It might be a good idea to have a special case for Values when producing error messages. Currently, even though the documentation mentions that Values is only valid as the return type of a function, the error messages can be a little confusing. For example, using Values as an expression produces the error “type name used out of context,” but it isn’t really a type.
> On Mar 18, 2015, at 14:01, Sam Tobin-Hochstadt <sa...@cs.indiana.edu> wrote: > > Type variables range only over types, and `(Values A B)` isn't a type. > If your program type checked, then the following program would too: > > (: my-force : (All (A) (-> A) -> A)) > (define (my-force f) > (let ([tmp : A (f)]) tmp)) > > (my-force (lambda () (values (void) (void))) > > but this program has a runtime error with the wrong number of values. > > Sam > > On Wed, Mar 18, 2015 at 4:48 PM, Benjamin Greenman <bl...@cornell.edu> wrote: >> Today I got a surprising type error that I think is worth sharing. >> >> (: my-force (All (A) (-> (-> A) A)) >> (define (my-force x) (x)) >> >> (my-force (lambda () (values (void) (void))) >> ;; ERROR! 'my-force' cannot be applied to argument. >> ;; expected "(-> A)", got "(-> (values Void Void))" >> ;; result type "A", expected result "AnyValues" >> >> (inst my-force (Values Void Void)) >> ;; Parse Error in type: "Values" is unbound >> >> That is all. I see now that Values is only allowed in result positions, but >> I'd still expected this to work. >> >> >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users >> > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
