[Haskell-cafe] class default method proposal

Luke Palmer lrpalmer at gmail.com
Wed Dec 12 23:10:50 EST 2007

On Dec 11, 2007 3:19 PM, David Menendez <dave at zednenem.com> wrote:
> 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?

Isn't a type which is both a Monad and a Comonad just Identity?

(I'm actually not sure, I'm just conjecting)


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