Re: [Haskell-cafe] class default method proposal

David Menendez wrote:

On Dec 11, 2007 9:20 AM, Duncan Coutts mailto:duncan.coutts@worc.ox.ac.uk> wrote:

So my suggestion is that we let classes declare default implementations of methods from super-classes.

Does this proposal have any unintended consequences? I'm not sure. Please discuss :-)

It creates ambiguity if two classes declare defaults for a common superclass.

My standard example involves Functor, Monad, and Comonad. Both Monad and Comonad could provide a default implementation for fmap. But let's say I have a type which is both a Monad and a Comonad: which default implementation gets used?

I'm disappointed to see this objection isn't listed on the wiki.

Doesn't sound like a very big problem. That would just be a compile time error ("More than one default for fmap possible for Foo, please reslve ambiguity"). Jules