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80 lines
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5.4 KiB
HTML
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<title>CLiki: bind</title>
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href="https://www.cliki.net/site/feed/article.atom?title=bind">
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<span class="hidden">CLiki - bind</span>
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<div id="content"><div id="content-area"><div id="article-title">bind</div><div id="article">Bind combines <a href="https://www.cliki.net/site/HyperSpec/Body/speope_letcm_letst.html" class="hyperspec">let*</a>, <a href="https://www.cliki.net/site/HyperSpec/Body/mac_destructuring-bind.html" class="hyperspec">destructuring-bind</a> and <a href="https://www.cliki.net/site/HyperSpec/Body/mac_multiple-value-bind.html" class="hyperspec">multiple-value-bind</a> into a single form. Simple bindings are as in let*. Destructuring is done if the first item in a binding is a list. Multiple value binding is done if the first item in a binding is a list and the first item in the list is 'values'. More information about bind can be found on <a href="http://common-lisp.net/project/metabang-bind/">common-lisp.net</a>. <p>An example is probably the best way to describe its syntax:
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<pre>
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(bind ((a 2)
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((b &rest args &key (c 2) &allow-other-keys) '(:a :c 5 :d 10 :e 54))
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((:values d e) (truncate 4.5)))
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(list a b c d e args))
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==> (2 :A 5 4 0.5 (:C 5 :D 10 :E 54))
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</pre>
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Bind is especially handy when you have more than one layer of
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multiple-value-bind or destructuring-bind. Since bind is a single form,
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you don't end up too far off to the right in editor land.<p>Bind works by parsing the bindings and rewriting them as nested
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let, multiple-value-bind and destructuring-bind forms. Bind handles
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declarations correctly -- putting each at the appropriate level.<p>Bind is released under the MIT license. The most recent version (as of 1 October 2005) reflects this explicitly.<p>I like bind because it (a). presents a uniform means of three of CL's most typical binding constructs and (b). because you don't have to worry about nesting! I like the more explicit syntax with "values" because it makes it clear what it is doing and is analogous to something like (setf (values a b) (foo)). <p>Bind was written by <a href="Gary King.html" class="internal">Gary King</a>.<p>An alternative syntax -- which bind does not support -- that doesn't rely on explicitly saying "values" is:
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<pre>
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(bind ((d e (truncate 4.5)))
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(list d e))
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=>
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(4 0.5)
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</pre>
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Implementation-wise, what it's doing is using the last item in the
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list, (truncate 4.5), as the form to evaluate. Then the first item, d,
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gets the primary value, and each item in between gets one of the
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other values; in this case, that's just e.<p>This is particularly nice because it makes it possible to combine
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multiple-value binding and destructuring binding. For example:<p><pre>
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(bind (((this &rest that) the-other (values (list 1 2 3 4) 5)))
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(list this that the-other))
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=>
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(1 (2 3 4) 5)
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</pre><p>See also: <a href="X.LET-STAR.html" class="internal">X.LET-STAR</a> <a href="https://www.cliki.net/WITH%20macro" class="new">WITH macro</a>
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<a href="pattern matching.html" class="category">pattern matching</a> <a href="utilities.html" class="category">utilities</a></div></div>
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