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214 lines
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>VHashes (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="Data-Types.html" rel="up" title="Data Types">
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<div class="subsection-level-extent" id="VHashes">
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<div class="nav-panel">
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<p>
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Next: <a href="Hash-Tables.html" accesskey="n" rel="next">Hash Tables</a>, Previous: <a href="Association-Lists.html" accesskey="p" rel="prev">Association Lists</a>, Up: <a href="Data-Types.html" accesskey="u" rel="up">Data Types</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="subsection" id="VList_002dBased-Hash-Lists-or-_0060_0060VHashes_0027_0027"><span>6.6.21 VList-Based Hash Lists or “VHashes”<a class="copiable-link" href="#VList_002dBased-Hash-Lists-or-_0060_0060VHashes_0027_0027"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-VList_002dbased-hash-lists"></a>
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<a class="index-entry-id" id="index-VHash"></a>
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<p>The <code class="code">(ice-9 vlist)</code> module provides an implementation of <em class="dfn">VList-based
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hash lists</em> (see <a class="pxref" href="VLists.html">VLists</a>). VList-based hash lists, or <em class="dfn">vhashes</em>, are an
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immutable dictionary type similar to association lists that maps <em class="dfn">keys</em> to
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<em class="dfn">values</em>. However, unlike association lists, accessing a value given its
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key is typically a constant-time operation.
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</p>
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<p>The VHash programming interface of <code class="code">(ice-9 vlist)</code> is mostly the same as
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that of association lists found in SRFI-1, with procedure names prefixed by
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<code class="code">vhash-</code> instead of <code class="code">alist-</code> (see <a class="pxref" href="SRFI_002d1-Association-Lists.html">Association Lists</a>).
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</p>
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<p>In addition, vhashes can be manipulated using VList operations:
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</p>
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<div class="example">
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<pre class="example-preformatted">(vlist-head (vhash-consq 'a 1 vlist-null))
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⇒ (a . 1)
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(define vh1 (vhash-consq 'b 2 (vhash-consq 'a 1 vlist-null)))
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(define vh2 (vhash-consq 'c 3 (vlist-tail vh1)))
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(vhash-assq 'a vh2)
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⇒ (a . 1)
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(vhash-assq 'b vh2)
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⇒ #f
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(vhash-assq 'c vh2)
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⇒ (c . 3)
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(vlist->list vh2)
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⇒ ((c . 3) (a . 1))
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</pre></div>
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<p>However, keep in mind that procedures that construct new VLists
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(<code class="code">vlist-map</code>, <code class="code">vlist-filter</code>, etc.) return raw VLists, not vhashes:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define vh (alist->vhash '((a . 1) (b . 2) (c . 3)) hashq))
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(vhash-assq 'a vh)
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⇒ (a . 1)
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(define vl
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;; This will create a raw vlist.
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(vlist-filter (lambda (key+value) (odd? (cdr key+value))) vh))
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(vhash-assq 'a vl)
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⇒ ERROR: Wrong type argument in position 2
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(vlist->list vl)
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⇒ ((a . 1) (c . 3))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-vhash_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-vhash_003f"> ¶</a></span></dt>
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<dd><p>Return true if <var class="var">obj</var> is a vhash.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-vhash_002dcons"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-cons</strong> <var class="def-var-arguments">key value vhash [hash-proc]</var><a class="copiable-link" href="#index-vhash_002dcons"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-vhash_002dconsq"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-consq</strong> <var class="def-var-arguments">key value vhash</var><a class="copiable-link" href="#index-vhash_002dconsq"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-vhash_002dconsv"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-consv</strong> <var class="def-var-arguments">key value vhash</var><a class="copiable-link" href="#index-vhash_002dconsv"> ¶</a></span></dt>
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<dd><p>Return a new hash list based on <var class="var">vhash</var> where <var class="var">key</var> is associated with
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<var class="var">value</var>, using <var class="var">hash-proc</var> to compute the hash of <var class="var">key</var>.
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<var class="var">vhash</var> must be either <code class="code">vlist-null</code> or a vhash returned by a previous
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call to <code class="code">vhash-cons</code>. <var class="var">hash-proc</var> defaults to <code class="code">hash</code> (see <a class="pxref" href="Hash-Table-Reference.html"><code class="code">hash</code> procedure</a>). With <code class="code">vhash-consq</code>, the
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<code class="code">hashq</code> hash function is used; with <code class="code">vhash-consv</code> the <code class="code">hashv</code>
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hash function is used.
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</p>
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<p>All <code class="code">vhash-cons</code> calls made to construct a vhash should use the same
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<var class="var">hash-proc</var>. Failing to do that, the result is undefined.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-vhash_002dassoc"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-assoc</strong> <var class="def-var-arguments">key vhash [equal? [hash-proc]]</var><a class="copiable-link" href="#index-vhash_002dassoc"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-vhash_002dassq"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-assq</strong> <var class="def-var-arguments">key vhash</var><a class="copiable-link" href="#index-vhash_002dassq"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-vhash_002dassv"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-assv</strong> <var class="def-var-arguments">key vhash</var><a class="copiable-link" href="#index-vhash_002dassv"> ¶</a></span></dt>
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<dd><p>Return the first key/value pair from <var class="var">vhash</var> whose key is equal to <var class="var">key</var>
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according to the <var class="var">equal?</var> equality predicate (which defaults to
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<code class="code">equal?</code>), and using <var class="var">hash-proc</var> (which defaults to <code class="code">hash</code>) to
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compute the hash of <var class="var">key</var>. The second form uses <code class="code">eq?</code> as the equality
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predicate and <code class="code">hashq</code> as the hash function; the last form uses <code class="code">eqv?</code>
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and <code class="code">hashv</code>.
