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[Caml-list] Sumtypes of records
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| Date: | -- (:) |
| From: | Ville-Pertti Keinonen <will@e...> |
| Subject: | Re: [Caml-list] Sumtypes of records |
> I disagree, it would be useful, it's far more self-documenting than a
> tuple (which usually need a record-like comment right next to their
> declaration to tell what's what), and you could match the record like
>
> Bar x -> x.age
>
> and it wouldn't need a type name. I've wanted this feature myself a
> number of times.
If you can say Bar x -> x.age, you can say Bar x -> x, which implies
that the type of x should be a type in its own right, independent of
Bar.
I think this is consistent.
> It has type { age : int; }, just like int * int has type int * int and
> doesn't need a name. Does the record need a name for some other
> reason?
int * int denotes a type, { age : int } defines a type. One { age :
int } is not compatible with a different { age : int }.
It would probably be possible for { age : int } to be made a valid type
expression, but it would change the language significantly and provide
you with fairly little gain (it would save you one type definition when
using it as part of a sum type).
Also note that sum types of records are less storage-efficient than sum
types with multiple arguments (the record is indirect, because you must
be able to write Bar x -> x). Incidentally, if the record weren't
indirect, Bar x could be Obj.magic-compatible with x, but that's just a
detail of the implementation...
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