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Date: Tue, 14 Oct 2003 17:11:12 +0800
From: "Rafael 'Dido' Sevilla" <dido@imperium.ph>
To: caml-list@inria.fr
Subject: [Caml-list] implementing the visitor pattern in ocaml
Message-ID: <20031014091112.GB26517@imperium.ph>
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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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