[Haskell-cafe] ByteString.pack behavior

[Don Stewart]

aslatter:
> On Tue, May 20, 2008 at 8:44 AM, Antoine Latter <aslatter at gmail.com> wrote:
> >
> > It's up on hackage here:
> > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/checked
> >
> 
> One more thing - there's no "Data" instance for these types.
> Presumably it wouldn't be hard to add, I just wasn't familiar with the
> class, and the technique (if any) for lifting an instance of Data
> through a newtype wasn't immediately obvious form the class interface.

With generalised newtype deriving, it should be fine to derive Data and
Typeable:

    {-# LANGUAGE DeriveDataTypeable         #-}
    {-# LANGUAGE GeneralizedNewtypeDeriving #-}

    module M where

    import Data.Generics

    newtype T = T Int
        deriving (Eq,Ord,Show,Read,Bounded,Enum,Num,Data,Typeable)

    x :: T
    x = 1 + 2

-- Don