Try with existentials and GADT

deques.ml

@@ -0,0 +1,21 @@
+module Buffers = struct
+  type ('a, _) buffer =
+    | Buffer0 :                           ('a,    [`Red]) buffer
+    | Buffer1 : 'a                     -> ('a, [`Yellow]) buffer
+    | Buffer2 : 'a * 'a                -> ('a,  [`Green]) buffer
+    | Buffer3 : 'a * 'a * 'a           -> ('a,  [`Green]) buffer
+    | Buffer4 : 'a * 'a * 'a * 'a      -> ('a, [`Yellow]) buffer
+    | Buffer5 : 'a * 'a * 'a * 'a * 'a -> ('a,    [`Red]) buffer
+end
+open Buffers
+
+type 'a somebuffer =
+  | Buffer : ('a, 'x) buffer -> 'a somebuffer
+type z = int somebuffer
+(* let f b = match b with
+ *     Buffer0 -> 42
+ *   | Buffer1 x -> 42
+ *   | Buffer2 (x, y) -> 42
+ *   | Buffer3 (x, y, z) -> 42
+ *   | Buffer4 (x, y, z, t) -> 42
+ *   | Buffer5 (x, y, z, t, u) -> 42 *)