On 29 June 2014 15:49, Grégoire Henry <gregoire.henry@ocamlpro.com> wrote:

> Your first example is a particular case where we could assume that
> 'W.a' and 'W.b' are distinct. They are 'fully abstract types', they
> have no inhabitants. They were introduced in current typing context
> (and not imported from the signature of another module) and we known
> they are distinct.
>

This has bitten me before as well. It seems to me that whether or not a
type is abstract and whether or not it has any values should be essentially
independent characteristics, but it seems impossible to state in a
signature that a type has no constructors. So there's this property of
types that you can't preserve across module boundaries.