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    <p>Tail-call optimization is where you are able to avoid allocating a new stack frame for a function because the calling function will simply return the value that it gets from the called function. The most common use is tail-recursion, where a recursive function written to take advantage of tail-call optimization can use constant stack space.</p> <p>Scheme is one of the few programming languages that guarantee in the spec that any implementation must provide this optimization <em>(JavaScript does also, starting with ES6)</em>, so here are two examples of the factorial function in Scheme:</p> <pre class="lang-lisp prettyprint-override"><code>(define (fact x) (if (= x 0) 1 (* x (fact (- x 1))))) (define (fact x) (define (fact-tail x accum) (if (= x 0) accum (fact-tail (- x 1) (* x accum)))) (fact-tail x 1)) </code></pre> <p>The first function is not tail recursive because when the recursive call is made, the function needs to keep track of the multiplication it needs to do with the result after the call returns. As such, the stack looks as follows:</p> <pre class="lang-scheme prettyprint-override"><code>(fact 3) (* 3 (fact 2)) (* 3 (* 2 (fact 1))) (* 3 (* 2 (* 1 (fact 0)))) (* 3 (* 2 (* 1 1))) (* 3 (* 2 1)) (* 3 2) 6 </code></pre> <p>In contrast, the stack trace for the tail recursive factorial looks as follows:</p> <pre class="lang-scheme prettyprint-override"><code>(fact 3) (fact-tail 3 1) (fact-tail 2 3) (fact-tail 1 6) (fact-tail 0 6) 6 </code></pre> <p>As you can see, we only need to keep track of the same amount of data for every call to fact-tail because we are simply returning the value we get right through to the top. This means that even if I were to call (fact 1000000), I need only the same amount of space as (fact 3). This is not the case with the non-tail-recursive fact, and as such large values may cause a stack overflow.</p>
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    1. POWhat Is Tail Call Optimization?
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    1. USKyle Cronin
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    1. COVery clear (even if it is in Scheme!) and a nice workthrough of the steps too. Thanks!
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      1. USmajelbstoat
    2. COIf you want to learn more about this, I suggest reading the first chapter of Structure and Interpretation of Computer Programs.
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      1. USKyle Cronin
    3. COI guess an inner function which will tail-recurse and an accumulator as parameter to that function are typical in most cases.
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      1. USVigneshwaran
 

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