Re: [Caml-list] Strange behavior from type inference after functor application

[Gabriel Scherer <gabriel.scherer@gmail.com>]

The problem with
  module C = Combi (S) (TypeWithConf (struct let v = 1 end))
is that the resulting type has no "name": (struct let v = 1 end) can
not be part of a type path.

On the contrary, TypeWithConf(Conf) has a type t with name
TypeWithConf(Conf).t, so C.t can be equal to S.t *
TypeWithConf(Conf).t. There would be no such type as S.t *
TypeWIthConf(struct let v = 1 end).t.

The other fix, forcing TypeWithConf to return a signature with (t =
unit), makes the "naming problem" go away: whatever functor you pass
as a parameter (either a path such as "Conf" or a struct exrpession),
the resulting type is equal to "unit", you don't have to try to name
it from the way it was constructed.

On Thu, May 31, 2012 at 10:05 PM,  <rixed@happyleptic.org> wrote:
> Given these definitions:
>
> -- v1 --
>
> module type TYPE = sig type t end
>
> module Combi (S1 : TYPE) (S2 : TYPE) :
>    TYPE with type t = S1.t * S2.t =
> struct
>    type t = S1.t * S2.t
> end
>
> module TypeWithConf (Conf : sig val v : int end) :
>        TYPE =
> struct
>    type t = unit
> end
>
> module S = struct type t = unit end
>
> module C = Combi (S) (TypeWithConf (struct let v = 1 end))
>
> let f (x : C.t) = fst x
>
> -----
>
> Here, the compiler fails to infer that C.t is indeed the product of two types
> (as stated in the Combi signature), and complains that:
>
> Error: This expression has type C.t but an expression was expected of type 'a * 'b
> (pointing at the argument of fst)
>
> There is two ways to satisfies it:
>
> - either, state that the type of TypeWithConf is 'TYPE with type t = unit'
>  instead of merely 'TYPE'
>
> - or, more surprisingly, to define the Conf structure as in:
>
> -- v2 --
> (* ... *)
>
> module Conf = struct let v = 1 end
> module C = Combi (S) (TypeWithConf (Conf))
>
> (* ... *)
> -----
>
> Can someone help me find an explanation to this behavior?
>
>
>
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