added List.counts, related to List.count

src/core/CCList.ml

@@ -382,6 +382,19 @@ let count f l =
   count (fun x -> x mod 2 = 0) [2; 6; 9; 4] = 3
 *)
 
+let counts p l =
+  fold_left (fun (ok, ko) x ->
+      if p x then (ok + 1, ko)
+      else (ok, ko + 1)
+    ) (0, 0) l
+
+(*$T
+  counts (fun x -> x mod 2 = 0) [] = (0, 0)
+  counts (fun x -> x mod 2 = 0) [0; 0; 2; 4] = (4, 0)
+  counts (fun x -> x mod 2 = 0) [1; 3; 5; 7] = (0, 4)
+  counts (fun x -> x mod 2 = 0) [2; 6; 9; 4] = (3, 1)
+*)
+
 let product f l1 l2 =
   flat_map (fun x -> map (fun y -> f x y) l2) l1
 

src/core/CCList.mli

@@ -86,9 +86,14 @@ val fold_flat_map : ('acc -> 'a -> 'acc * 'b list) -> 'acc -> 'a list -> 'acc *
     @since 0.14 *)
 
 val count : ('a -> bool) -> 'a list -> int
-(** [count f l] counts how much elements of [l] comply with the function [f].
+(** [count p l] counts how many elements of [l] satisfy predicate [p].
     @since 1.5 *)
 
+val counts : ('a -> bool) -> 'a list -> int * int
+(** [let ok_count, ko_count = counts p l in ...]
+    counts how many elements of [l] satisfy (resp. violate) predicate [p].
+    @since NEXT_RELEASE *)
+
 val init : int -> (int -> 'a) -> 'a t
 (** [init len f] is [f 0; f 1; ...; f (len-1)].
     @raise Invalid_argument if len < 0.

src/core/CCListLabels.mli

@@ -86,6 +86,9 @@ val fold_flat_map : f:('acc -> 'a -> 'acc * 'b list) -> init:'acc -> 'a list ->
 val count : f:('a -> bool) -> 'a list -> int
 (** @since 2.2 *)
 
+val counts : f:('a -> bool) -> 'a list -> int * int
+(** @since NEXT_RELEASE *)
+
 val init : int -> f:(int -> 'a) -> 'a t
 (** [init len ~f] is [f 0; f 1; ...; f (len-1)].
     @raise Invalid_argument if len < 0.