1 Feb
2011
1 Feb
'11
7:18 a.m.
On 1 February 2011 11:47, Ozgur Akgun
So, is there a way to declare an AbGroup instance for the types with num instances only?
No - as Henning says its then no more useful than simply a function: add :: (Num u) => a -> a -> a add = (+) 'Overarching instances' i.e. classes with one implementation are a design flaw, not a virtue [*] so this hypothetical extension isn't welcome... {-# LANGUAGE OverarchingInstances #-} [*] Of course, there might be some valid cases for things in Oleg Kiselyov's typecast style, but modelling numericals doesn't look like one of them.