[Haskell-cafe] Partial instance of a class

[Will Yager]

To avoid the hellish nightmare of Java’s UnsupportedOperationException-riddled ecosystem, I think we should *very strongly* discourage partial implementations of typeclasses as much as humanly possible. If a type doesn’t fit the typeclass, please don’t pretend it does. 

If it turns out that our typeclasses are divided along bad semantic boundaries (which they clearly are in some cases), we should fix that instead of turning the typeclass landscape into a minefield. 

—Will