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+(***********************************************************************)
+(*  v      *   The Coq Proof Assistant  /  The Coq Development Team    *)
+(* <O___,, *        INRIA-Rocquencourt  &  LRI-CNRS-Orsay              *)
+(*   \VV/  *************************************************************)
+(*    //   *      This file is distributed under the terms of the      *)
+(*         *       GNU Lesser General Public License Version 2.1       *)
+(***********************************************************************)
+
+
+(** Combinators on monadic computations. *)
+
+
+(** A definition of monads, each of the combinators is used in the
+    [Make] functor. *)
+module type Def = sig
+
+  type +'a t
+  val return : 'a -> 'a t
+  val (>>=) : 'a t -> ('a -> 'b t) -> 'b t
+  val (>>) : unit t -> 'a t -> 'a t
+  val map : ('a -> 'b) -> 'a t -> 'b t
+
+(** The monadic laws must hold:
+    - [(x>>=f)>>=g] = [x>>=fun x' -> (f x'>>=g)]
+    - [return a >>= f] = [f a]
+    - [x>>=return] = [x]
+
+    As well as the following identities:
+    - [x >> y] = [x >>= fun () -> y]
+    - [map f x] = [x >>= fun x' -> f x'] *)
+
+end
+
+
+(** List combinators *)
+module type ListS = sig
+
+  type 'a t
+
+  (** [List.map f l] maps [f] on the elements of [l] in left to right
+      order. *)
+  val map : ('a -> 'b t) -> 'a list -> 'b list t
+
+  (** [List.map f l] maps [f] on the elements of [l] in right to left
+      order. *)
+  val map_right : ('a -> 'b t) -> 'a list -> 'b list t
+
+  (** Like the regular [List.fold_right]. The monadic effects are
+      threaded right to left.
+
+      Note: many monads behave poorly with right-to-left order. For
+      instance a failure monad would still have to traverse the
+      whole list in order to fail and failure needs to be propagated
+      through the rest of the list in binds which are now
+      spurious. It is also the worst case for substitution monads
+      (aka free monads), exposing the quadratic behaviour.*)
+  val fold_right : ('a -> 'b -> 'b t) -> 'a list -> 'b -> 'b t
+
+  (** Like the regular [List.fold_left]. The monadic effects are
+      threaded left to right. It is tail-recursive if the [(>>=)]
+      operator calls its second argument in a tail position. *)
+  val fold_left : ('a -> 'b -> 'a t) -> 'a -> 'b list -> 'a t
+
+  (** Like the regular [List.iter]. The monadic effects are threaded
+      left to right. It is tail-recurisve if the [>>] operator calls
+      its second argument in a tail position. *)
+  val iter : ('a -> unit t) -> 'a list -> unit t
+
+
+  (** {6 Two-list iterators} *)
+
+  (** [fold_left2 r f s l1 l2] behaves like {!fold_left} but acts
+      simultaneously on two lists. Runs [r] (presumably an
+      exception-raising computation) if both lists do not have the
+      same length. *)
+  val fold_left2 : 'a t ->
+    ('a -> 'b -> 'c -> 'a t) -> 'a -> 'b list -> 'c list -> 'a t
+
+end
+
+module type S = sig
+
+  include Def
+
+  module List : ListS with type 'a t := 'a t
+
+end
+
+(** Expands the monadic definition to extra combinators. *)
+module Make (M:Def) : S with type +'a t = 'a M.t