The standard class hierarchy is a consequence of Haskell's historical development, rather than logic. The
Monad type classes could be defined 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
This would eliminate the necessity of declaring a Monad instance for every Applicative, and eliminate the need for sets of duplicate functions such as [
ap], and [
fail should be removed from Monad; a failed pattern match should error in the same way as is does for pure code, while in
MonadPlus, the current behaviour could be maintained with
mzero. This would, however, complicate the Monad machinery.
Pointed 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,
listMap :: (a -> b) -> [a] -> [b] listMap = map
Context alias would also be a great help with backwards compatibility.
Another variant might be to split a
Pointed 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
Pointed 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.