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[Caml-list] Module equivalence across definitions
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 Date: 2001-10-17 (07:42) From: Tom Hirschowitz Subject: [Caml-list] Module equivalence across definitions
```

Hi,

I think it is more a question of datatypes than of
applicative functors, in the sense that if you replace
the module B with something like

module B(X:T) = struct
type t = int
let v = 2
end

then it works.

It's just because sum types are some kind of abstract
types, the constructors interfacing them with the outer
world. For instance :

# module A = struct type t = A end;;

# module B = struct type t = A end;;

# A.A = B.A;;
This expression has type A.t but is here used with type B.t

Do you agree?

Lauri Alanko writes:
> [Pardon for possible duplicates]
>
> Hello. Given:
>
>
> module type T = sig
>   type t
> end
>
> module A : T = struct
>   type t = A
> end
>
> module B(X:T) = struct
>   type t = B
>   let v = B
> end
>
> module type CT = sig
>   val a : B(A).t
> end
>
> module C : CT = struct
>   module Aalias = A
>   module BA = B(Aalias)
>   let a = BA.v
> end
>
>
> I get an error:
>
>
> Signature mismatch:
> Modules do not match:
>   sig
>     module Aalias : sig type t = A.t end
>     module BA : sig type t = B(Aalias).t = B val v : t end
>     val a : BA.t
>   end
> is not included in
>   CT
> Values do not match: val a : BA.t is not included in val a : B(A).t
>
>
> But changing "module BA = B(Aalias)" to "module BA = B(A)" makes everything
> work.
>
> What gives? I'm used to seeing structural equivalence being used everywhere
> in O'Caml's type system, but here simply renaming a module seems to give it
> a distinct new identity. Is this a bug or a feature?
>
>
> Lauri Alanko
> la@iki.fi
> -------------------
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