Re: [Caml-list] Proposal for extensible open datatypes

[Leo P White <lpw25@cam.ac.uk>]

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