11 Feb
2015
11 Feb
'15
5:14 p.m.
On Wed, 11 Feb 2015, Gabriel Gonzalez wrote:
Actually, I don't consider this instance a hack. One of the things that convinced me of the elegance of this `Writer`-like instance for tuples is that you can use it to implement another instance:
instance (Monoid a, Monoid b) => Monoid (a, b) where mempty = pure mempty mappend = liftA2 mappend
Funny, but without the Writer instance the implementation would be only slightly longer and it would be more comprehensible and more symmetric. If it would not be the Monoid instance you could not use 'pure' and 'liftA2' anyway.