From the R5RS standard:
Values might be defined as follows:
(define (values . things)
(call-with-current-continuation
(lambda (cont) (apply cont things))))
My first interpretation of this was that an expression like (+ (values 1 2)) is equivalent to (apply + '(1 2)) and would yield the result 3. But, according to my tests, this interpretation is not correct. Here is my interpretation of the code above: values is a function taking any number of arguments, bundled into a list called things. Then, the current continuation (the place where values is used) is called with the list things "unbundled".
What am I missing? The example above (+ (values 1 2)) gives an error or 1 depending on the interpreter I used.
See, when you type
(+ (values 1 2))
the continuation of the call to values is actually a single argument to +. So, it is either treated as 1 (the first element to the list, the first value produced by the procedure), or an error. R5RS says in this regard:
Except for continuations created by the call-with-values procedure, all continuations take exactly one value. The effect of passing no value or more than one value to continuations that were not created by call-with-values is unspecified.
On the other hand, call-with-values would correctly bind your list's elements to its consumer argument's formal arguments:
Calls its producer argument with no values and a continuation that, when passed some values, calls the consumer procedure with those values as arguments.