boolean
\defunx{char-set-any?}{pred cs}\object
\begin{desc}
The \ex{char-set-every?} procedure returns true if predicate \var{pred}
returns true of every character in the character set \var{cs}.

Likewise, \ex{char-set-any?} applies \var{pred} to every character in
character set \var{cs}, and returns the first true value it finds.
If no character produces a true value, it returns false.

The order in which these procedures sequence through the elements of
\var{cs} is not specified.
\end{desc}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Character-set algebra}
\defun {char-set-invert}{char-set}{char-set}
\defunx{char-set-union}{\vari{char-set}1\ldots}{char-set}
\defunx{char-set-intersection}{\vari{char-set}1 \vari{char-set}2\ldots}{char-set}
\defunx{char-set-difference}{\vari{char-set}1 \vari{char-set}2\ldots}{char-set}
\begin{desc}
These procedures implement set complement, union, intersection, and difference
for character sets.
The union, intersection, and difference operations are n-ary, associating
to the left; the difference function requires at least one argument, while
union and intersection may be applied to zero arguments.
\end{desc}

\defun {char-set-adjoin}{cs \vari{char}1\ldots}{char-set}
\defunx{char-set-delete}{cs \vari{char}1\ldots}{char-set}
\begin{desc}
Add/delete the \vari{char}i characters to/from character set \var{cs}.
\end{desc}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Standard character sets}
\label{sec:std-csets}
Several character sets are predefined for convenience:

\begin{center}
\newcommand{\entry}[1]{\ex{#1}\index{#1}}
\begin{tabular}{|ll|}
\hline
\entry{char-set:lower-case}	&	Lower-case alphabetic chars \\
\entry{char-set:upper-case}	&	Upper-case alphabetic chars \\
\entry{char-set:alphabetic}	&	Alphabetic chars \\
\entry{char-set:numeric}	&	Decimal digits: 0--9 \\
\entry{char-set:alphanumeric}	&	Alphabetic or numeric \\
\entry{char-set:graphic}	&	Printing characters except space \\
\entry{char-set:printing}	&	Printing characters including space \\
\entry{char-set:whitespace}	&	Whitespace characters \\
\entry{char-set:control}	&	Control characters \\
\entry{char-set:punctuation}	&	Punctuation characters \\
\entry{char-set:hex-digit}	&       A hexadecimal digit: 0--9, A--F, a--f \\
\entry{char-set:blank}		&	Blank characters \\
\entry{char-set:ascii}		&	A character in the ASCII set. \\
\entry{char-set:empty}		&	Empty set \\
\entry{char-set:full}		&	All characters \\
\hline
\end{tabular}
\end{center}
The first eleven of these correspond to the character classes defined in
Posix.
Note that there may be characters in \ex{char-set:alphabetic} that are
neither upper or lower case---this might occur in implementations that
use a character type richer than ASCII, such as Unicode.
A ``graphic character'' is one that would put ink on your page.
While the exact composition of these sets may vary depending upon the
character type provided by the Scheme system upon which scsh is running,
here are the definitions for some of the sets in an ASCII character set:
\begin{center}
\newcommand{\entry}[1]{\ex{#1}\index{#1}}
\begin{tabular}{|ll|}
\hline
char-set:alphabetic	&	A--Z and a--z \\
char-set:lower-case	&	a--z \\
char-set:upper-case	&	A--Z \\
char-set:graphic	&	Alphanumeric + punctuation \\
char-set:whitespace	&	Space, newline, tab, page, 
					vertical tab, carriage return \\
char-set:blank		&	Space and tab \\
char-set:control	&	ASCII 0--31 and 127 \\
char-set:punctuation	&	\verb|!"#$%&'()*+,-./:;<=>|\verb#?@[\]^_`{|}~# \\
\hline
\end{tabular}
\end{center}


