Keiko Nakata <keiko@kurims.kyoto-u.ac.jp> writes:

   Why I cannot have *any types* in signatures of functor arguments?
   Concretely, the following F is not typable.
   
   module F(X:sig type t = [`A of _ ] val f : t -> int end) = 
     struct
       let f = function `A x as a -> X.f a | `B -> 0
     end
   

The problem is rather that your type abbreviation is illegal:

  # type t = [`A of _ ];;
  A type variable is unbound in this type declaration.
  In case `A of 'a the variable 'a is unbound

Use 

  module F(X:sig val f : [`A of _ ] -> int end) = 
    struct
      let f = function `A x as a -> X.f a | `B -> 0
  end
   
or a type abbreviation that works. Either 

  type 'a t = [`A of _ ] as 'a;;

or 

  type 'a t = [`A of _ as 'a ];;

Where the latter is the same as the already proposed

  type 'a t = [ `A of 'a ];;

Bye,

Hendrik