Re: [Caml-list] Re: OCAML Downcasting?

[skaller <skaller@users.sourceforge.net>]

On Wed, 2004-09-22 at 08:06, Michael Vanier wrote:

> Um, no, they didn't.  In fact, it's a completely different mechanism.  The
> compiler erases the generic information so that the JVM sees only old-style
> java classes without parameterization and adds casts where needed.  OK,
> this wasn't the greatest example in the world, because it relies massively
> on RTTI.

Ocaml does this too. However, the polymorphism uses *static*
type checking to erase the type information, and doesn't
need to use RTTI.

BTW: Ocaml *does* provide limited RTTI and 
dynamic dispatch on that RTTI. If you examine:

type num = Int of int | Float of float

match x with
| Int x -> print_int x
| Float x -> print_float x

you can see that the match must use RTTI and it does.
However it isn't a full description of the type,
just an integral 'tag' allowing a switch to the 
correct case. Not enough information to check
the type -- the static type system has
checked already 'up to which variant' and the
rest is done at runtime.

Similarly, RTTI is used to do array bounds
checks in Ocaml: arrays typing is weaker
than in Pascal, which never needs array
bound checks, or Modula, where advanced
analysis can elide about 60% of such checks.

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net



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