[Haskell-cafe] class default method proposal

Jules Bean jules at jellybean.co.uk
Tue Dec 11 10:26:36 EST 2007

David Menendez wrote:
> On Dec 11, 2007 9:20 AM, Duncan Coutts <duncan.coutts at worc.ox.ac.uk 
> <mailto:duncan.coutts at worc.ox.ac.uk>> wrote:
>     So my suggestion is that we let classes declare default implementations
>     of methods from super-classes.
> <snip.
>     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 


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