[Haskell-cafe] Trying to make a Typeable instance

7 Jul 2007

      Hello,

I'm trying to make the type (ListGT map k a) an instance of Typeable,
where map is kind (* -> *).

data ListGT map k a
  = Empt
  | BraF ![k] a !(map (ListGT map k a))
  | BraE ![k]   !(map (ListGT map k a))

I thought I'd cracked it with something like this..

instance (Typeable (map (ListGT map k a)), Typeable k, Typeable a) =>
          Typeable (ListGT map k a) where
    typeOf lgt = mkTyConApp (mkTyCon "Data.Trie.General.ListGT")
                [mTypeRep, kTypeRep, aTypeRep]
      where BraF [k] a m = lgt -- This is just to get types for k a m !!
            kTypeRep = typeOf k
            aTypeRep = typeOf a
            mTypeRep = typeOf m

However, showing the resulting TypRep gives a stack overflow. I wasn't
too surprised about this, so I tried replacing the last line with..
            mTypeRep = mkTyConApp (typeRepTyCon (typeOf m)) []
..thinking that this would make it terminate. But it doesn't.

Could someone explain how to do this?

Thanks
--
Adrian Hey