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</p>
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<p>Note that it is important to consistently use the same hash function for
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<var class="var">hash-proc</var> as was passed to <code class="code">vhash-cons</code>. Failing to do that, the
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result is unpredictable.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-vhash_002ddelete"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-delete</strong> <var class="def-var-arguments">key vhash [equal? [hash-proc]]</var><a class="copiable-link" href="#index-vhash_002ddelete"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-vhash_002ddelq"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-delq</strong> <var class="def-var-arguments">key vhash</var><a class="copiable-link" href="#index-vhash_002ddelq"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-vhash_002ddelv"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-delv</strong> <var class="def-var-arguments">key vhash</var><a class="copiable-link" href="#index-vhash_002ddelv"> ¶</a></span></dt>
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<dd><p>Remove all associations from <var class="var">vhash</var> with <var class="var">key</var>, comparing keys with
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<var class="var">equal?</var> (which defaults to <code class="code">equal?</code>), and computing the hash of
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<var class="var">key</var> using <var class="var">hash-proc</var> (which defaults to <code class="code">hash</code>). The second
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form uses <code class="code">eq?</code> as the equality predicate and <code class="code">hashq</code> as the hash
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function; the last one uses <code class="code">eqv?</code> and <code class="code">hashv</code>.
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</p>
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<p>Again the choice of <var class="var">hash-proc</var> must be consistent with previous calls to
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<code class="code">vhash-cons</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-vhash_002dfold"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-fold</strong> <var class="def-var-arguments">proc init vhash</var><a class="copiable-link" href="#index-vhash_002dfold"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-vhash_002dfold_002dright"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-fold-right</strong> <var class="def-var-arguments">proc init vhash</var><a class="copiable-link" href="#index-vhash_002dfold_002dright"> ¶</a></span></dt>
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<dd><p>Fold over the key/value elements of <var class="var">vhash</var> in the given direction,
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with each call to <var class="var">proc</var> having the form <code class="code">(<var class="var">proc</var> key value
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result)</code>, where <var class="var">result</var> is the result of the previous call to
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<var class="var">proc</var> and <var class="var">init</var> the value of <var class="var">result</var> for the first call
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to <var class="var">proc</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-vhash_002dfold_002a"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-fold*</strong> <var class="def-var-arguments">proc init key vhash [equal? [hash]]</var><a class="copiable-link" href="#index-vhash_002dfold_002a"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-vhash_002dfoldq_002a"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-foldq*</strong> <var class="def-var-arguments">proc init key vhash</var><a class="copiable-link" href="#index-vhash_002dfoldq_002a"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-vhash_002dfoldv_002a"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">vhash-foldv*</strong> <var class="def-var-arguments">proc init key vhash</var><a class="copiable-link" href="#index-vhash_002dfoldv_002a"> ¶</a></span></dt>
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<dd><p>Fold over all the values associated with <var class="var">key</var> in <var class="var">vhash</var>, with each
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call to <var class="var">proc</var> having the form <code class="code">(proc value result)</code>, where
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<var class="var">result</var> is the result of the previous call to <var class="var">proc</var> and <var class="var">init</var> the
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value of <var class="var">result</var> for the first call to <var class="var">proc</var>.
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</p>
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<p>Keys in <var class="var">vhash</var> are hashed using <var class="var">hash</var> are compared using <var class="var">equal?</var>.
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The second form uses <code class="code">eq?</code> as the equality predicate and <code class="code">hashq</code> as
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the hash function; the third one uses <code class="code">eqv?</code> and <code class="code">hashv</code>.
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</p>
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<p>Example:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define vh
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(alist->vhash '((a . 1) (a . 2) (z . 0) (a . 3))))
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(vhash-fold* cons '() 'a vh)
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⇒ (3 2 1)
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(vhash-fold* cons '() 'z vh)
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⇒ (0)
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</pre></div>
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</dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-alist_002d_003evhash"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">alist->vhash</strong> <var class="def-var-arguments">alist [hash-proc]</var><a class="copiable-link" href="#index-alist_002d_003evhash"> ¶</a></span></dt>
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<dd><p>Return the vhash corresponding to <var class="var">alist</var>, an association list, using
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<var class="var">hash-proc</var> to compute key hashes. When omitted, <var class="var">hash-proc</var> defaults
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to <code class="code">hash</code>.
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</p></dd></dl>
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</div>
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<hr>
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<p>
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Next: <a href="Hash-Tables.html">Hash Tables</a>, Previous: <a href="Association-Lists.html">Association Lists</a>, Up: <a href="Data-Types.html">Data Types</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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