Re: [Caml-list] reverse coercions

[Gerd Stolpmann <info@gerd-stolpmann.de>]

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Am Freitag, den 10.06.2016, 14:12 -0400 schrieb Hendrik Boom:
> In http://caml.inria.fr/pub/docs/u3-ocaml/ocaml-objects.html it says:
> 
> reverse coercions from a supertype to a supertype are never possible in 
> OCaml.
> 
> (a) Might one of these 'supertype's  actually be a 'subtype'?

The second should be a subtype.

> (b) Is there any way to do a run-time type-test on a value of a 
> specific statically known supertype to test whether it is actually a 
> value of a known specified subtype, and if so to proceed to use it as 
> that subtype?

This is not possible (essentially because subtyping is not bound back to
the class hierarchy (a subclass is not necessarily a subtype), and there
is no runtime representation of the types).

To some extent you can emulate what you want with open variants, e.g.
(untested):

type leaftype = ..

class type supertype =
  object
     ...
     method leaftype : leaftype
  end

class type subtype = ...

type leaftype +=
  | Subtype of subtype

class subclass : subtype =
  object(self)
    inherit supertype
    method leaftype = Leaftype self
    ...
  end

For every subclass, define a new variant for leaftype. Then, the test is

let leafobj =
  match obj#leaftype with
    | Subtype x -> x
    | _ -> failwith "coercion failed"


Gerd

> If so, I'd like to use this in a site of pattern-directed rule 
> invocations.
> 
> If not, I'd like to know if there are any well-known efficient ways 
> around this.  Of course I can effectively Godel-encode my data 
> structures using lists, and interpret all my patterns and the 
> application types I woyd have rather expressed as OCaml types, but I'm 
> looking for something more elegant, statically checked,  and 
> efficient.
> 
> -- hendrik
> 

-- 
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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
My OCaml site:          http://www.camlcity.org
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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