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From: Oleg Trott <oleg_trott@columbia.edu>
To: skaller@ozemail.com.au, caml-list@inria.fr
Subject: Re: [Caml-list] tpyping question
Date: Sun, 10 Aug 2003 22:51:56 -0400
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On Sunday 10 August 2003 09:13 pm, skaller wrote:
> type x = [`A | `B of int | `V of float];;
> let g (a:x) = a;;
> let f a = match a with
>
> | `A -> g a
>
> (* | `B _ *) | _ -> g a;;
>
> Compiles fine, but gives an error if the comments are
> removed.
>
> File "xx.ml", line 4, characters 10-11:
> This expression has type [< `A | `B of 'a ] but is here used with type
>   x = [ `A | `B of int | `V of float ]
> The first variant type does not allow tag(s) `V
>
> I can fix this with a coercion/annotation:
> | (`A:x) -> g a
>
> but that's a bit messy. Of course an annotation on the
> argument doesn't work (since such annotations are only
> consulted *after* the body type is infered).
>
> Is there a better way to do this?

This is indeed very odd, because the equivalent

let f a = match a with `A -> g a | `B _ -> g a | _ -> g a

works fine.

-- 
Oleg Trott <oleg_trott@columbia.edu>

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