decreasing int range operator

sequence.ml

@@ -541,6 +541,10 @@ let int_range ~start ~stop =
   fun k ->
     for i = start to stop do k i done
 
+let int_range_dec ~start ~stop =
+  fun k ->
+    for i = start downto stop do k i done
+
 (** Convert the given set to a sequence. The set module must be provided. *)
 let of_set (type s) (type v) m set =
   let module S = (val m : Set.S with type t = s and type elt = v) in
@@ -681,6 +685,8 @@ end
 module Infix = struct
   let (--) i j = int_range ~start:i ~stop:j
 
+  let (--^) i j = int_range_dec ~start:i ~stop:j
+
   let (|>) x f = f x
 
   let (@@) a b = append a b
@@ -688,6 +694,10 @@ module Infix = struct
   let (>>=) x f = flatMap f x
 end
 
+let (--) = Infix.(--)
+
+let (--^) = Infix.(--^)
+
 (** {2 Pretty printing of sequences} *)
 
 (** Pretty print a sequence of ['a], using the given pretty printer

sequence.mli

@@ -318,7 +318,12 @@ val to_buffer : char t -> Buffer.t -> unit
   (** Copy content of the sequence into the buffer *)
 
 val int_range : start:int -> stop:int -> int t
-  (** Iterator on integers in [start...stop] by steps 1 *)
+  (** Iterator on integers in [start...stop] by steps 1. Also see
+      {!(--)} for an infix version. *)
+
+val int_range_dec : start:int -> stop:int -> int t
+  (** Iterator on decreasing integers in [stop...start] by steps -1.
+      See {!(--^)} for an infix version *)
 
 val of_set : (module Set.S with type elt = 'a and type t = 'b) -> 'b -> 'a t
   (** Convert the given set to a sequence. The set module must be provided. *)
@@ -414,12 +419,23 @@ end
 
 module Infix : sig
   val (--) : int -> int -> int t
+    (** [a -- b] is the range of integers from [a] to [b], both included,
+        in increasing order. It will therefore be empty if [a > b]. *)
+
+  val (--^) : int -> int -> int t
+    (** [a --^ b] is the range of integers from [b] to [a], both included,
+        in decreasing order (starts from [a]).
+        It will therefore be empty if [a < b]. *)
 
   val (|>) : 'a -> ('a -> 'b) -> 'b
 
   val (@@) : 'a t -> 'a t -> 'a t
 end
 
+val (--) : int -> int -> int t
+
+val (--^) : int -> int -> int t
+
 (** {2 Pretty printing of sequences} *)
 
 val pp_seq : ?sep:string -> (Format.formatter -> 'a -> unit) ->

tests/test_sequence.ml

@@ -182,6 +182,13 @@ let test_buff () =
   OUnit.assert_equal "DLROW OLLEH" (Buffer.contents b);
   ()
 
+let test_int_range () =
+  OUnit.assert_equal ~printer:pp_ilist [1;2;3;4] S.(to_list (1--4));
+  OUnit.assert_equal ~printer:pp_ilist [10;9;8;7;6] S.(to_list (10 --^ 6));
+  OUnit.assert_equal ~printer:pp_ilist [] S.(to_list (10--4));
+  OUnit.assert_equal ~printer:pp_ilist [] S.(to_list (10 --^ 60));
+  ()
+
 let suite =
   "test_sequence" >:::
     [ "test_empty" >:: test_empty;
@@ -208,4 +215,5 @@ let suite =
       "test_rev" >:: test_rev;
       "test_unfoldr" >:: test_unfoldr;
       "test_hashtbl" >:: test_hashtbl;
+      "test_int_range" >:: test_int_range;
     ]