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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} *)
(** Raised when an combinator expects several lists of the same size
but finds that they are not. Exceptions must be raised inside
the monad, so two-list combinators take an extra argument to
raise the exception. *)
exception SizeMismatch
(** [fold_left2 r f s l1 l2] behaves like {!fold_left} but acts
simultaneously on two lists. Returns [r SizeMismatch] if both lists
do not have the same length. *)
val fold_left2 : (exn->'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
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