stk/Doc/Reference/Appendix-D.tex

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