[Haskell-cafe] Inheritance and Wrappers

[Stephen Tetley]

On 1 February 2011 11:47, Ozgur Akgun <ozgurakgun at gmail.com> wrote:

>
> 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.