((( [logo]Racket )))Need Help?
AboutDownloadDocumentationPLaneTCommunityLearning

[plt-scheme] typed-scheme: conditional branches and parametric polymorphism

From: Carl Eastlund (carl.eastlund at gmail.com)
Date: Thu Jan 28 14:58:06 EST 2010		 
2010/1/28 Jon Zeppieri <zeppieri at gmail.com>:
> 2010/1/28 Carl Eastlund <carl.eastlund at gmail.com>:
>> Try giving r1 a type in terms of A and T, without wrapping it in (All
>> (A T) ...).  Tell Typed Scheme what type it is for a given
>> instantiation, and it will hopefully be able to figure out what your
>> function is doing a little better.
>
> That's exactly what I tried:
>
> (let: ((r1 : (Parser-Result A T) (p1 tks ncur nmax))) ...)
>
> I also tried to annotate r1 inside the cond branch:
>
> (cond ((Ok? r1) #{r1 : (Parser-Result A T)}) ...)

Ah ha!  I think I found what you need.  Use either:

(define: [A T] (|| [p1 : (Parser A T)] [p2 : (Parser A T)]) : (Parser A T)
  <BODY>)

...or:

(: || (All [A T] ((Parser A T) (Parser A T) -> (Parser A T)))
(define ||
  (plambda: [A T] ([p1 : (Parser A T)] [p1 : (Parser A T)])
    <BODY>))

The plambda: form and the define: variant with the variables up front
both bind polymorphic type that are available in the input types,
result type, and in the body of the function.  Use those, and you
should be able to give a type to r1 in terms of A and T.  You can find
both define: and plambda: in the Typed Scheme reference:

http://docs.plt-scheme.org/ts-reference/index.html

--Carl
Posted on the users mailing list.