add CCKList.{zip, unzip}

src/iter/CCKList.ml

@@ -303,6 +303,26 @@ let rec merge cmp l1 l2 () = match l1(), l2() with
       then `Cons (x1, merge cmp l1' l2)
       else `Cons (x2, merge cmp l1 l2')
 
+let rec zip a b () = match a(), b() with
+  | `Nil, _
+  | _, `Nil -> `Nil
+  | `Cons (x, a'), `Cons (y, b') -> `Cons ((x,y), zip a' b')
+
+let unzip l =
+  let rec first l () = match l() with
+    | `Nil -> `Nil
+    | `Cons ((x,_), tl) -> `Cons (x, first tl)
+  and second l () = match l() with
+    | `Nil -> `Nil
+    | `Cons ((_, y), tl) -> `Cons (y, second tl)
+  in
+  first l, second l
+
+(*$Q
+  Q.(list (pair int int)) (fun l -> \
+    let l = CCKList.of_list l in let a, b = unzip l in equal (=) l (zip a b))
+*)
+
 (** {2 Implementations} *)
 
 let return x () = `Cons (x, nil)

src/iter/CCKList.mli

@@ -171,6 +171,16 @@ val exists2 : ('a -> 'b -> bool) -> 'a t -> 'b t -> bool
 val merge : 'a ord -> 'a t -> 'a t -> 'a t
 (** Merge two sorted iterators into a sorted iterator *)
 
+val zip : 'a t -> 'b t -> ('a * 'b) t
+(** Combine elements pairwise. Stops as soon as one of the lists stops.
+    @since NEXT_RELEASE *)
+
+val unzip : ('a * 'b) t -> 'a t * 'b t
+(** Splits each tuple in the list
+    @since NEXT_RELEASE *)
+
+(** {2 Misc} *)
+
 val sort : ?cmp:'a ord -> 'a t -> 'a t
 (** Eager sort. Requires the iterator to be finite. O(n ln(n)) time
     and space.