On 21-Sep-08, at 3:25 PM, Arthur Smyles wrote:

(if (values #f) #t #f)

=> #f

(if (values #f #f) #t #f)

=> #t

I was surprised to discover this. Although technically not a bug since the scheme spec considers this situation unspecified, it makes using multiple-values unreliable.

In what way is it "unreliable"? The situation is defined as an error by the Scheme standard, which means a user cannot expect any specific behavior.

Gambit handles multiple values by treating the "values" procedure as a data structure constructor (just like "vector" except with a different type tag). The exception is that when it is given a single argument it behaves like the identity function, so (values 123) = 123 . This explains why (if (values #f #f) 11 22) gives 11, just like (if (vector #f #f) 11 22) gives 11, but (if (values #f) 11 22) = (if #f 11 22) = 22 .

This affects pp as well

(pp (values #f #f))

=> #<unknown>

What would you expect this to give? I'm not sure this is related to what you want to do, but these operations are available:

(##values? (values 11 22))

#t

(##vector->list (values 11 22))

(11 22)

(for-each pp (##vector->list (values 11 22)))

11 22

Marc