]> Hello, I have the following definition: module rec H : sig type 'a node = | Node of 'a node J.t | Leaf of 'a type 'a t = 'a node val equal : 'a t -> 'a t -> bool val hash : 'a t -> int end = struct type 'a node = | Node of 'a node J.t | Leaf of 'a type 'a t = 'a node let equa = (==) let hash = Hashtbl.hash end and J : Hashtbl.S with type 'a key = 'a H.node = Hashtbl.Make( H ) ;; The data type 'a node is polymorphic. It is also a key used by the hash-table. Note that H now does not comply with Hashtbl.HashedType (because Hashtbl.HashedType is not polymorphic). By adding the constraint "with type" also does not help. Is it possible to make H comply with Hashtbl.HashedType i.e: make J.Key = 'a H.node ? TIA, Hugo F.