Re: [Caml-list] Type inference and marshalling

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2011/07/05, at 22:59, malc wrote:

> Perhaps someone could explain why following behaves the way it does:
> 
> ~$ ocaml
>        Objective Caml version 3.11.2
> 
> # let f ic = let i = input_value ic in let j = i + 1 in LargeFile.seek_in ic i;;
> Warning Y: unused variable j.
> val f : in_channel -> unit = <fun>

The return type of input_value being 'a, which gets generalized by the
relaxed value restriction, i gets the polymorphic type "forall 'a. 'a".
So you can use it both as an int and an int64.
==> input_value is an unsafe function, you should always write a type
	annotation on its return type.

> # let f ic = let i = input_value ic in let j = i + 1 in LargeFile.seek_in ic j;;
> Error: This expression has type int but an expression was expected of type
>         int64

j being the result of an integer addition, it has type int, and cannot be used
as int64.

Jacques