[Caml-list] coercions from superclass to subclass?

[Caml-list] coercions from superclass to subclass?

[Michael Vanier]

I understand that there is no mechanism to coerce from a superclass to a
subclass in ocaml (i.e. the equivalent of C++'s run-time type
identification or java's instanceof operator).  Are there any plans to add
such a capability in the future?

Mike
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Re: [Caml-list] coercions from superclass to subclass?

[Alessandro Baretta]

Michael Vanier wrote:
> I understand that there is no mechanism to coerce from a superclass to a
> subclass in ocaml (i.e. the equivalent of C++'s run-time type
> identification or java's instanceof operator).  Are there any plans to add
> such a capability in the future?
> 
> Mike

Jacques Garrigue gave the following explanation a week ago.

 > By the way, here is a simple way to do downcasts, when
 > really needed.
 >
 > # let tbl : (< >, impl) Hashtbl.t = Hashtbl.create 17
 >   let register obj = Hashtbl.add tbl (obj :> < >) obj
 >   let recover obj = Hashtbl.find tbl (obj :> < >) ;;
 > val tbl : (<  >, impl) Hashtbl.t = <abstr>
 > val register : impl -> unit = <fun>
 > val recover : < .. > -> impl = <fun>


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Re: [Caml-list] coercions from superclass to subclass?

[Jacques Garrigue]

From: Michael Vanier <mvanier@cs.caltech.edu>

> I understand that there is no mechanism to coerce from a superclass to a
> subclass in ocaml (i.e. the equivalent of C++'s run-time type
> identification or java's instanceof operator).  Are there any plans to add
> such a capability in the future?

The workaround was already posted. Look out Remi Vanicat's message
for a space-safe implementation.
Note that you need a coercion if you want to do it in an initializer
  class c () = object (self)
    initializer register (self :> c)
    method m = 1
  end
However this does not work with ocaml 3.04. You must write
   (self : < m : _ ; .. > :> c)
enumerating all the methods.
(self :> c) will work in ocaml 3.05.

You can also choose to write a wrapper to the constructor
  let new_c () = let o = new c () in register o; o
but this won't register automatically all subclasses.

The moral reason for the absence of downcasts is not that this is
unacceptable to use downcasts (yet, better to avoid it).  The problem
is that subtyping in ocaml is not by name, but by structure. Checking
subtyping would require a complex run-time type (not available
currently), and would be costly.  Moreover, you may produce objects
whose type is not fully known, making it even worse.

Jacques Garrigue
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