[Caml-list] Re: Caml-list References to polymorphic variants

[Remi Vanicat <remi.vanicat@laposte.net>]

nickgrey@softhome.net writes:

> Hi,
>
> I want to be able to store an arbitrary polymorphic variant in a
> reference:
>
> let (x : ([> ] option) ref ) = ref None ;;
>
> (Not my actual code, I've boiled it down to a minimal example.)
>
> This won't compile because it "contains a type variable which cannot
> be generalized".  I understand this error message in other contexts.
> If my code were
>
> let x = ref None
>
> then I could break type safety by putting (Some 1) into the reference,
> and later reading the reference with type (string option) thus casting
> a string to an int.
>
> But as I'm dealing with a polymorphic variant, surely this should not
> apply. My understanding of polymorphic variants is that you do not
> need to specify at the time of declaration what types it might
> contain.

you can break type safety with your example :

`Foo 10
and
`Foo "string"
are both some polymorphic variant, and if your generalized type would
be ok, nothing could forbid you to "convert" one to another, having
the same effect that the (Some 1) and (string option) example.
 


-- 
Rémi Vanicat

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