add seq_list and seq_list_map

src/Sequence.ml

@@ -181,6 +181,19 @@ let flat_map f seq k = seq (fun x -> f x k)
 let flat_map_l f seq k =
   seq (fun x -> List.iter k (f x))
 
+let rec seq_list_map f l k = match l with
+ | [] -> k []
+ | x :: tail ->
+   f x (fun x' -> seq_list_map f tail (fun tail' -> k (x'::tail')))
+
+let seq_list l = seq_list_map (fun x->x) l
+
+(*$= & ~printer:Q.Print.(list @@ list int)
+  [[1;2];[1;3]] (seq_list [singleton 1; doubleton 2 3] |> to_list)
+  [] (seq_list [singleton 1; empty; doubleton 2 3] |> to_list)
+  [[1;2;4];[1;3;4]] (seq_list [singleton 1; doubleton 2 3; singleton 4] |> to_list)
+*)
+
 let filter_map f seq k =
   seq (fun x -> match f x with
       | None -> ()

src/Sequence.mli

@@ -200,6 +200,16 @@ val flat_map_l : ('a -> 'b list) -> 'a t -> 'b t
 (** Convenience function combining {!flat_map} and {!of_list}
     @since 0.9 *)
 
+val seq_list : 'a t list -> 'a list t
+(** [seq_list l] returns all the ways to pick one element in each sub-sequence
+    in [l]. Assumes the sub-sequences can be iterated on several times.
+    @since NEXT_RELEASE *)
+
+val seq_list_map : ('a -> 'b t) -> 'a list -> 'b list t
+(** [seq_list_map f l] maps [f] over every element of [l],
+    then calls {!seq_list}
+    @since NEXT_RELEASE *)
+
 val filter_map : ('a -> 'b option) -> 'a t -> 'b t
 (** Map and only keep non-[None] elements
     Formerly [fmap]