[racket] problem (?) with typed racket

Looks like the docs are still in collects/typed-scheme/scribblings/

Robby

On Mon, Jul 5, 2010 at 9:17 AM, Matthias Felleisen <matthias at ccs.neu.edu> 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
>
>
>
>
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