# User:Zzo38/Proposal about classes

### From HaskellWiki

## Revision as of 20:34, 3 September 2011

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

## Contents |

## 1 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.

## 2 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.

## 3 Restrictive classes

For example, make a restrictive monad that only contains instances of `Ord`. I am not sure about syntax or other details.

## 4 Classes of classes

See proposal of kinds.

## 5 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.