[Haskell-cafe] Trying to make a Typeable instance

[Hugh Perkins]

Can you just write:

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

?

On 7/7/07, Adrian Hey <ahey at iee.org> 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?
>
> Thanks
> --
> Adrian Hey
>
>
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