\defun {char-alphabetic?}\character\boolean
\defunx{char-lower-case?}\character\boolean
\defunx{char-upper-case?}\character\boolean
\defunx{char-numeric?	}\character\boolean
\defunx{char-alphanumeric?}\character\boolean
\defunx{char-graphic?}\character\boolean
\defunx{char-printing?}\character\boolean
\defunx{char-whitespace?}\character\boolean
\defunx{char-blank?}\character\boolean
\defunx{char-control?}\character\boolean
\defunx{char-punctuation?}\character\boolean
\defunx{char-hex-digit?}\character\boolean
\defunx{char-ascii?}\character\boolean
\begin{desc}
These predicates are defined in terms of the above character sets.
\end{desc}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Linear-update character-set operations}
These procedures have a hybrid pure-functional/side-effecting semantics:
they are allowed, but not required, to side-effect one of their parameters
in order to construct their result.
An implementation may legally implement these procedures as pure,
side-effect-free functions, or it may implement them using side effects,
depending upon the details of what is the most efficient or simple to 
implement in terms of the underlying representation.

What this means is that clients of these procedures \emph{may not} rely
upon these procedures working by side effect.
For example, this is not guaranteed to work:
\begin{verbatim}
(let ((cs (char-set #\a #\b #\c)))
  (char-set-adjoin! cs #\d)
  cs) ; Could be either {a,b,c} or {a,b,c,d}.
\end{verbatim}
However, this is well-defined:
\begin{verbatim}
(let ((cs (char-set #\a #\b #\c)))
  (char-set-adjoin! cs #\d)) ; {a,b,c,d}
\end{verbatim}
So clients of these procedures write in a functional style, but must
additionally be sure that, when the procedure is called, there are no
other live pointers to the potentially-modified character set (hence the term
``linear update'').

There are two benefits to this convention:
\begin{itemize}
\item Implementations are free to provide the most efficient possible
      implementation, either functional or side-effecting.
\item Programmers may nonetheless continue to assume that character sets
      are purely functional data structures: they may be reliably shared
      without needing to be copied, uniquified, and so forth.
\end{itemize}

In practice, these procedures are most useful for efficiently constructing
character sets in a side-effecting manner, in some limited local context, 
before passing the character set outside the local construction scope to be
used in a functional manner.

Scsh provides no assistance in checking the linearity of the potentially
side-effected parameters passed to these functions --- there's no linear
type checker or run-time mechanism for detecting violations.

\defun{char-set-copy}{cs}{char-set}
\begin{desc}
Returns a copy of the character set \var{cs}.
``Copy'' means that if either the input parameter or the
result value of this procedure is passed to one of the linear-update
procedures described below, the other character set is guaranteed
not to be altered.
(A system that provides pure-functional implementations of the rest of
the linear-operator suite could implement this procedure as the 
identity function.)
\end{desc}

\defun{char-set-adjoin!}{cs \vari{char}1\ldots}{char-set}
\begin{desc}
Add the \vari{char}i characters to character set \var{cs}, and
return the result. 
This procedure is allowed, but not required, to side-effect \var{cs}.
\end{desc}

\defun{char-set-delete!}{cs \vari{char}1\ldots}{char-set}
\begin{desc}
Remove the \vari{char}i characters to character set \var{cs}, and
return the result. 
This procedure is allowed, but not required, to side-effect \var{cs}.
\end{desc}

\defun {char-set-invert!}{char-set}{char-set}
\defunx{char-set-union!}{\vari{char-set}1 \vari{char-set}2\ldots}{char-set}
\defunx{char-set-intersection!}{\vari{char-set}1 \vari{char-set}2\ldots}{char-set}
\defunx{char-set-difference!}{\vari{char-set}1 \vari{char-set}2\ldots}{char-set}
\begin{desc}
These procedures implement set complement, union, intersection, and difference
for character sets.
They are allowed, but not required, to side-effect their first parameter.
The union, intersection, and difference operations are n-ary, associating
to the left.
\end{desc}
