User:Zzo38/Proposal about classes
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This document is proposal relating to various things dealing with classes in Haskell.
Monads
Move join into the Monad class, to allow monads to defined in terms of unit/join/fmap. Together with things described below, it can automatically be Functor as well, and define fmap if you are using return/bind definitions.
Default superclass definitions
For example, for monads you can have:
class Functor m => Monad (m :: * -> *) where {
(>>=) :: forall a b. m a -> (a -> m b) -> m b;
(>>) :: forall a b. m a -> m b -> m b;
return :: a -> m a;
join :: m (m a) -> m a;
fail :: String -> m a;
a >>= f = join $ fmap f a;
m >> k = m >>= \_ -> k;
join = (>>= id);
fail = error;
fmap f m = m >>= return . f;
};
In this case, you can define the defaults for Functor if they are not already defined.
Restrictive classes
For example, make a restrictive monad that only contains instances of Ord. I am not sure about syntax or other details.
Classes of classes
See proposal of kinds.