User:Zzo38/Proposal about classes
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.
Automatic instances
You can have a Template Haskell command, that takes a name of class, and a function that uses the definition of a type to make up a instance declaration for that class. If there is no such instance, it tries to make up the instance using that function. If it results in fail, then it is error as if there is no instance normally, but the fail message is also displayed.