Am Sonntag 21 Juni 2009 21:24:24 schrieb Andrew Coppin:
> I have a datatype with about a dozen constructors. I'd like to find a
> way to use the type system to prevent one of the constructors from being
> used in certain places. But I can't think of a way to do that.
>
> data Foobar =
> Foo Foobar |
> Bar Foobar |
> Zoo Foobar
>
> I want the type system to track whether or not Zoo has been used in a
> specific value. Sure, you can check for it at runtime, but I'd be
> happier if the type system can guarantee its absence when required.
>
> I tried this:
>
> data Zoo x =
> Zoo x |
> NoZoo x
>
> data GeneralFoobar f =
> Foo f |
> Bar f
>
> type Foobar = GeneralFoobar Foobar
> type FoobarZ = GeneralFoobar (Zoo FoobarZ)
>
> but the type checker seems to not like it. (Plus I now have to wade
> through miles of NoZoo wrappers everywhere.)
>
> The other alternative is to just duplicate the entire data declaration,
> sans the Zoo constructor... but I'd obviously prefer not to have to do
> that. (E.g., if I ever need to alter the declaration, I now have two
> copies to keep in sync.)
>
> Suggestions?
GADTs?
data Okay
data HasZoo
data Aye
data Nay
class IsOkay a b | a -> b where
instance IsOkay Okay Aye where
instance IsOkay HasZoo Nay where
data Foobar where
Bling :: Foobar a
Foo :: Foobar a -> Foobar a
Bar :: Foobar a -> Foobar a
Zoo :: Foobar a -> Foobar HasZoo
use :: (IsOkay a Aye) => Foobar a -> whatever
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