Re: [Caml-list] dynamic runtime cast in Ocaml

[Xavier Leroy <xavier.leroy@inria.fr>]

> I am sometimes lacking the dynamic cast (with a runtime test) in
> Ocaml. Assuming that I have the usual point and colored_point classes
> (as in the 3.06 Manual), I want to deal with point array-s and test if
> the 3rd point is in fact a colored point.
> Is there some type-theoritical issues here?

There was a long thread on this topic on this list not long ago.
Several approaches were discussed: using datatypes (Point of point |
CPoint of cpoint), extensible datatypes (via the type "exn" of
exceptions), etc.  See also Coca-ML:
http://www.pps.jussieu.fr/~emmanuel/Public/Dev/coca-ml/index-en.html

This said, my opinion on downcasts is that they are a low-level,
dangerous feature that is best replaced by advanced, statically-typed
constructs like OCaml provides.  (Some day, I'll have a bumper sticker
custom-made that reads "Caml programmers do it statically".)

In my (admittedly limited) experience with Java, 95% of downcasts in
Java code fall in one of the following scenarios:
- when using container classes (program puts a Foo in a Hashtable,
  reads it back as Object, needs downcast to Foo);
- when using binary methods (equal(), compare())
- to encode sum types (a Foo is a Bar or a Gee).

The OCaml type system is able to type-check all three scenarios
statically, using parameterized classes, "mytype" specialization,
and datatypes / variants (respectively).

The remaining 5% of downcasts were kludges where the programmer
managed to ruin a perfectly good class hierarchy by sprinkling type
tests everywhere, rather than subclassing and redefining virtual
methods adequately in classic OO fashion.

To be fair, I may have missed some legitimate uses of downcasts in
complicated situations.  But I believe these legitimate uses are
uncommon enough that it is best to find another way to express the
desired behavior, rather than extend OCaml with downcasts.

That you can do self-modifying code in assembler, Duff's device in C,
or downcasts in Java doesn't mean that you want to do it in OCaml as
well :-)

- Xavier Leroy
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