[Haskell-cafe] Trying to make a Typeable instance

[Adrian Hey]

Adrian Hey wrote:
> 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?

(Answering my own question) this seems to do the trick..

instance (Typeable1 map, 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 = typeOf1 m

Regards
--
Adrian Hey