add CCList.hd_tl

src/core/CCList.ml

@@ -454,6 +454,15 @@ let rec drop n l = match l with
   | _ when n=0 -> l
   | _::l' -> drop (n-1) l'
 
+let hd_tl = function
+  | [] -> failwith "hd_tl"
+  | x :: l -> x, l
+
+(*$T
+  try ignore (hd_tl []); false with Failure _ -> true
+  hd_tl [1;2;3] = (1, [2;3])
+*)
+
 let take_drop n l = take n l, drop n l
 
 let split = take_drop

src/core/CCList.mli

@@ -106,6 +106,11 @@ val take : int -> 'a t -> 'a t
 val drop : int -> 'a t -> 'a t
 (** Drop the [n] first elements, keep the rest *)
 
+val hd_tl : 'a t -> 'a * 'a t
+(** [hd_tl (x :: l)] returns [hd, l].
+    @raise Failure if the list is empty
+    @since NEXT_RELEASE *)
+
 val take_drop : int -> 'a t -> 'a t * 'a t
 (** [take_drop n l] returns [l1, l2] such that [l1 @ l2 = l] and
     [length l1 = min (length l) n] *)