Try adding
deriving instance Typeable 'Zero
deriving instance Typeable a => Typeable ('Succ a)
to your module.
(I haven't tested it — you might need to tweak it a bit.)
Roman
* TP <paratribulati...@free.fr> [2013-06-02 18:08:02+0200]
> Hi all,
>
> I try to play with the "Typeable" typeclass, and I have some difficulties to
> make it work in this simple case where I use type-level naturals:
>
> ---------------------
> {-# LANGUAGE GADTs #-}
> {-# LANGUAGE DeriveDataTypeable #-}
> {-# LANGUAGE DataKinds #-}
> {-# LANGUAGE PolyKinds #-}
> {-# LANGUAGE StandaloneDeriving #-}
>
> import Data.Typeable
>
> data Nat = Zero | Succ Nat
> deriving ( Show, Eq, Ord, Typeable )
>
> data Box where
> Box :: (Typeable s, Show s, Eq s) => s -> Box
> deriving Typeable
>
> data Proxy a = P deriving Typeable
>
> 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
> ---------------------
>
> I obtain:
>
> ---------------------
> No instance for (Typeable Nat 'Zero) arising from a use of ‛Box’
> In the expression: Box one
> In an equation for ‛foo’: foo = Box one
> In the expression:
> do { let one = ...;
> let foo = Box one;
> print foo }
> ---------------------
>
> Note that it is necessary to use the HEAD version of GHC to obtain this
> message (I use version 7.7.20130525); with GHC 7.6.2, the message is
> different because recent improvements in Typeable are not present (1).
>
> What is the problem? I've tried different things without success.
> Tell me if the "beginners" diffusion list is more suitable than Haskell-
> Cafe.
>
> Thanks,
>
> TP
>
>
> (1):
> http://hauptwerk.blogspot.fr/2012/11/coming-soon-in-ghc-head-poly-kinded.html
>
>
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