Three' Traversable instance

output

traversable:
  fmap:    +++ OK, passed 500 tests.
  foldMap: *** Failed! Falsifiable (after 6 tests):
<function>
Three' (1,4,[-4,3,-5,5]) 1 (-2)
RHS
RHS
RHS
LHS

TraversableExercises.hs

module TraversableExercises where

import Test.QuickCheck
import Test.QuickCheck.Checkers
import Test.QuickCheck.Classes

data Three' a b =
    Three' a b b
    deriving (Eq, Ord, Show)

instance (Arbitrary a, Arbitrary b) => Arbitrary (Three' a b) where
    arbitrary = do
        a <- arbitrary
        b <- arbitrary
        b' <- arbitrary
        return (Three' a b b')

instance (Eq a, Eq b) => EqProp (Three' a b) where
    (=-=) = eq

instance Functor (Three' a) where
    fmap f (Three' a b b') = Three' a (f b) (f b')

instance Foldable (Three' a) where
    foldr f v (Three' _ b b') = f b' (f b v)

instance Traversable (Three' a) where
    traverse f (Three' a b b') = Three' a <$> f b <*> f b'
  
main = do
    quickBatch $ traversable (undefined :: Three' (Int, Int, [Int]) (Int, Int, [Int]))

The problem was with the order of b/b' in foldr. Thank you to Daniel Díaz‏ for figuring it out.

https://twitter.com/DiazCarrete/status/847908639952207872