[Haskell-cafe] Redefining superclass default methods in a subclass

Brian Hulley brianh at metamilk.com
Thu Jan 4 14:00:05 EST 2007

Looking at some of the ideas in 
http://www.haskell.org/haskellwiki/The_Other_Prelude , it struck me that the 
class system at the moment suffers from the problem that as hierarchies get 
deeper, the programmer is burdened more and more by the need to 
cut-and-paste method definitions between instances because Haskell doesn't 
allow a superclass (or ancestor class) method default to be redefined in a 

For example, consider this part of a proposal for Functor => Applicative => 

    -- I've just used 'm' so it's easy to see what parts are relevant to 
    class Functor m where
            fmap :: (a -> b) -> m a -> m b

    class Functor m => Applicative m where
            return :: a -> m a

            (<*>) :: m (a -> b) -> m a -> m b

            (>>) :: m a -> m b -> m b
            ma >> mb = -- left as exercise for a rainy day!

    class Applicative m => Monad m where
            (>>=) :: m a -> (a -> m b) -> m b

The problem with this is that whereas someone defining a Monad at the moment 
only needs to define (return) and (>>=), with the above, though it gives 
obvious advantages in flexibility, generality etc, defining a new Monad 
involves providing methods (in instance decls) for fmap and (<*>) as well, 
and the default method for (>>) is

        ma >> mb = (fmap (const id) ma) <*> mb

(from that page above) which I'm sure everyone will agree is a *lot* more 
complicated than:

        ma >> mb = ma >>= (\_ -> mb)

Not only is the first definition for (>>) more complicated, it obscures the 
simple fact that for monads it's just a trivial special-use case of >>= 
where the bound argument is ignored.

Therefore I'm wondering if it would be possible to allow default methods for 
a superclass to be defined, or redefined, in a subclass, so we could write:

    class Applicative m => Monad m where
            (>>=) :: m a -> (a -> m b) -> m b

            mf <*> ma = mf >>= \f -> ma >>= \a -> return (f a)

            ma >> mb = ma >>= \_ = -> mb

            fmap f ma = ma >>= \a -> return (f a)

(I know the above can be written in a more point-free style but I wrote it 
like that to make it easy to understand what's happening.)

The essential point here (excuse the pun :-) ) is that it is impossible to 
write the default methods in the class in which the operation is defined, 
because the implementation depends on methods of the relevant subclass (and 
will therefore be different for different subclasses though not for each 
particular instance of a given ancestor class of a  particular subclass). As 
Haskell stands at the moment, we are forced to cut and paste identical 
methods for each individual instance of each ancestor class of a particular 
subclass because we can't override an ancestor class method in the *class* 
decl for a subclass.

The type class system at present is based on the idea that you can define 
related methods together and in terms of each other, at one level of the 
hierarchy. However as the above example shows, related methods sometimes 
need to be spread over the hierarchy but we still want to be able to define 
default implementations of them in terms of each other.

Perhaps there is some reason this can't be done?


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