[Haskell-cafe] newtype deriving Alternative

[Ryan Ingram]

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 at 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.
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