186 lines
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186 lines
9.8 KiB
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>SRFI-41 Stream Primitives (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="SRFI_002d41.html" rel="up" title="SRFI-41">
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<link href="SRFI_002d41-Stream-Library.html" rel="next" title="SRFI-41 Stream Library">
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<link href="SRFI_002d41-Stream-Fundamentals.html" rel="prev" title="SRFI-41 Stream Fundamentals">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="SRFI_002d41-Stream-Primitives">
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<div class="nav-panel">
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<p>
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Next: <a href="SRFI_002d41-Stream-Library.html" accesskey="n" rel="next">SRFI-41 Stream Library</a>, Previous: <a href="SRFI_002d41-Stream-Fundamentals.html" accesskey="p" rel="prev">SRFI-41 Stream Fundamentals</a>, Up: <a href="SRFI_002d41.html" accesskey="u" rel="up">SRFI-41 - Streams</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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<hr>
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<h4 class="subsubsection" id="SRFI_002d41-Stream-Primitives-1"><span>7.5.28.2 SRFI-41 Stream Primitives<a class="copiable-link" href="#SRFI_002d41-Stream-Primitives-1"> ¶</a></span></h4>
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<p>This library provides eight operators: constructors for
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<code class="code">stream-null</code> and <code class="code">stream-pair</code>s, type predicates for streams
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and the two kinds of streams, accessors for both fields of a
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<code class="code">stream-pair</code>, and a lambda that creates procedures that return
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streams.
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</p>
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<dl class="first-defvr">
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<dt class="defvr" id="index-stream_002dnull"><span class="category-def">Scheme Variable: </span><span><strong class="def-name">stream-null</strong><a class="copiable-link" href="#index-stream_002dnull"> ¶</a></span></dt>
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<dd><p>A promise that, when forced, is a single object, distinguishable from
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all other objects, that represents the null stream. <code class="code">stream-null</code>
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is immutable and unique.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-stream_002dcons"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">stream-cons</strong> <var class="def-var-arguments">object-expr stream-expr</var><a class="copiable-link" href="#index-stream_002dcons"> ¶</a></span></dt>
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<dd><p>Creates a newly-allocated stream containing a promise that, when forced,
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is a <code class="code">stream-pair</code> with <var class="var">object-expr</var> in its <code class="code">stream-car</code>
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and <var class="var">stream-expr</var> in its <code class="code">stream-cdr</code>. Neither
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<var class="var">object-expr</var> nor <var class="var">stream-expr</var> is evaluated when
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<code class="code">stream-cons</code> is called.
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</p>
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<p>Once created, a <code class="code">stream-pair</code> is immutable; there is no
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<code class="code">stream-set-car!</code> or <code class="code">stream-set-cdr!</code> that modifies an
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existing stream-pair. There is no dotted-pair or improper stream as
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with lists.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-stream_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">stream?</strong> <var class="def-var-arguments">object</var><a class="copiable-link" href="#index-stream_003f"> ¶</a></span></dt>
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<dd><p>Returns true if <var class="var">object</var> is a stream, otherwise returns false. If
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<var class="var">object</var> is a stream, its promise will not be forced. If
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<code class="code">(stream? obj)</code> returns true, then one of <code class="code">(stream-null? obj)</code>
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or <code class="code">(stream-pair? obj)</code> will return true and the other will return
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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-stream_002dnull_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">stream-null?</strong> <var class="def-var-arguments">object</var><a class="copiable-link" href="#index-stream_002dnull_003f"> ¶</a></span></dt>
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<dd><p>Returns true if <var class="var">object</var> is the distinguished null stream, otherwise
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returns false. If <var class="var">object</var> is a stream, its promise will be forced.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-stream_002dpair_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">stream-pair?</strong> <var class="def-var-arguments">object</var><a class="copiable-link" href="#index-stream_002dpair_003f"> ¶</a></span></dt>
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<dd><p>Returns true if <var class="var">object</var> is a <code class="code">stream-pair</code> constructed by
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<code class="code">stream-cons</code>, otherwise returns false. If <var class="var">object</var> is a
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stream, its promise will be forced.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-stream_002dcar"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">stream-car</strong> <var class="def-var-arguments">stream</var><a class="copiable-link" href="#index-stream_002dcar"> ¶</a></span></dt>
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<dd><p>Returns the object stored in the <code class="code">stream-car</code> of <var class="var">stream</var>. An
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error is signaled if the argument is not a <code class="code">stream-pair</code>. This
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causes the <var class="var">object-expr</var> passed to <code class="code">stream-cons</code> to be
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evaluated if it had not yet been; the value is cached in case it is
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needed again.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-stream_002dcdr"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">stream-cdr</strong> <var class="def-var-arguments">stream</var><a class="copiable-link" href="#index-stream_002dcdr"> ¶</a></span></dt>
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<dd><p>Returns the stream stored in the <code class="code">stream-cdr</code> of <var class="var">stream</var>. An
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error is signaled if the argument is not a <code class="code">stream-pair</code>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-stream_002dlambda"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">stream-lambda</strong> <var class="def-var-arguments">formals body …</var><a class="copiable-link" href="#index-stream_002dlambda"> ¶</a></span></dt>
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<dd><p>Creates a procedure that returns a promise to evaluate the <var class="var">body</var> of
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the procedure. The last <var class="var">body</var> expression to be evaluated must
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yield a stream. As with normal <code class="code">lambda</code>, <var class="var">formals</var> may be a
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single variable name, in which case all the formal arguments are
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collected into a single list, or a list of variable names, which may be
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null if there are no arguments, proper if there are an exact number of
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arguments, or dotted if a fixed number of arguments is to be followed by
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zero or more arguments collected into a list. <var class="var">Body</var> must contain
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at least one expression, and may contain internal definitions preceding
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any expressions to be evaluated.
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</p></dd></dl>
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<div class="example">
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<pre class="example-preformatted">(define strm123
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(stream-cons 1
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(stream-cons 2
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(stream-cons 3
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stream-null))))
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(stream-car strm123) ⇒ 1
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(stream-car (stream-cdr strm123) ⇒ 2
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(stream-pair?
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(stream-cdr
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(stream-cons (/ 1 0) stream-null))) ⇒ #f
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(stream? (list 1 2 3)) ⇒ #f
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(define iter
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(stream-lambda (f x)
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(stream-cons x (iter f (f x)))))
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(define nats (iter (lambda (x) (+ x 1)) 0))
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(stream-car (stream-cdr nats)) ⇒ 1
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(define stream-add
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(stream-lambda (s1 s2)
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(stream-cons
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(+ (stream-car s1) (stream-car s2))
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(stream-add (stream-cdr s1)
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(stream-cdr s2)))))
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(define evens (stream-add nats nats))
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(stream-car evens) ⇒ 0
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(stream-car (stream-cdr evens)) ⇒ 2
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(stream-car (stream-cdr (stream-cdr evens))) ⇒ 4
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</pre></div>
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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_002d41-Stream-Library.html">SRFI-41 Stream Library</a>, Previous: <a href="SRFI_002d41-Stream-Fundamentals.html">SRFI-41 Stream Fundamentals</a>, Up: <a href="SRFI_002d41.html">SRFI-41 - Streams</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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</body>
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</html>
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