28 Feb
2014
28 Feb
'14
11:14 a.m.
On Fri, Feb 28, 2014 at 9:44 AM, Casey McCann
The restriction to distinct type variables, and the resulting issues with unification, apply equally to instances for applied type constructors. This disallows e.g. writing a Monoid instance for (a -> a) directly without a newtype wrapper, not that we'd want to do that anyway.
This was once the instance defined in Data.Monoid. It was changed to the current one in GHC 6.6. -- Dan