119 lines
2.8 KiB
TeX
119 lines
2.8 KiB
TeX
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% STk Reference manual (Appendix: Modules)
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%
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% Author: Erick Gallesio [eg@unice.fr]
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% Creation date: 6-Apr-1998 11:19
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% Last file update: 8-Apr-1998 10:10
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%
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This appendix shows some usages of the {\stk} modules. Most of the
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examples which are exhibited here are derived from the Tung and Dybvig paper
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\cite{Tung-Dybvig-96}.
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\subsection*{Interactive Redefinition}
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Consider first the definitions,
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\begin{scheme}
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(define-module A
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(export square)
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(define square
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(lambda (x) (+ x x))))
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(define-module B
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(import A)
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(define distance
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(lambda (x y)
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(sqrt (+ (square x) (square y))))))
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\end{scheme}
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Obviously, the \texttt{square} function exported from \texttt{A} is
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incorrect, as we can see in its usage below:
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\begin{scheme}
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(with-module B (round (distance 3 4))) \lev 4.0
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\end{scheme}
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The function can be redefined (\textit{corrected}) by the following expression:
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\begin{scheme}
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(with-module A
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(set! square
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(lambda (x) (* x x))))
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\end{scheme}
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And now,
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\begin{scheme}
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(with-module B (round (distance 3 4))) \lev 5
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\end{scheme}
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which is correct.
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\subsection*{Lexical principle}
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This example reuses the modules \texttt{A} and \texttt{B} of previous
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section and adds a \texttt{Compare} module that exports the
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\texttt{less-than-4?} predicates, which states if the distance from a
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point to the origin is less than 4.
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\begin{scheme}
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(define-module A
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(export square)
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(define square (lambda (x) (* x x))))
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(define-module B
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(import A)
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(export distance)
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(define distance
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(lambda (x y) (sqrt (+ (square x) (square y))))))
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(define-module Compare
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(import B)
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(define less-than-4? (lambda (x y) (< (distance x y) 4)))
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(define square (lambda (x) (+ x x))))
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\end{scheme}
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Consider now the call,
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\begin{scheme}
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(with-module compare (less-than-4? 3 4)) \lev \schfalse
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\end{scheme}
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The call to \texttt{distance} done from \texttt{less-than-4?}
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indirectly calls the \texttt{square} procedure of module \texttt{A} rather than
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the one defined locally in module \texttt{Compare}.
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\subsection*{Mutually Referential Modules}
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This example uses two mutually referential modules taht import and
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export to each other to implement mutually recursive even? and odd?
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procedures
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\begin{scheme}
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(define-module Odd) ;; Forward declaration
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(define-module Even
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(import Odd)
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(export even?)
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(define even? (lambda (x) (if (zero? x) \schtrue (odd? (- x 1))))))
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(define-module Odd
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(import Even)
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(export odd?)
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(define odd? (lambda (x) (if (zero? x) \schfalse (even? (- x 1))))))
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\end{scheme}
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Hereafter are some usages of theses procedures:
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\begin{scheme}
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(with-module Odd (odd? 3)) \lev \schtrue
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(with-module Odd (odd? 10)) \lev \schfalse
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(with-module Even (even? 3)) \lev \schfalse
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(with-module Even (even? 10)) \lev \schtrue
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\end{scheme}
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%%% Local Variables:
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%%% mode: latex
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%%% TeX-master:"manual"
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%%% End:
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