Re: [Caml-list] Match error with abstract types in modules

[Leo White <lpw25@cam.ac.uk>]

> Please could someone remind me what it is about types in modules which means
> that
>
>   module Foo = struct
>     type 'a foo
>   end
>
>   type _ t = A : int Foo.foo t
>            | B : float Foo.foo t
>
>   let foo A = ()
>
> is non-exhaustive pattern matching, but:
>
>   type 'a bar
>
>   type _ u = C : int bar u
>            | D : float bar u
>
>   let bar C = ()
>
> is exhaustive? Or is it actually a bug (given that foo B is a type error)?
>

If Foo is replaced with:

    module Foo : sig
      type 'a foo
    end = struct
      type 'a foo = unit
    end

so that `int foo` is equal to `float foo`, then your match isn't really
exhaustive.

In the second version, the compiler cheats. Since `bar` is definied in
the same module, it knows that the abstract `bar` type it can see is
actually the definition, not just an abstraction of a type defined
elsewhere, so it knows that `int bar` cannot equal `float bar`.

Personally, I dislike this behaviour and would prefer both cases to give
an error.

Regards,

Leo