(Difference between revisions)
(pure -> return)
(Note about the possibility of splitting off a Pointed class)
Revision as of 15:48, 7 December 2010The standard class hierarchy is a consequence of Haskell's historical development, rather than logic. The
type classes could be defined as:
This would eliminate the necessity of declaring a Monad instance for every Applicative, and eliminate the need for sets of duplicate functions such as [
class Functor f where map :: (a -> b) -> f a -> f b class Functor f => Applicative f where return :: a -> f a (<*>) :: f (a -> b) -> f a -> f b (*>) :: f a -> f b -> f b (<*) :: f a -> f b -> f a class Applicative m => Monad m where (>>=) :: m a -> (a -> m b) -> m b f >>= x = join $ map f x join :: m (m a) -> m a join x = x >>= id
], and [
should be removed from Monad; a failed pattern match could error in the same way as is does for pure code. The only sensible uses for fail seem to be synonyms for
has not been included due to controversy as to whether it should be a subclass of Functor, a superclass of Functor, independent of Functor, or perhaps it is not sufficiently useful to include at all.
Backward compatibility could be eased with a legacy module, such as:
module Legacy where fmap :: Functor f => (a -> b) -> f a -> f b fmap = map liftA :: Applicative f => (a -> b) -> f a -> f b liftA = map liftM :: Monad m => (a -> b) -> m a -> m b liftM = map ap :: Monad m => m (a -> b) -> m a -> m b ap = (<*>) (>>) :: Monad m => m a -> m b -> m b (>>) = (*>) concat :: [[a]] -> [a] concat = join etc.
And for those who really want a list map,
Another variant might be to split a
listMap :: (a -> b) -> [a] -> [b] listMap = map
class from the
class Pointed f where return :: a -> f a class (Functor f, Pointed f) => Applicative f where (<*>) :: f (a -> b) -> f a -> f b (*>) :: f a -> f b -> f b (<*) :: f a -> f b -> f a
functionality by itself could be useful, for example, in a DSL in which it is only possible to embed values and not to lift functions to functions over those embedded values.