Erik Hesselink wrote:
>
> If you want to use types instead of modules and hiding as Chris
> suggested, you can use a type index like this:
>
> {-# LANGUAGE EmptyDataDecls, GADTs, KindSignatures #-}
> data Nothing
> data Just a
>
> data Subject :: * -> * where
> NoName :: Subject Nothing
> Name :: String -> Subject (Just String)
>
> data Certificate a = Certificate
> { subject :: Subject a }
>
> validate :: Certificate a -> Maybe (Certificate (Just String))
> validate c =
> case subject c of
> NoName -> Nothing
> Name n -> Just c
>
> A "Certificate (Just String)" now always holds a name, and a
> "Certificate Nothing" doesn't. A "Certificate a" can hold either.
>
> Erik
>
Thanks, both of you. The GADT version seems slightly easier
to work with in our case.
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