Add a two-list monadic fold_left iterator.

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/monad.mli b/lib/monad.mli
index 768fb739d3..c72e584db1 100644
--- a/lib/monad.mli
+++ b/lib/monad.mli
@@ -54,6 +54,21 @@ module type ListS = sig
       operator calls its second argument in a tail position. *)
   val fold_left : ('a -> 'b -> 'a t) -> 'a -> 'b list -> 'a 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