[Haskell-cafe] class default method proposal

[David Menendez]

On Dec 11, 2007 9:20 AM, Duncan Coutts <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.

-- 
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>
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