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    <p>Let's disambiguate "tail calls" first.</p> <p>A <em>call in tail position</em> is a function call whose result is immediately returned as the value of the enclosing function. Tail position is a static property.</p> <p>A call in tail position can be implemented without pushing anything onto the stack, because the old stack frame is essentially useless (under assumptions that are generally true in functional languages but not necessarily in C, etc). As Guy Steele put it, a tail call is a jump that passes arguments.</p> <p>Roughly, a language implementation is <em>properly tail recursive</em> if it has the same asymptotic space usage as one that implements all calls in tail position as jumps without stack growth. That's a really rough simplification. If you want the full story, see Clinger's <a href="http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.50.4500" rel="noreferrer">Proper Tail Recursion and Space Efficiency</a>.</p> <p>Note that just handling <em>tail-recursive</em> functions specially is not enough to achieve proper tail recursion (<em>any</em> tail call must be specially handled). The terminology is somewhat misleading.</p> <p>Also note that there are other ways to achieve that asymptotic space efficiency without implementing tail calls as jumps. For example, you might implement them as normal calls and then periodically compact the stack by removing useless frames (somehow). See Baker's <a href="http://www.pipeline.com/~hbaker1/CheneyMTA.html" rel="noreferrer">Cheney on the MTA</a>.</p>
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    1. POWhat is difference between tail calls and tail recursion?
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    1. USWill Ness
    1. USRyan Culpepper
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    1. COgreat answer! I'd just really like to see some elaboration on what exactly did you mean by *"just handling tail-recursive functions specially is not enough to achieve proper tail recursion"*. What does this refer to?
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      1. USWill Ness
    2. CO@WillNess, see Andreas's answer, which talks about this and gives links for more details.
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    3. COone more thing. Is the old stack frame really that useless? If "tail call is a jump that passes arguments", where does it passes them? On the old stack frame, is it not? (at least when it calls itself). I think TCO is as much about stack frame reuse as direct jumps. In case of say three mutually recursive functions the three frames might coexist in the stack, too. Looks like TCO is about recovering the forsaken GOTO of unstructured programming. :)
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