DefaultSignatures extension can be used to make Monoid default

[HaskellZhangSong]

{-# LANGUAGE DefaultSignatures #-}

import Data.Monoid
class Default a where
    def :: a
    default def :: Monoid a => a
    def = mempty

[mauke]

I've just merged https://github.com/mauke/data-default/pull/9, which uses default signatures to make a Generic instance instead. I have to play around with that first.