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<title>CLiki: Named multiple-values</title>
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<span class="hidden">CLiki - Named multiple-values</span>
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<div id="content"><div id="content-area"><div id="article-title">Named multiple-values</div><div id="article">Named-multiple-values is a piece of macrology that enables one to deal with
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Common Lisp's multiple-values somewhat abstractly. That is, you may
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refer to a set of values by name, and also particular values by name.<p><a href="http://web.archive.org/web/20070205214300/http://www.cs.uit.no/~frodef/sw/named-multiple-values.lisp">
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named-multiple-values.lisp</a> might be
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considered to be a <a href="convenience library.html" class="category">convenience library</a>. It defines a single macro, called
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<em>named-multiple-values:define-named-multiple-values</em>. This
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macro generates two macros, whose names it returns. See the example.<p><h4>Example</h4><p>Here is an example of what it can look like:<p><pre>
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(define-named-multiple-values foo-values (bar zot zut)
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(:all &all))
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=> FOO-VALUES, FOO-VALUES-BIND
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(foo-values () :bar 'barney :zut 'zutteklut)
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=> BARNEY, NIL, ZUTTEKLUT
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(foo-values-bind (&zut my-zut &bar local-bar)
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(values 1 2 3)
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(list my-zut local-bar))
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=> (3 1)
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(foo-values-bind (&all default-foos)
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(values 1 2 3)
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(foo-values (:defaults default-foos)
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:zot 'special-zot))
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=> 1, SPECIAL-ZOT, 3
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</pre><p>However, named-multiple-values is not expected to be particularly
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useful for interactive work, see the "background" section.<p><h4>Description</h4><p>I will offer two analogies to the concept of named-multiple-values:<p><ul>
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<li> Multiple-values are similar to functions' argument-lists, where the
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latter is "down-stream", and the former is "up-stream" flow of
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information. Much like Common Lisp---by use of <a href="https://www.cliki.net/site/HyperSpec/Body/sec_3-4-1.html" class="hyperspec">&key</a>---allow you to
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pass arguments (down-stream) to functions by name rather than position,
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you can now generate up-stream multiple-values by name. </li><p> <li> Common Lisp's <a href="https://www.cliki.net/site/HyperSpec/Body/mac_defstruct.html" class="hyperspec">defstruct</a> offers the possibility of defining
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abstract types whose concrete type is (for example) a list.
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So while the type abstractly consits only of members a, b, and c,
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in reality there will be a list of length three, whose first
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element is a, second element is b, and third is c.
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Named multiple-values are very much the same thing, except
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the set of values only exists as an abstract entity: No list
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or anything else is ever consed up, since the value-sets as such
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never exist as lisp objects. Consequently, they cannot be passed
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around between functions. </li>
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</ul>
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<h4>Background</h4><p>What is named-multiple-values useful for? I'm not quite sure. This
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code is factored out from a project where I had many tens of functions
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that followed the same protocol, both down-stream and up-stream. That
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is, the functions accepted the same parameters, and returned the same
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values. Being the lisper that I am, I took advantage of this
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regularity in the code by having macros that dealt with both
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directions of the protocol. This added greatly to the clarity of the
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code, and saved me a lot of editing.<p>While the down-stream part of my original project's macrology was
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rather trivial, and didn't really provide that much beyond what &key
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already does, the up-stream part I thought might be more interesting
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for more general-purpose use. So here it is, available as finished
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code, or just as and idea you might find usable in your own code.<p>Because this code is factored out and modified quite a bit from my
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original project with well-tested code, and abstracted into what I
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suppose might be called a higher-order-macro (a macro-writing macro,
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that is), I fully expect there to be bugs and quirks. So this code
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should perhaps be considered more as a description of an idea than a
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ready-to-use library.<p>Comments are welcome.<p>I (<a href="Gary King.html" class="internal">Gary King</a>) am not sure that I follow but are named-multiple-values similar to returning property lists but without the consing? <p>Yes, I think that would describe it reasonably. If you want to return numerous values and would prefer to refer to each value by name rather than ordering, you can use ``Named multiple-values''.<p><hr>
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