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. -- View this message in context: http://old.nabble.com/Statically-tracking-%22validity%22---suggestions--tp29... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.