Editing Scheme (programming language) (section)

===Proper tail recursion===
{{Further|Tail recursion}}
Scheme has an iteration construct, <code>do</code>, but it is more [[Programming idiom|idiomatic]] in Scheme to use [[tail recursion]] to express [[iteration]]. Standard-conforming Scheme implementations are required to optimize tail calls so as to support an unbounded number of active tail calls (R5RS sec. 3.5)<ref name="r5rs"/>—a property the Scheme report describes as ''proper tail recursion''—making it safe for Scheme programmers to write iterative algorithms using recursive structures, which are sometimes more intuitive. Tail recursive procedures and the ''named <code>let</code>'' form provide support for iteration using tail recursion.

<syntaxhighlight lang="Scheme">
;; Building a list of squares from 0 to 9:
;; Note: loop is simply an arbitrary symbol used as a label. Any symbol will do.

(define (list-of-squares n)
  (let loop ((i n) (res '()))
    (if (< i 0)
        res
        (loop (- i 1) (cons (* i i) res)))))

(list-of-squares 9)
===> (0 1 4 9 16 25 36 49 64 81)
</syntaxhighlight>