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[Caml-list] strange effect of type annotation
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Date: -- (:)
From: fis@w...
Subject: [Caml-list] strange effect of type annotation


hi all,

I was just stumbling over something that is surely not a bug but a
slightly counterintuitive effect in the ocaml type system and thought
somebody might have an instructive comment.  (I know I should go read
the papers on ocaml typing, though. :-)

>   let good : (int * float) = let x                  = fst in (x (0,1), x (0., 1.));;
>   let bad  : (int * float) = let x: ('a * 'a) -> 'a = fst in (x (0,1), x (0., 1.));;

The type checker sais:

>   let bad  : (int * float) = let x: ('a * 'a) -> 'a = fst in (x (0,1), x (0., 1.));;
>                                                                          ^^^^^^^^
> This expression has type float * float but is here used with type int * int

I know that the type of fst is more general than this, but it looks
like it would have done the job for me here.  Why doesn't it?  And is
there a type annotation for x that does?

cheers,
matthias

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