[Haskell-cafe] class default method proposal

[David Menendez]

On Dec 11, 2007 1:29 PM, apfelmus <apfelmus at quantentunnel.de> wrote:

> Without the automatic search, this is already possible
>
>     class Functor f where
>         fmap :: (a -> b) -> f a -> f b
>
>     class Functor m => Monad m where
>         return :: a -> m a
>         (>>=)  :: m a -> (a -> m b) -> m b
>
>        -- aka liftM
>     fmapDefault :: Monad m => (a -> b) -> m a -> m b
>     fmapDefault f m = m >>= (return . f)
>
>     instance Monad [] where
>         return x = [x]
>         (>>=)    = flip concatMap
>
>     instance Functor [] where
>         fmap = fmapDefault
>
>  fmap  is already written for you, the instance declaration is only
> boilerplate. I first saw this in  Data.Traversable .
>

This is pretty much how I define Functor and Applicative instances for my
monads. It is admittedly irritating to have to write out the boilerplate,
but  it doesn't seem irritating enough to require a language extension to
eliminate.

-- 
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>
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