make count lazy

src/Sequence.ml

@@ -446,14 +446,16 @@ let count (type k) ?(hash=Hashtbl.hash) ?(eq=(=)) seq =
       let hash = hash
     end) in
   (* compute group table *)
+  let tbl = lazy (
     let tbl = Tbl.create 32 in
     seq
       (fun x ->
          let n = try Tbl.find tbl x with Not_found -> 0 in
-       Tbl.replace tbl x (n+1)
-    );
+         Tbl.replace tbl x (n+1));
+    tbl
+  ) in
   fun yield ->
-    Tbl.iter (fun x n -> yield (x,n)) tbl
+    Tbl.iter (fun x n -> yield (x,n)) (Lazy.force tbl)
 
 (*$R
   [1;2;3;3;2;2;3;4]

src/Sequence.mli

@@ -276,6 +276,7 @@ val count : ?hash:('a -> int) -> ?eq:('a -> 'a -> bool) ->
   'a t -> ('a * int) t
 (** Map each distinct element to its number of occurrences in the whole seq.
     Similar to [group_by seq |> map (fun l->List.hd l, List.length l)]
+    precondition: for any [x] and [y], if [eq x y] then [hash x=hash y] must hold.
     @since 0.10 *)
 
 val uniq : ?eq:('a -> 'a -> bool) -> 'a t -> 'a t