[Caml-list] implementing the visitor pattern in ocaml

[Caml-list] implementing the visitor pattern in ocaml

[Rafael 'Dido' Sevilla]

I've been experimenting with OCaml's object-oriented features, and have
tried to implement the visitor pattern in OCaml.  Here's one attempt:

class virtual abstractvisitor =
object
  method virtual visit_foo : foo -> unit
end
and foo (x_init : int) =
object (self)
  val x = x_init
  method get_x = x
  method accept (v : abstractvisitor) = v#visit_foo(self)
end
and visitor =
object
  inherit abstractvisitor
  method visit_foo f = print_string ("x = " ^ (string_of_int f#get_x))
end
;;

but I get this:

This expression has type < accept : abstractvisitor -> 'a; get_x : int;
.. >
but is here used with type foo = < .. >
Self type cannot escape its class

What does this mean?  What's the right way to implement the visitor
design pattern in OCaml?

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Re: [Caml-list] implementing the visitor pattern in ocaml

[Samuel Lacas]

Hi, you need to use coercions here (at least, that's the way it works
with me). Simply declare

method accept (v : abstractvisitor) = v#visit_foo(self :> foo)

in class foo.
Have a look at section 3.11 of the manual for more information.

Hope this helps,

sL

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Re: [Caml-list] implementing the visitor pattern in ocaml

[Didier Remy]

Samuel Lacas <Samuel.Lacas@trusted-logic.fr> writes:

> Have a look at section 3.11 of the manual for more information.

And section 5.3, which also comes with more details at
http://cristal.inria.fr/~remy/cours/appsem/ocaml-objects.html#toc13

Didier

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