add CCKlist.memoize for costly computations

doc/intro.txt

@@ -72,6 +72,7 @@ CCFQueue
 CCFlatHashtbl
 CCHashSet
 CCHashTrie
+CCImmutArray
 CCIntMap
 CCMixmap
 CCMixset

src/core/CCFormat.mli

@@ -67,7 +67,7 @@ val stdout : t
 val stderr : t
 
 val sprintf : ('a, t, unit, string) format4 -> 'a
-  (** print into a string *)
+(** print into a string *)
 
 val to_file : string -> ('a, t, unit, unit) format4 -> 'a
-  (** Print to the given file *)
+(** Print to the given file *)

src/iter/CCKList.ml

@@ -441,6 +441,36 @@ let sort_uniq ?(cmp=Pervasives.compare) l =
   let l = to_list l in
   uniq (fun x y -> cmp x y = 0) (of_list (List.sort cmp l))
 
+type 'a memoize =
+  | MemoThunk
+  | MemoSave of [`Nil | `Cons of 'a * 'a t]
+
+let rec memoize f =
+  let r = ref MemoThunk in
+  fun () -> match !r with
+    | MemoSave l -> l
+    | MemoThunk ->
+        let l = match f() with
+          | `Nil -> `Nil
+          | `Cons (x, tail) -> `Cons (x, memoize tail)
+        in
+        r := MemoSave l;
+        l
+
+(*$R
+  let printer = Q.Print.(list int) in
+  let gen () =
+    let rec l = let r = ref 0 in fun () -> incr r; `Cons (!r, l) in l
+  in
+  let l1 = gen () in
+  assert_equal ~printer [1;2;3;4] (take 4 l1 |> to_list);
+  assert_equal ~printer [5;6;7;8] (take 4 l1 |> to_list);
+  let l2 = gen () |> memoize in
+  assert_equal ~printer [1;2;3;4] (take 4 l2 |> to_list);
+  assert_equal ~printer [1;2;3;4] (take 4 l2 |> to_list);
+*)
+
+
 (** {2 Fair Combinations} *)
 
 let rec interleave a b () = match a() with

src/iter/CCKList.mli

@@ -191,6 +191,10 @@ val sort_uniq : ?cmp:'a ord -> 'a t -> 'a t
     finite. O(n ln(n)) time and space.
     @since 0.3.3 *)
 
+val memoize : 'a t -> 'a t
+(** Avoid recomputations by caching intermediate results
+    @since NEXT_RELEASE *)
+
 (** {2 Fair Combinations} *)
 
 val interleave : 'a t -> 'a t -> 'a t