Hi Uri,
Here's how it might look.
{-# LANGUAGE DeriveDataTypeable, DataKinds, KindSignatures, ScopedTypeVariables #-}
module Example where
import Data.Typeable
import Data.Proxy
data Tag = TagV | TagE | TagA | TagL
deriving Typeable
class TypeableTag (t :: Tag) where
tagRep :: Proxy t -> TypeRep
instance TypeableTag TagV where
tagRep _ = mkTyConApp (mkTyCon3 "mypkg" "Example" "'TagV") []
-- ... same for the other tags
newtype TaggedVar (t :: Tag) = TaggedVar Int
instance TypeableTag t => Typeable (TaggedVar t) where
typeOf _ =
mkTyConApp
(mkTyCon3 "mkpkg" "Example" "TaggedVar")
[tagRep (Proxy :: Proxy t)]
Roman
* Uri Braun
I've read the recent posting titled "Non-derivable Typeable" (http://www.mail-archive.com/haskell-cafe@haskell.org/msg103616.html) which explains that Typeable cannot be automatically derived for cases where the kind is constrained.
I'm very impressed that a solution is imminent. In the interim, can somebody kindly suggest a workaround? I'm okay with a manual instance, but I'd appreciate some help as to how to write one. I'm looking for a Typeable instance for TaggedVar for the following example below (extracted from my code).
Thank you in advance!
+Uri
{-# LANGUAGE DeriveDataTypeable, DataKinds, KindSignatures #-} module Example where
import Data.Typeable
data Tag = TagV | TagE | TagA | TagL deriving Typeable
newtype TaggedVar (t :: Tag) = TaggedVar Int
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