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Adds Bifunctor laws #75

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Adds Bifunctor laws #75

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29 changes: 28 additions & 1 deletion src/Test/QuickCheck/Classes.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,7 @@ module Test.QuickCheck.Classes
( ordRel, ord, ordMorphism, semanticOrd
, semigroup
, monoid, monoidMorphism, semanticMonoid
, functor, functorMorphism, semanticFunctor, functorMonoid
, functor, bifunctor, functorMorphism, semanticFunctor, functorMonoid
, apply, applyMorphism, semanticApply
, applicative, applicativeMorphism, semanticApplicative
, bind, bindMorphism, semanticBind, bindApply
Expand All @@ -30,6 +30,7 @@ module Test.QuickCheck.Classes

import Data.Bifoldable (Bifoldable (..))
import Data.Bifunctor hiding (first, second)
import qualified Data.Bifunctor as Bifunctor
import Data.Foldable (Foldable(..))
import Data.Functor.Apply (Apply ((<.>)))
import Data.Functor.Alt (Alt ((<!>)))
Expand Down Expand Up @@ -233,6 +234,32 @@ functor = const ( "functor"
identityP = fmap id =-= (id :: m a -> m a)
composeP g f = fmap g . fmap f =-= (fmap (g.f) :: m a -> m c)

-- | Properties to check that the 'Bifunctor' @m@ satisfies the
-- functor laws.
bifunctor :: forall m a b c d.
( Bifunctor m
, Arbitrary c, Arbitrary d
, CoArbitrary a, CoArbitrary b
, Show (m a b), Arbitrary (m a b), EqProp (m a b), EqProp (m c d)) =>
m (a,b,c,d) (a,b,c,d) -> TestBatch
bifunctor = const ( "bifunctor"
, [ ("bimap id id ≡ id", property identityP)
, ("first id ≡ id", property identityFirstP)
, ("second id ≡ id", property identitySecondP)
, ("bimap f g ≡ first f . second g", property bimapFirstSecondP)
]
)
where
identityP :: Property
identityFirstP :: Property
identitySecondP :: Property
bimapFirstSecondP :: (b -> d) -> (a -> c) -> Property

identityP = bimap id id =-= (id :: m a b -> m a b)
identityFirstP = Bifunctor.first id =-= (id :: m a b -> m a b)
identitySecondP = Bifunctor.second id =-= (id :: m a b -> m a b)
bimapFirstSecondP g f = bimap f g =-= (Bifunctor.first f . Bifunctor.second g :: m a b -> m c d)

-- Note the similarity between 'functor' and 'monoidMorphism'. The
-- functor laws say that 'fmap' is a homomorphism w.r.t '(.)':
--
Expand Down