User:Zzo38/Proposal about classes

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This document is proposal relating to various things dealing with classes in Haskell.


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.

Universal class

There can be one universal class that all types automatically belong to. This is a polymorphic class (see proposal of kinds) of kind forall k. [k]; that is, it is for any kind (including other classes).

Generalize an existing class

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.

See also