How to write Semigroup instance for this data type?

Question

I\'m trying to do one of the Semigroup exercises in Haskell Book (Chapter 15, \"Monoid, Semigroup\") but I\'m stuck. The following is given:

newtype Combine          


        

Answer 1

You can shadow Monoid's (<>) this way:

import Data.Monoid hiding ((<>))

Then when you import Data.Semigroup, you will have just one (<>) in scope: the one from Data.Semigroup.

Answer 2

What they want is probably the addition of functions. For that, type b needs to be a Semigroup :

import Data.Semigroup

newtype Combine a b =
  Combine { unCombine :: (a -> b) }

instance Semigroup b => Semigroup (Combine a b) where
  (Combine f) <> (Combine g) = Combine (\x -> f x <> g x)