Hi Daniel,
2014-02-10 17:10 GMT+01:00 Daniel Trstenjak
On Mon, Feb 10, 2014 at 04:55:52PM +0100, Dominique Devriese wrote:
Hm. Interesting point, I guess this is the same problem as the whole orphan instances debate... I didn't think of the connection to that problem. Still, I'm convinced there are situations where local instances are *exactly* what we need, so there must be some way to avoid this problem...
If a type class has a clear semantical meaning, what should then be the point of having multiple instances for the same data type?
A clear semantical meaning contradicts multiple instances, they would only make reasoning about your code harder.
I disagree. Just think about the Ord type class. A decidable order relation for a type is definitely something I would consider a clear semantical meaning, but different sensible instances can be defined for many types... The same thing goes for Eq or Monoid or MonadState or... Regards, Dominique