Works for me on GHC6.10.4: {-# LANGUAGE GeneralizedNewtypeDeriving #-} module NewtypeDerive where import Control.Applicative newtype Foo f a = Foo (f a) deriving (Functor, Applicative, Alternative) newtype Bar a = Bar [a] deriving (Functor, Applicative, Alternative) -- ryan On Wed, Oct 14, 2009 at 2:16 PM, Martijn van Steenbergen < martijn@van.steenbergen.nl> wrote:
Hello café,
I've never written an Alternative instance for a newtype yet that doesn't look like this:
instance Alternative T where
empty = T empty T x <|> T y = T (x <|> y)
Why does newtype deriving not work for Alternative? (It works fine for Monoid.)
Thanks,
Martijn. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe