Re: [Haskell-cafe] Typeable typeclass and type-level naturals

6 Jun 2013

      Hi,

On Wed, Jun 5, 2013 at 11:21 PM, TP  wrote:
...
José Pedro Magalhães wrote:
...
...
Oh, it should probably be simply
deriving instance Typeable 'Zero
  deriving instance Typeable 'Succ
Yes, that's how it should be. Please let me know if that
doesn't work.
Thanks, it works perfectly like that.
I don't understand exactly why the previous syntax did not work, but maybe
it will be clearer when I finish the paper "Scrap your boilerplate: a
practical design pattern for generic programming" (anyway, this paper seems
very interesting).
It's an interesting paper, but I'm afraid it won't help you understand
what's going
on. Typeable is changing in GHC 7.8 to become poly-kinded. You are using
Typeable instances for things which are not of kind *, so you need this new
poly-kinded variant. The SYB paper says nothing about polykinds (it was
written
way before polykinds came up).

You can find some more information on the new Typeable here:
http://hackage.haskell.org/trac/ghc/wiki/GhcKinds/PolyTypeable

By the way, you can also get those Typeable instances derived automatically
if you use the LANGUAGE pragma AutoDeriveTypeable. This is also documented
in the HEAD user's guide.

Cheers,
Pedro
...
Output of the code:
---------------------
$ runghc-head test_typeable.hs
Box test_typeable.hs: Prelude.undefined
---------------------
Maybe the "Box " in front of the line is strange, but it is OK: "one" is
undefined, not "Box one".
This is the full tested code, for sake of reference:
-----------------------
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE StandaloneDeriving #-}
import Data.Typeable
data Nat = Zero | Succ Nat
    deriving ( Show, Eq, Ord )
deriving instance Typeable 'Zero
deriving instance Typeable 'Succ
data Box where
    Box :: (Typeable s, Show s, Eq s) => s -> Box
    deriving Typeable
data Proxy a = P deriving (Typeable, Show, Eq)
deriving instance Show Box
instance Eq Box where
(Box s1) == (Box s2) = Just s1 == cast s2
main = do
let one = undefined :: Main.Proxy ('Succ 'Zero)
let foo = Box one
print foo
----------------------
Thanks a lot,
TP
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