Hi Ben, Thank you for your answer.
On Thursday, February 20, 2020 at 3:29:15 AM UTC+1, Ben Greenman wrote: > > On 2/19/20, unlimitedscolobb <unlimite...@gmail.com <javascript:>> wrote: > > > > [....] > > > > ;Type Checker: Type a could not be converted to a contract because it > > contains free variables. > > ; in: a > > > > Does this mean that I can never cast to types containing type variables? > > Yes > Ah, okay, got it. > > My original problem comes from playing around with eval, which returns > > AnyValues. Ideally, I would like to be able to retrieve the first > returned > > > > value (done), and then convert it to a type variable bound by one of the > > arguments: > > > > (: my-super-func (All (a) (-> (Type1 a) a))) > > (define (my-super-func arg) > > ... > > (get-first-value (eval some-stuff))) ; how to cast to a ? > > You can ask the caller to supply a cast function that turns an Any to an > A. > Ahaa, that's a clever idea! But depending on what you want to cast to, occurrence typing may be > more convenient. For example: > > #lang typed/racket > > (: my-super-func (All (a) (-> (-> Any) (-> Any Boolean : #:+ a) a))) > (define (my-super-func get-v check-v) > (assert (get-v) check-v)) > > (ann (my-super-func (lambda () 42) exact-integer?) Integer); > ;; 42 > (ann (my-super-func (lambda () '(A)) (lambda (x) (and (list? x) > (andmap symbol? x)))) (Listof Symbol)) > ;; '(A) > This does look quite a bit like lifting contracts to types, very nice. Thank you very much for clarifying and giving a detailed example. I'm coming over from Haskell, so I'm still in process of wrapping my head around types and contracts in Racket. - Sergiu -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/3fdb2744-fd5b-4668-9da0-ab853fa0efaa%40googlegroups.com.
