Looks like the docs are still in collects/typed-scheme/scribblings/ Robby
On Mon, Jul 5, 2010 at 9:17 AM, Matthias Felleisen <[email protected]> wrote: > > On Jul 5, 2010, at 7:19 AM, Jose A. Ortega Ruiz wrote: > >> >> Hi, >> >> The following (contrived) typed/racket program doesn't typecheck: >> >> (define-struct: (E) Arrow >> ([arrow : (E -> (Listof E))])) >> >> (: compose-arrows (All (E) ((Arrow E) (Arrow E) -> (Arrow E)))) >> (define (compose-arrows a b) >> (Arrow (lambda (e) ((Arrow-arrow a) (car ((Arrow-arrow b) e)))))) > > As Noel points out, all that's missing is a type declaration for > the parameter e in the lambda: > > (lambda ({e : E}) ...) > > will do the trick. > > Why? TR does not infer types. It checks them. When a type declaration > is missing, TR assumes that e is of type TheRacketType, aka Any. For > a parameter of type Any, it makes no sense to be used in a position > where it is supposed to be a number, a boolean, or an E. Your program > must somehow 'narrow' the type of e down to what the operation expects, > in your case E. > > If I knew where the TR guide lives, I'd add the appropriate two-line > paragraph to clarify this point. > > ;; --- > >> Is the failure to type check a genuine limitation of the type inference >> engine, or is it me expecting too much from type inference in general? >> And, is there a more compact way of indicating the type of that lambda? > > It is not quite true that TR doesn't infer types. It does so but in > an extremely limited manner. Technically, this is called LOCAL Inference. > > What does this mean? > > When you write (lambda (x) ...) TR can't know what this function will be > applied to. It must really assume x ranges over Any. > > When you write (let ((x 7)) ...) however, TR assumes you want x to range > over numbers in the body of let because the initial value has type Number. > To see how this fails, try > > (let ([x 7]) (set! x "hello") x) > > Again, I wish I could clarify this bit in the docs. > > -- Matthias > > > > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
