81 lines
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81 lines
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<title>CLiki: Tail Recursion</title>
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<span class="hidden">CLiki - Tail Recursion</span>
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<div id="content"><div id="content-area"><div id="article-title">Tail Recursion</div><div id="article">"Proper" Tail Recursion as found in Scheme is a feature that some miss in CL. It means carefully written recursive function calls can execute in constant space.<p>Most high-performance CL compilers can already do significant tail call elimination (see their respective manuals). In a <a href="http://groups.google.ie/groups?dq=&hl=en&lr=&ie=UTF-8&group=comp.lang.lisp&selm=cavzb.3175%24nm6.18005%40news.indigo.ie"> thread</a>
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on c.l.l., a vague idea of pseudostandardising across implementations was raised.
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To do this, some stuff would need to be defined.<p>What would constitute a "tail context" in CL? <p>It's all very well saying do tail "elimination where possible", but you'd
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need a list of where, like in Scheme R5RS section 3.5, which defines where
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tail contexts are in terms of a subset of rules of the scheme grammar. The
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Scheme ones are pretty common-sense I guess, but in CL may not be just
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"intuitively obvious" to everyone. Or maybe they would. The
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CMUCL manual section 5.5 about tail recursion in CMUCL talks about "tail
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recursive positions" - but AFAICS it does not exhaustively define what all
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the "tail recursive positions" / "tail contexts" might be in CL. <p>The CMUCL user manual explains in what situations tail-call elimination has
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to be suppressed for other features' sake, at least in CMUCL. Various
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dynamic binding issues mainly. These caveats are almost certainly not
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"intuitively obvious" to someone freshly arriving in CL from Scheme.<p>The major concern would be that an implementation with a feature of
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:tail-whatever and providing a known declaration of (optimise
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(tail-whatever 3)) to turn it on, would at minimum comply with a
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particular list of valid tail contexts, provided the programmer bore in
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mind the list of known caveats. <p>So you could just #- and bomb out if you knew you were writing code that
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pretty much depended on it - if a fellow with your source wanted to just
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risk it on a non-compliant implementation, they'd have to make a conscious
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decision to overrule your #-<p><hr>
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So, we need:<p>
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<b>A specification of tail contexts for all forms in CL</b><p>TODO :-)<p><b>A list of caveats</b><p>See e.g. CMUCL User Manual, 5.5<p>
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<b>A *features* entry</b><p>:x-tail-rec or whatever.<p>
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<b>A definition of a new declaration</b><p>e.g. from Joe Marshall in c.l.l. thread.<p>(declaim (optimize (dynamic-space 3)))
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= Tail call elimination where possible<p>(declaim (optimize (dynamic-space 2)))
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= Tail call elimination (when possible) for self-calls and locally
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defined (i.e. FLET or LABELS) functions only, no elimination on
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non-self calls.<p>(declaim (optimize (dynamic-space 1)))
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= Tail call elimination (when possible) for self-calls only.<p>(declaim (optimize (dynamic-space 0)))
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= No tail call elimination anywhere.<p><hr>
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<a href="programming tips.html" class="category">programming tips</a></div></div>
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