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Subject: [Caml-list] coping with circular dependencies

In a simplified form I've the following setup:

module a defines a type with some functions:

(* file a.mli *)
  type t
  ...
  val of_string: string -> t

module b is a parser, defining function parse, that can produce values of types A.t from, for example, strings:

(* file b.mli *)
  type t = A.t
  ...
  val parse_a : string -> t
  
When I try to compile this simple example, compiler refuses to accept it, reasoning that :

Error: The implementation a.ml does not match the interface a.cmi:
       Values do not match:
         val of_string : string -> B.t
       is not included in
         val of_string : string -> t


Are there're any solutions,  satisfying the following constraints:
1) I want A.t to be an abstract type.
2) IRL a.mli and b.mli are rather large files (b is produces from mll and mly files)