On 2016-02-25 at 12:12:33 +0100, Jeremy wrote: [...]
Index Int wrote
And given that `Traversable` gives a rise to a `Foldable` similarly to how a `Monad` gives a rise to an `Applicative`, it is the same stupid mistake not to reflect this relation in the type system.
The Monad definition actually uses the Applicative instance. Traversable makes no use of Foldable.
What do you mean by that exactly? Haskell 2010 defines the `Monad` class as: class Monad m where return :: a -> m a (>>=) :: m a -> (a -> m b) -> m b (>>) :: m a -> m b -> m b m >> k = m >>= \_ -> k fail :: String -> m a fail = error whereas with AMP+MRP+MFP we would end up with the following quite minimal `Monad` class definition: class Applicative m => Monad m where (>>=) :: m a -> (a -> m b) -> m b In neither case `Applicative`'s methods are used. Maybe a better example you could argue is that the `Ord` class *currently* uses its `Eq` superclass to support its default implementation of `compare`. But other than that you could just as easily define an `Ord` class which does not use `(==)` in any of its default method implementations, and therefore would exhibit a redundant superclass constraint on `Eq`. But even then I guess nobody would want to remove the `Eq` superclass from `Ord` either?