[Haskell-cafe] Trying to make a Typeable instance
ndmitchell at gmail.com
Sat Jul 7 05:31:47 EDT 2007
You can use Data.Derive to do this for you:
Or DrIFT: http://repetae.net/~john/computer/haskell/DrIFT/
On 7/7/07, Adrian Hey <ahey at iee.org> wrote:
> 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
> Adrian Hey
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