The call-with-values and values procedures were described in an earlier Scheme of Things ( Lisp Pointers, volume IV, number 1), but I'll review them here. The following is adapted from John Ramsdell's concise description:
)
essential procedure
values delivers all of its arguments to its continuation.
(call-with-values thunk receiver) essential procedure
call-with-values calls its thunk argument with a continuation that, when passed some values, calls the receiver procedure with those values as arguments. The continuation for the call to receiver is the continuation of the call to call-with-values.
Except for continuations created by the call-with-values procedure, all continuations take exactly one value, as now; the effect of passing no value or more than one value to continuations that were not created by call-with-values is unspecified (as indeed it is unspecified now).
values might be defined as follows: =0pt=0pt=0pt =0pt[] (define (values . things) (call-with-current-continuation (lambda (cont) (apply cont things))))
That is, the procedures supplied by call-with-current-continuation must be passed the same number of arguments as values expected by the continuation.
Because the behavior of a number-of-values mismatch between a continuation and its invoker is unspecified, some implementations may assign some specific meaning to such situations; for example, extra values might be ignored, or defaults might be supplied for missing values. Thus this multiple return value proposal is compatible with Common Lisp's multiple values, but strictly more conservative than it. The behavior of programs in such situations was a point of contention among the authors, which is why only the least common denominator behavior was specified.