Re: [Caml-list] Equality between abstract type definitions

[Jonathan Protzenko <jonathan.protzenko@gmail.com>]

On 10/25/2013 02:35 PM, Roberto Di Cosmo wrote:
> Thanks Ivan,
>        the potential confusion between type annotations
> in the code and type specifications in module interfaces
> is a very good point.
>
> Writing
>
>  let f : 'a -> 'a = fun x -> x+1 
>
> will just boil down to defining 
>
>  val f : int -> int = <fun>
>
> On the other side, declaring 
>
>  val f : 'a -> 'a 
>
> in a module signature actually *requires* the implementation
> to be at least as generic as 'a -> 'a, so a definition
> let f = fun x -> x+1 in the body will not work.
You should also remember that the top-level, which is what most
beginners use, also favors the confusion.

# let f x = x;;
val f : 'a -> 'a = <fun>
# let f : 'a -> 'a = fun x -> x + 1;;
val f : int -> int = <fun>

What OCaml is actually telling you is: "this function has type 'a -> 'a,
but if you write 'a -> 'a yourself, I'm going to give it a totally
different meaning".

~ jonathan