Re: Many functions can be generalised

On Dec 4, 2016 12:22 AM, "David Menendez" wrote: You can also use it with contra-variant or invariant type constructors, e.g., instance Siftable a (a -> Bool) where sift f g = \x -> f x && g x That looks backwards for your composition law, but I'm a bit tired so I wouldn't swear to it. Surely you can do the same with the constructor class. newtype Ab a = Ab (a -> Bool) instance Siftable Ab where siftAway _ = Ab (const False) sift p (Ab g) = Ab ... I'm not sure if my siftAway excludes anything it shouldn't....