Re: [Caml-list] References and polymorphism

[Romain Bardou <bardou@lsv.ens-cachan.fr>]

Le 10/01/2012 16:29, Dario Teixeira a écrit :
> Hi,
>
> Consider functions foobar1 and foobar2:
>
>
> type 'a t = {id: int; x: 'a}
>
> let foobar1: 'a ->  'a t =
>          fun x ->  {id = 0; x}
>
> let foobar2: 'a ->  'a t =
>          let ctr = ref 0 in
>          fun x ->  incr ctr; {id = !ctr; x}
>
>
>
> I would expect them to have the same type, because foobar2's
> use of a reference cell is kept private.  However, they don't.
> In fact, foobar2 is not really polymorphic:
>
>
> type 'a t = { id : int; x : 'a; }
> val foobar1 : 'a ->  'a t
> val foobar2 : '_a ->  '_a t
>
>
> It's easy to get around this issue by putting the reference cell
> outside of foobar2.  Function foobar3 does just this, and behaves
> as expected:
>
>
> let next =
>          let ctr = ref 0 in
>          fun () ->  incr ctr; !ctr
>
> let foobar3: 'a ->  'a t =
>          fun x ->  {id = next (); x}
>
>
>
> Could someone point me to a good explanation of what's going on?
> (I have the feeling I've read about this restriction before.)
>
> Best regards,
> Dario Teixeira
>
>
>

Hello,

This is, basically, the value restriction: you cannot let-generalize 
something which is not *syntactically* a value. Function foobar1 is 
syntactically a function, and a function is a value. Function foobar2 is 
not: it starts with a let-binding. It computes something before 
returning a function. It cannot be generalized.

Cheers,

-- 
Romain Bardou