[plt-scheme] Typed-Scheme and car

Would this

> #lang typed-scheme
>
> (define x (list (cons 'one 1) (cons 'two 2) (cons 'three 3)))
>
> (define y ({inst map Symbol (Pair Symbol Integer)} car x))

make you happier? Local inference can't infer at which type map is  
used here. That's why you need to declare it.

-- Matthias



On Apr 5, 2009, at 1:57 PM, Paulo J. Matos wrote:

> Hi all,
>
> Just trying this example I got two unexpected error:
> #lang typed-scheme
>
> (: x (Listof (Pair Symbol Integer)))
> (define x '((one . 1) (two . 2) (three . 3)))
>
> (: y (Listof Symbol))
> (define y (map car x))
>
> . typecheck: Expected (Listof (Pair Symbol Integer)), but got (List
> Any Any Any) in: (quote ((one . 1) (two . 2) (three . 3)))
> . typecheck: Polymorphic function map could not be applied to  
> arguments:
> Domain: (a b ... b -> c) (Listof a) (Listof b) ... b
> Arguments: (All (a b) (case-lambda ((Pair a b) -> a) ((Listof a) ->
> a))) (Listof (Pair Symbol Integer))
> Result type: (Listof c)
> Expected result: (Listof Symbol)
>  in: (map car x)
>
> if I replace x define with:
> (define x (list (cons 'one 1) (cons 'two 2) (cons 'three 3)))
>
> the first error disappears but the second remains. The second error
> goes away with:
> (define y (map (lambda: ((p : (Pair Symbol Integer))) (car p)) x))
>
> But this is MUCH more annoying to write and the program gets much more
> verbose and I have to say that after rewriting a couple of 'big'
> modules in typed-scheme, the scheme simplicity and elegance starts to
> fade. Making typed-scheme support things such as a (map car x) is for
> me a big requirement. Currently my code is full of lambda: in maps,
> filters, etc for no good reason. I hope that there is work to improve
> this in the future. I guess it is possible to make the type inference
> mechanism to detect this.
>
> Still, cheers and kudos to Sam and other for the current state of
> typed-scheme. It is a very nice language to program in even though its
> current state still has some quirks.
>
>
> -- 
> Paulo Jorge Matos - pocmatos at gmail.com
> Webpage: http://www.personal.soton.ac.uk/pocm
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