On Wed, Mar 18, 2015 at 5:11 PM Alexis King <lexi.lam...@gmail.com> wrote:
> 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. > Yes, this is a good idea. Sam > > > 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 > >
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