A agreed upon technique for dealing with typeclass hierarchies has
been slow to arrive. For instance, all monads are functors, but
providing a monad instance for your type doesn't automatically make it
a functor as well.
All monads are also applicative functors, and Control.Applicative does
have a newtype to recognize them as such:
Prelude> :m + Control.Applicative
Prelude Control.Applicative> :i WrapMonad
newtype WrappedMonad m a = WrapMonad {unwrapMonad :: m a}
-- Defined in Control.Applicative
Prelude Control.Applicative> :i Applicative
class (Functor f) => Applicative f where
pure :: a -> f a
(<*>) :: f (a -> b) -> f a -> f b
-- Defined in Control.Applicative
instance (Monad m) => Applicative (WrappedMonad m)
-- Defined in Control.Applicative
...
Just "wrap" up your monad in WrapMonad, treat it like an applicative
functor, and then unwrap it with unwrapMonad.
HTH,
Nick
On 12/1/06, Greg Fitzgerald <[EMAIL PROTECTED]> wrote:
> Text.ParserCombinators.ReadP.(+++) :: ReadP a -> ReadP a -> ReadP a
Wow, fast and complete, Thanks Don! :)
Would it make sense to derive instances of Applicable and Alternative
for ReadP? Something like this maybe:
instance Applicative ReadP where
pure = return
(<*>) = ap
instance Alternative ReadP where
empty = pfail
(<|>) = (<++)
Thanks,
Greg
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