add filter_count

src/Sequence.ml

@@ -207,6 +207,17 @@ let filter_mapi f seq k =
       | None -> ()
       | Some y -> k y)
 
+let filter_count f seq =
+  let i = ref 0 in
+  seq (fun x -> if f x then incr i);
+  !i
+
+(*$Q
+  Q.(list int) (fun l -> \
+    let seq = of_list l and f x = x mod 2 = 0 in \
+    filter_count f seq = (filter f seq |> length))
+*)
+
 let intersperse elem seq k =
   let first = ref true in
   seq (fun x -> (if !first then first := false else k elem); k x)

src/Sequence.mli

@@ -224,6 +224,10 @@ val filter_mapi : (int -> 'a -> 'b option) -> 'a t -> 'b t
 (** Map with indices, and only keep non-[None] elements
     @since 0.11 *)
 
+val filter_count : ('a -> bool) -> 'a t -> int
+(** Count how many elements satisfy the given predicate
+    @since NEXT_RELEASE *)
+
 val intersperse : 'a -> 'a t -> 'a t
 (** Insert the single element between every element of the sequence *)
 

src/SequenceLabels.mli

@@ -197,6 +197,10 @@ val seq_list_map : f:('a -> 'b t) -> 'a list -> 'b list t
     then calls {!seq_list}
     @since 0.11 *)
 
+val filter_count : f:('a -> bool) -> 'a t -> int
+(** Count how many elements satisfy the given predicate
+    @since NEXT_RELEASE *)
+
 val intersperse : x:'a -> 'a t -> 'a t
 (** Insert the single element between every element of the sequence *)