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From: Hendrik Boom <hendrik@topoi.pooq.com>
To: caml-list@inria.fr
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Subject: [Caml-list] reverse coercions

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'?

(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?

If so, I'd like to use this in a suite 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