[kernel] Add `nest` and `flatten` to `Extlib` to manipulate tuples

src/libraries/stdlib/extlib.ml

@@ -77,6 +77,14 @@ let uncurry f x = f (fst x) (snd x)
 let iter_uncurry2 iter f v =
   iter (fun a b -> f (a, b)) v
 
+(* ************************************************************************* *)
+(** {2 Tuples} *)
+(* ************************************************************************* *)
+
+let nest b (a, c) = (a, b), c
+
+let flatten ((a, b), c) = a, b, c
+
 (* ************************************************************************* *)
 (** {2 Lists} *)
 (* ************************************************************************* *)

src/libraries/stdlib/extlib.mli

@@ -77,6 +77,17 @@ val iter_uncurry2:
   (('a -> 'b -> unit) -> 'c -> unit) ->
   (('a * 'b -> unit) -> 'c -> unit)
 
+(* ************************************************************************* *)
+(** {2 Tuples} *)
+(* ************************************************************************* *)
+
+val nest: 'b -> 'a * 'c -> ('a * 'b) * 'c
+(** Nest the first argument with the first element of the pair given as second
+    argument. *)
+
+val flatten: ('a * 'b) * 'c -> 'a * 'b * 'c
+(** Flatten the pairs into a triplet. *)
+
 (* ************************************************************************* *)
 (** {2 Lists} *)
 (* ************************************************************************* *)