28 Feb
2014
28 Feb
'14
9:44 a.m.
On Thu, Feb 27, 2014 at 8:32 PM, Karl Voelker
The spec doesn't allow multi-parameter type classes at all, so I don't see how you could run into this problem while writing extensionless Haskell.
I guess it's still a valid question whether the MultiParamTypeClasses extension should allow repeated type parameters.
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. - C.