148 lines
6.2 KiB
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148 lines
6.2 KiB
HTML
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>An Introduction to CPS (Guile Reference Manual)</title>
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<link href="Continuation_002dPassing-Style.html" rel="up" title="Continuation-Passing Style">
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<div class="subsubsection-level-extent" id="An-Introduction-to-CPS">
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<div class="nav-panel">
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<p>
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Next: <a href="CPS-in-Guile.html" accesskey="n" rel="next">CPS in Guile</a>, Up: <a href="Continuation_002dPassing-Style.html" accesskey="u" rel="up">Continuation-Passing Style</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
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<h4 class="subsubsection" id="An-Introduction-to-CPS-1"><span>9.4.4.1 An Introduction to CPS<a class="copiable-link" href="#An-Introduction-to-CPS-1"> ¶</a></span></h4>
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<p>Consider the following Scheme expression:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(begin
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(display "The sum of 32 and 10 is: ")
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(display 42)
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(newline))
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</pre></div>
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<p>Let us identify all of the sub-expressions in this expression,
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annotating them with unique labels:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(begin
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(display "The sum of 32 and 10 is: ")
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|k1 k2
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k0
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(display 42)
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|k4 k5
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k3
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(newline))
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|k7
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k6
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</pre></div>
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<p>Each of these labels identifies a point in a program. One label may be
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the continuation of another label. For example, the continuation of
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<code class="code">k7</code> is <code class="code">k6</code>. This is because after evaluating the value of
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<code class="code">newline</code>, performed by the expression labelled <code class="code">k7</code>, we
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continue to apply it in <code class="code">k6</code>.
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</p>
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<p>Which expression has <code class="code">k0</code> as its continuation? It is either the
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expression labelled <code class="code">k1</code> or the expression labelled <code class="code">k2</code>.
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Scheme does not have a fixed order of evaluation of arguments, though it
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does guarantee that they are evaluated in some order. Unlike general
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Scheme, continuation-passing style makes evaluation order explicit. In
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Guile, this choice is made by the higher-level language compilers.
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</p>
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<p>Let us assume a left-to-right evaluation order. In that case the
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continuation of <code class="code">k1</code> is <code class="code">k2</code>, and the continuation of
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<code class="code">k2</code> is <code class="code">k0</code>.
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</p>
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<p>With this example established, we are ready to give an example of CPS in
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Scheme:
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</p>
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<div class="example smalllisp lisp">
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<pre class="lisp-preformatted">(lambda (ktail)
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(let ((k1 (lambda ()
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(let ((k2 (lambda (proc)
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(let ((k0 (lambda (arg0)
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(proc k4 arg0))))
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(k0 "The sum of 32 and 10 is: ")))))
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(k2 display))))
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(k4 (lambda _
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(let ((k5 (lambda (proc)
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(let ((k3 (lambda (arg0)
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(proc k7 arg0))))
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(k3 42)))))
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(k5 display))))
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(k7 (lambda _
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(let ((k6 (lambda (proc)
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(proc ktail))))
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(k6 newline)))))
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(k1))
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</pre></div>
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<p>Holy code explosion, Batman! What’s with all the lambdas? Indeed, CPS
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is by nature much more verbose than “direct-style” intermediate
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languages like Tree-IL. At the same time, CPS is simpler than full
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Scheme, because it makes things more explicit.
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</p>
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<p>In the original program, the expression labelled <code class="code">k0</code> is in effect
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context. Any values it returns are ignored. In Scheme, this fact is
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implicit. In CPS, we can see it explicitly by noting that its
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continuation, <code class="code">k4</code>, takes any number of values and ignores them.
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Compare this to <code class="code">k2</code>, which takes a single value; in this way we
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can say that <code class="code">k1</code> is in a “value” context. Likewise <code class="code">k6</code> is
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in tail context with respect to the expression as a whole, because its
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continuation is the tail continuation, <code class="code">ktail</code>. CPS makes these
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details manifest, and gives them names.
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="CPS-in-Guile.html">CPS in Guile</a>, Up: <a href="Continuation_002dPassing-Style.html">Continuation-Passing Style</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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