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<title>SRFI-45 (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="SRFI_002d45">
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<p>
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Next: <a href="SRFI_002d46.html" accesskey="n" rel="next">SRFI-46 Basic syntax-rules Extensions</a>, Previous: <a href="SRFI_002d43.html" accesskey="p" rel="prev">SRFI-43 - Vector Library</a>, Up: <a href="SRFI-Support.html" accesskey="u" rel="up">SRFI Support Modules</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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<h4 class="subsection" id="SRFI_002d45-_002d-Primitives-for-Expressing-Iterative-Lazy-Algorithms"><span>7.5.31 SRFI-45 - Primitives for Expressing Iterative Lazy Algorithms<a class="copiable-link" href="#SRFI_002d45-_002d-Primitives-for-Expressing-Iterative-Lazy-Algorithms"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-SRFI_002d45"></a>
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<p>This subsection is based on <a class="uref" href="http://srfi.schemers.org/srfi-45/srfi-45.html">the
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specification of SRFI-45</a> written by André van Tonder.
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</p>
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<p>Lazy evaluation is traditionally simulated in Scheme using <code class="code">delay</code>
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and <code class="code">force</code>. However, these primitives are not powerful enough to
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express a large class of lazy algorithms that are iterative. Indeed, it
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is folklore in the Scheme community that typical iterative lazy
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algorithms written using delay and force will often require unbounded
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memory.
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</p>
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<p>This SRFI provides set of three operations: {<code class="code">lazy</code>, <code class="code">delay</code>,
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<code class="code">force</code>}, which allow the programmer to succinctly express lazy
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algorithms while retaining bounded space behavior in cases that are
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properly tail-recursive. A general recipe for using these primitives is
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provided. An additional procedure <code class="code">eager</code> is provided for the
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construction of eager promises in cases where efficiency is a concern.
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</p>
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<p>Although this SRFI redefines <code class="code">delay</code> and <code class="code">force</code>, the
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extension is conservative in the sense that the semantics of the subset
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{<code class="code">delay</code>, <code class="code">force</code>} in isolation (i.e., as long as the
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program does not use <code class="code">lazy</code>) agrees with that in R5RS. In other
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words, no program that uses the R5RS definitions of delay and force will
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break if those definition are replaced by the SRFI-45 definitions of
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delay and force.
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</p>
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<p>Guile also adds <code class="code">promise?</code> to the list of exports, which is not
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part of the official SRFI-45.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-promise_003f-1"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">promise?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-promise_003f-1"> ¶</a></span></dt>
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<dd><p>Return true if <var class="var">obj</var> is an SRFI-45 promise, otherwise return false.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-delay-1"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">delay</strong> <var class="def-var-arguments">expression</var><a class="copiable-link" href="#index-delay-1"> ¶</a></span></dt>
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<dd><p>Takes an expression of arbitrary type <var class="var">a</var> and returns a promise of
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type <code class="code">(Promise <var class="var">a</var>)</code> which at some point in the future may be
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asked (by the <code class="code">force</code> procedure) to evaluate the expression and
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deliver the resulting value.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-lazy"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">lazy</strong> <var class="def-var-arguments">expression</var><a class="copiable-link" href="#index-lazy"> ¶</a></span></dt>
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<dd><p>Takes an expression of type <code class="code">(Promise <var class="var">a</var>)</code> and returns a
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promise of type <code class="code">(Promise <var class="var">a</var>)</code> which at some point in the
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future may be asked (by the <code class="code">force</code> procedure) to evaluate the
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expression and deliver the resulting promise.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-force-1"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">force</strong> <var class="def-var-arguments">expression</var><a class="copiable-link" href="#index-force-1"> ¶</a></span></dt>
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<dd><p>Takes an argument of type <code class="code">(Promise <var class="var">a</var>)</code> and returns a value
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of type <var class="var">a</var> as follows: If a value of type <var class="var">a</var> has been computed
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for the promise, this value is returned. Otherwise, the promise is
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first evaluated, then overwritten by the obtained promise or value, and
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then force is again applied (iteratively) to the promise.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-eager"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">eager</strong> <var class="def-var-arguments">expression</var><a class="copiable-link" href="#index-eager"> ¶</a></span></dt>
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<dd><p>Takes an argument of type <var class="var">a</var> and returns a value of type
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<code class="code">(Promise <var class="var">a</var>)</code>. As opposed to <code class="code">delay</code>, the argument is
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evaluated eagerly. Semantically, writing <code class="code">(eager expression)</code> is
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equivalent to writing
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(let ((value expression)) (delay value)).
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</pre></div>
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<p>However, the former is more efficient since it does not require
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unnecessary creation and evaluation of thunks. We also have the
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equivalence
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(delay expression) = (lazy (eager expression))
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</pre></div>
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</dd></dl>
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<p>The following reduction rules may be helpful for reasoning about these
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primitives. However, they do not express the memoization and memory
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usage semantics specified above:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(force (delay expression)) -> expression
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(force (lazy expression)) -> (force expression)
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(force (eager value)) -> value
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</pre></div>
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<h4 class="subsubheading" id="Correct-usage"><span>Correct usage<a class="copiable-link" href="#Correct-usage"> ¶</a></span></h4>
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<p>We now provide a general recipe for using the primitives {<code class="code">lazy</code>,
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<code class="code">delay</code>, <code class="code">force</code>} to express lazy algorithms in Scheme. The
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transformation is best described by way of an example: Consider the
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stream-filter algorithm, expressed in a hypothetical lazy language as
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (stream-filter p? s)
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(if (null? s) '()
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(let ((h (car s))
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(t (cdr s)))
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(if (p? h)
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(cons h (stream-filter p? t))
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(stream-filter p? t)))))
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</pre></div>
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<p>This algorithm can be expressed as follows in Scheme:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (stream-filter p? s)
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(lazy
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(if (null? (force s)) (delay '())
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(let ((h (car (force s)))
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(t (cdr (force s))))
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(if (p? h)
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(delay (cons h (stream-filter p? t)))
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(stream-filter p? t))))))
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</pre></div>
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<p>In other words, we
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</p>
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<ul class="itemize mark-bullet">
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<li>wrap all constructors (e.g., <code class="code">'()</code>, <code class="code">cons</code>) with <code class="code">delay</code>,
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</li><li>apply <code class="code">force</code> to arguments of deconstructors (e.g., <code class="code">car</code>,
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<code class="code">cdr</code> and <code class="code">null?</code>),
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</li><li>wrap procedure bodies with <code class="code">(lazy ...)</code>.
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</li></ul>
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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="SRFI_002d46.html">SRFI-46 Basic syntax-rules Extensions</a>, Previous: <a href="SRFI_002d43.html">SRFI-43 - Vector Library</a>, Up: <a href="SRFI-Support.html">SRFI Support Modules</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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