[Haskell-cafe] Manually instantiating Typeable w/DataKinds

[Roman Cheplyaka]

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 <uribraun at eecs.harvard.edu> [2013-01-24 16:14:53-0500]
> 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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