setisnotafunctor.hs

{-# LANGUAGE DeriveFunctor #-}

module SetIsNotAFunctor where

import Data.Set (Set)
import qualified Data.Set as Set

-- | A type for annotating a value where equality and ordering only care
-- about the value.
data Ann ann a = Ann { annotation :: ann, value :: a }
    deriving (Show, Functor)

instance Eq a => Eq (Ann ann a) where
    Ann _ a == Ann _ b = a == b

instance Ord a => Ord (Ann ann a) where
    compare (Ann _ a) (Ann _ b) = compare a b

anns :: Set (Ann String Int)
anns = Set.fromList [Ann "Hello" 1, Ann "Foobar" 0]

g :: Ann String Int -> Ann String Int
g = fmap (`div` 2)

f :: Ann String Int -> String
f = annotation

main = do
    let lhs = Set.map (f . g) anns
    print lhs
    let rhs = (Set.map f . Set.map g) anns
    print rhs
    print (lhs == rhs)