On Jan 6 2012, Andreas Rossberg wrote:
On Jan 6, 2012, at 07.26 h, Andrej Bauer wrote:
I would be interested to hear what propeties of Ocaml you had to give
up to get this interesting extension working? For example, what
happens with checking for exhaustivness of match? Caml performs
various optimizations in pattern matching, why are those still ok now
that new alternatives may appear later?
One such type is already in ML, for historical reasons it happens to
be named exn. Consequently, you don't really give up anything, your
questions already apply to the exception type. Exhaustiveness simply
requires a catch-all in all pattern matches over this type. More
difficult is irredundancy, because constructors can be aliased without
the type system tracking that (and it cannot across module
boundaries). You have to give up there.
Generalising exn this way is an old idea, e.g. we implemented it in
Alice ML. The standard reply to requesting such an extension is that
it's not really needed, because you can already do everything using
exn (though without custom type distinctions, and minus GADTs in OCaml).
Yes, it uses the same same pattern matching as exn. This means that a
catch-all pattern is required for exhaustiveness, and if-then-elses are
used instead of jump tables.
The problem would become a bit more interesting if ordinary variant types
could also be made open, thus allowing types to have both ordinary
constructors and extensions, but the patch doesn't support that yet.
Leo
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