Scheme is “properly tail recursive”, meaning that tail calls or recursions from certain contexts do not consume stack space or other resources and can therefore be used on arbitrarily large data or for an arbitrarily long calculation. Consider for example,
(define (foo n) (display n) (newline) (foo (1+ n))) (foo 1) -| 1 2 3 ...
foo prints numbers infinitely, starting from the given n.
It's implemented by printing n then recursing to itself to print
n+1 and so on. This recursion is a tail call, it's the
last thing done, and in Scheme such tail calls can be made without
Or consider a case where a value is returned, a version of the SRFI-1
last function (see SRFI-1 Selectors) returning the last
element of a list,
(define (my-last lst) (if (null? (cdr lst)) (car lst) (my-last (cdr lst)))) (my-last '(1 2 3)) ⇒ 3
If the list has more than one element,
my-last applies itself
cdr. This recursion is a tail call, there's no code
after it, and the return value is the return value from that call. In
Scheme this can be used on an arbitrarily long list argument.
A proper tail call is only available from certain contexts, namely the following special form positions,
and— last expression
begin— last expression
case— last expression in each clause
cond— last expression in each clause, and the call to a
=>procedure is a tail call
do— last result expression
if— “true” and “false” leg expressions
lambda— last expression in body
letrec-syntax— last expression in body
or— last expression
The following core functions make tail calls,
apply— tail call to given procedure
call-with-current-continuation— tail call to the procedure receiving the new continuation
call-with-values— tail call to the values-receiving procedure
eval— tail call to evaluate the form
string-every— tail call to predicate on the last character (if that point is reached)
The above are just core functions and special forms. Tail calls in other modules are described with the relevant documentation, for example SRFI-1
every(see SRFI-1 Searching).
It will be noted there are a lot of places which could potentially be
tail calls, for instance the last call in a
for-each, but only
those explicitly described are guaranteed.