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Type inference question
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Date: 2005-04-26 (12:00)
From: Jean-Christophe Filliatre <Jean-Christophe.Filliatre@l...>
Subject: Re: [Caml-list] Type inference question

Julien Verlaguet writes:
 > I have the following function definition :
 > let myfun param=
 >    let res=Marshal.from_channel stdin [] in
 >      if res=param then
 >        res
 >      else res
 > I was expecting : myfun : 'a -> 'a
 > I got instead : myfun : 'a -> 'b
 > Is it normal ?

Yes.  "Marshal.from_channel stdin  []" has  type 'a  and this  type is
generalized  in the  let/in  construct, giving  res  the type  "forall
'a. 'a". In the test "res = param", the type of res is instanciated on
the type of param, but this does not affect the type of the result.

It would be better if  Marshal.from_channel would be given a type that
cannot  be generalized ('_a)  but I  don't think  that the  ocaml type
system can do this.

Anyway, it is  always a good idea to use a  type constraint when using
marshalling functions, as in

	let (x : tau) = Marshal.from_channel ...

Hope this helps,