add CCList.{head_opt,last_opt}

src/core/CCList.ml

@@ -569,6 +569,24 @@ let last n l =
   let len = List.length l in
   if len < n then l else drop (len-n) l
 
+let head_opt = function
+  | [] -> None
+  | x::_ -> Some x
+
+let rec last_opt = function
+  | [] -> None
+  | [x] -> Some x
+  | _ :: tail -> last_opt tail
+
+(*$= & ~printer:Q.Print.(option int)
+  (Some 1) (head_opt [1;2;3])
+  (Some 1) (head_opt [1])
+  None (head_opt [])
+  (Some 3) (last_opt [1;2;3])
+  (Some 1) (last_opt [1])
+  None (last_opt [])
+*)
+
 let rec find_pred p l = match l with
   | [] -> None
   | x :: _ when p x -> Some x

src/core/CCList.mli

@@ -134,6 +134,14 @@ val last : int -> 'a t -> 'a t
 (** [last n l] takes the last [n] elements of [l] (or less if
     [l] doesn't have that many elements *)
 
+val head_opt : 'a t -> 'a option
+(** First element.
+    @since NEXT_RELEASE *)
+
+val last_opt : 'a t -> 'a option
+(** Last element.
+    @since NEXT_RELEASE *)
+
 val find_pred : ('a -> bool) -> 'a t -> 'a option
 (** [find_pred p l] finds the first element of [l] that satisfies [p],
     or returns [None] if no element satisfies [p]