add CCLazy_list.take

src/iter/CCLazy_list.ml

@@ -50,8 +50,17 @@ let filter ~f l =
 
 (*$=
   [2;4;6] (of_list [1;2;3;4;5;6;7] |> filter ~f:(fun x -> x mod 2=0) |> to_list)
+  [2;4;6] (of_gen Gen.(1 -- max_int) |> filter ~f:(fun x -> x mod 2=0) |> take 3 |> to_list)
 *)
 
+let rec take n l =
+  lazy (
+    match l with
+      | _ when n=0 -> Nil
+      | lazy Nil -> Nil
+      | lazy (Cons (x,tl)) -> Cons (x, take (n-1) tl)
+  )
+
 let rec append a b =
   lazy (
     match a with

src/iter/CCLazy_list.mli

@@ -32,7 +32,11 @@ val map : f:('a -> 'b) -> 'a t -> 'b t
 (** Lazy map *)
 
 val filter : f:('a -> bool) -> 'a t -> 'a t
-(** Filter values
+(** Filter values.
+    @since NEXT_RELEASE *)
+
+val take : int -> 'a t -> 'a t
+(** Take at most n values.
     @since NEXT_RELEASE *)
 
 val append : 'a t -> 'a t -> 'a t