length and is_empty functions for Sequence

sequence.ml

@@ -120,6 +120,17 @@ let exists p seq =
     false
   with Exit -> true
 
+(** How long is the sequence? *)
+let length seq =
+  let r = ref 0 in
+  seq (fun _ -> incr r);
+  !r
+
+(** Is the sequence empty? *)
+let is_empty seq =
+  try seq (fun _ -> raise Exit); true
+  with Exit -> false
+
 module List =
   struct
     let of_seq seq = List.rev (fold (fun y x -> x::y) [] seq)

sequence.mli

@@ -66,6 +66,12 @@ val for_all : ('a -> bool) -> 'a t -> bool
 val exists : ('a -> bool) -> 'a t -> bool
   (** Exists there some element satisfying the predicate? *)
 
+val length : 'a t -> int
+  (** How long is the sequence? *)
+
+val is_empty : 'a t -> bool
+  (** Is the sequence empty? *)
+
 (** {2 Transform a sequence} *)
 
 val filter : ('a -> bool) -> 'a t -> 'a t