add Sequence.filter_mapi

src/Sequence.ml

@@ -197,8 +197,16 @@ let seq_list l = seq_list_map (fun x->x) l
 let filter_map f seq k =
   seq (fun x -> match f x with
       | None -> ()
-      | Some y -> k y
-    )
+      | Some y -> k y)
+
+let filter_mapi f seq k =
+  let i = ref 0 in
+  seq (fun x ->
+    let j = !i in
+    incr i;
+    match f j x with
+      | None -> ()
+      | Some y -> k y)
 
 let intersperse elem seq k =
   let first = ref true in

src/Sequence.mli

@@ -215,6 +215,10 @@ val filter_map : ('a -> 'b option) -> 'a t -> 'b t
     Formerly [fmap]
     @since 0.5 *)
 
+val filter_mapi : (int -> 'a -> 'b option) -> 'a t -> 'b t
+(** Map with indices, and only keep non-[None] elements
+    @since NEXT_RELEASE *)
+
 val intersperse : 'a -> 'a t -> 'a t
 (** Insert the single element between every element of the sequence *)
 

src/sequenceLabels.mli

@@ -176,6 +176,10 @@ val flat_map_l : f:('a -> 'b list) -> 'a t -> 'b t
 val filter_map : f:('a -> 'b option) -> 'a t -> 'b t
 (** Alias to {!fmap} with a more explicit name *)
 
+val filter_mapi : f:(int -> 'a -> 'b option) -> 'a t -> 'b t
+(** Map with indices, and only keep non-[None] elements
+    @since NEXT_RELEASE *)
+
 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.