]> From: Brian Hurt <brian.hurt@qlogic.com> > What I want to do is write a class interface like: > > class virtual ['a] foo : > object > method virtual doit : 'a > method map : 'b. ('a -> 'b) -> 'b foo > end > > The map function returns a new foo (not necessarily a new member of > whatever derived from foo class the function is actually being called on) > parameterized on the variant type. This precise type is not admissible in the ocaml type system. In ocaml recursive types must be regular: only 'a foo may occur in the expansion of 'a foo. This problem is discussed in an answer to PR#1730 in the caml bug database. This can be solved by introducing an explicit wrapper. I give two versions of the code: with a polymorphic reference and with a recursive modules. They are strictly equivalent: one is not safer than the other (at least currently). (* polymorphic reference *) type 'a c0 = C of < map : 'b. ('a -> 'b) -> 'b c0 > type m = {mutable new_c : 'a. 'a -> 'a c0} let m = {new_c = fun _ -> failwith "new_c"} class ['a] c (x : 'a) = object method map : 'b. ('a -> 'b) -> 'b c0 = fun f -> m.new_c (f x) end let () = m.new_c <- fun x -> C (new c x) (* recursive module *) module rec M : sig class ['a] c : 'a -> object method map : ('a -> 'b) -> 'b M.c_t end type 'a c_t = C of 'a c end = struct class ['a] c (x : 'a) = object (_ : _ #M.c) method map = fun f -> M.C (new M.c (f x)) end type 'a c_t = C of 'a c end Not that the fact the result of #map is a ['b c_t] rather than a ['b c] is not a practical problem: you can just write let C x' = x#map f If you prefer a [(x#map f).c] notation, use a one-field record rather than a one constructor sum. If you think (as I do) that all these examples are just too complicated in practice, there is a simpler way to go: only define a fold method in your class, and define map itself out of the class. Jacques Garrigue ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners