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Date: Sat, 6 Aug 2011 19:03:03 +0200
From: Guillaume Yziquel <guillaume.yziquel@citycable.ch>
To: Fabrice Le Fessant <Fabrice.Le_fessant@inria.fr>
Cc: caml-list@inria.fr, rixed@happyleptic.org
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References: <20110806125021.GB10154@ombreroze.happyleptic.org>
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Subject: Re: [Caml-list] typer strangeness (3.12.0)

Le Saturday 06 Aug 2011 à 14:58:49 (+0200), Fabrice Le Fessant a écrit :
> The type constraint that you specified does not constraint the
> polymorphism of the type. To declare a polymorphic constraint, you must
> use (with OCaml >= 3.12.0) :
> 
> let pipe : 'a 'b 'c. ('a, 'b) parzer -> ('c, 'a) parzer -> ('c, 'b) parzer =
>   fun p1 p2 ->
>     let p1_rem = ref [] in
>     fun bs -> match p1 (!p1_rem @ bs) with
>         | Fail -> Fail
>         | Wait -> Wait
>         | Res (res, rem) ->
>             p1_rem := rem ;
>             (match p2 [res] with
>                 | Res (res', rem') ->
>                     if rem' <> [] then Printf.printf "WRN: second end of
> a pipe did not consume eveything !\n" ;
>                     Res (res', !p1_rem)
>                 | x -> x)
> 
> In which case you get the following error :
> Error: This definition has type
>          'a 'b. ('b, 'b) parzer -> ('a, 'b) parzer -> ('a, 'b) parzer
>        which is less general than
>          'c 'd 'e. ('e, 'd) parzer -> ('c, 'e) parzer -> ('c, 'd) parzer
> 
> Fabrice

You can achieve a similar goal with stuff like:

# let f (type a) (type b) (x : a) = (x : b);;
Error: This expression has type a but an expression was expected of type b

The benefit of which is that you do not have f explicitely quantified as
opposed to the "let f : 'a 'b. ..." alternative. Feels more ocaml-ish to
me.

-- 
     Guillaume Yziquel