Copyright	(C) 2007-2015 Edward Kmett
License	BSD-style (see the file LICENSE)
Maintainer	libraries@haskell.org
Stability	provisional
Portability	portable
Safe Haskell	Trustworthy
Language	Haskell2010

Data.Functor.Contravariant

Contents

Contravariant Functors
Operators
Predicates
Comparisons
Equivalence Relations
Dual arrows

Description

Contravariant functors, sometimes referred to colloquially as Cofunctor, even though the dual of a Functor is just a Functor. As with Functor the definition of Contravariant for a given ADT is unambiguous.

Since: base-4.12.0.0

Synopsis

class Contravariant f where
- contramap :: (a' -> a) -> f a -> f a'
- (>$) :: b -> f b -> f a
phantom :: (Functor f, Contravariant f) => f a -> f b
(>$<) :: Contravariant f => (a -> b) -> f b -> f a
(>$$<) :: Contravariant f => f b -> (a -> b) -> f a
($<) :: Contravariant f => f b -> b -> f a
newtype Predicate a = Predicate {
- getPredicate :: a -> Bool
}
newtype Comparison a = Comparison {
- getComparison :: a -> a -> Ordering
}
defaultComparison :: Ord a => Comparison a
newtype Equivalence a = Equivalence {
- getEquivalence :: a -> a -> Bool
}
defaultEquivalence :: Eq a => Equivalence a
comparisonEquivalence :: Comparison a -> Equivalence a
newtype Op a b = Op {
- getOp :: b -> a
}

Contravariant Functors

class Contravariant f where Source #

The class of contravariant functors.

Whereas in Haskell, one can think of a Functor as containing or producing values, a contravariant functor is a functor that can be thought of as consuming values.

As an example, consider the type of predicate functions a -> Bool. One such predicate might be negative x = x < 0, which classifies integers as to whether they are negative. However, given this predicate, we can re-use it in other situations, providing we have a way to map values to integers. For instance, we can use the negative predicate on a person's bank balance to work out if they are currently overdrawn:

newtype Predicate a = Predicate { getPredicate :: a -> Bool }

instance Contravariant Predicate where
  contramap :: (a' -> a) -> (Predicate a -> Predicate a')
  contramap f (Predicate p) = Predicate (p . f)
                                         |   `- First, map the input...
                                         `----- then apply the predicate.

overdrawn :: Predicate Person
overdrawn = contramap personBankBalance negative

Any instance should be subject to the following laws:

Identity: contramap id = id
Composition: contramap (g . f) = contramap f . contramap g

Note, that the second law follows from the free theorem of the type of contramap and the first law, so you need only check that the former condition holds.

Minimal complete definition

contramap

Methods

contramap :: (a' -> a) -> f a -> f a' Source #

(>$) :: b -> f b -> f a infixl 4 Source #

Replace all locations in the output with the same value. The default definition is contramap . const, but this may be overridden with a more efficient version.

Instances

Instances details

Contravariant Comparison #	A `Comparison` is a `Contravariant` `Functor`, because `contramap` can apply its function argument to each input of the comparison function.
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Comparison a -> Comparison a' Source # (>$) :: b -> Comparison b -> Comparison a Source #
Contravariant Equivalence #	Equivalence relations are `Contravariant`, because you can apply the contramapped function to each input to the equivalence relation.
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Equivalence a -> Equivalence a' Source # (>$) :: b -> Equivalence b -> Equivalence a Source #
Contravariant Predicate #	A `Predicate` is a `Contravariant` `Functor`, because `contramap` can apply its function argument to the input of the predicate. Without newtypes `contramap f` equals precomposing with `f` (= `(. f)`). contramap :: (a' -> a) -> (Predicate a -> Predicate a') contramap f (Predicate g) = Predicate (g . f)
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Predicate a -> Predicate a' Source # (>$) :: b -> Predicate b -> Predicate a Source #
Contravariant (Op a) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a0) -> Op a a0 -> Op a a' Source # (>$) :: b -> Op a b -> Op a a0 Source #
Contravariant (Proxy :: Type -> Type) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Proxy a -> Proxy a' Source # (>$) :: b -> Proxy b -> Proxy a Source #
Contravariant (U1 :: Type -> Type) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> U1 a -> U1 a' Source # (>$) :: b -> U1 b -> U1 a Source #
Contravariant (V1 :: Type -> Type) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> V1 a -> V1 a' Source # (>$) :: b -> V1 b -> V1 a Source #
Contravariant (Const a :: Type -> Type) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a0) -> Const a a0 -> Const a a' Source # (>$) :: b -> Const a b -> Const a a0 Source #
Contravariant f => Contravariant (Alt f) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Alt f a -> Alt f a' Source # (>$) :: b -> Alt f b -> Alt f a Source #
Contravariant f => Contravariant (Rec1 f) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Rec1 f a -> Rec1 f a' Source # (>$) :: b -> Rec1 f b -> Rec1 f a Source #
(Contravariant f, Contravariant g) => Contravariant (Product f g) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Product f g a -> Product f g a' Source # (>$) :: b -> Product f g b -> Product f g a Source #
(Contravariant f, Contravariant g) => Contravariant (Sum f g) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Sum f g a -> Sum f g a' Source # (>$) :: b -> Sum f g b -> Sum f g a Source #
(Contravariant f, Contravariant g) => Contravariant (f :*: g) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> (f :: g) a -> (f :: g) a' Source # (>$) :: b -> (f :: g) b -> (f :: g) a Source #
(Contravariant f, Contravariant g) => Contravariant (f :+: g) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> (f :+: g) a -> (f :+: g) a' Source # (>$) :: b -> (f :+: g) b -> (f :+: g) a Source #
Contravariant (K1 i c :: Type -> Type) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> K1 i c a -> K1 i c a' Source # (>$) :: b -> K1 i c b -> K1 i c a Source #
(Functor f, Contravariant g) => Contravariant (Compose f g) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Compose f g a -> Compose f g a' Source # (>$) :: b -> Compose f g b -> Compose f g a Source #
(Functor f, Contravariant g) => Contravariant (f :.: g) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> (f :.: g) a -> (f :.: g) a' Source # (>$) :: b -> (f :.: g) b -> (f :.: g) a Source #
Contravariant f => Contravariant (M1 i c f) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> M1 i c f a -> M1 i c f a' Source # (>$) :: b -> M1 i c f b -> M1 i c f a Source #

phantom :: (Functor f, Contravariant f) => f a -> f b Source #

If f is both Functor and Contravariant then by the time you factor in the laws of each of those classes, it can't actually use its argument in any meaningful capacity.

This method is surprisingly useful. Where both instances exist and are lawful we have the following laws:

fmap      f ≡ phantom
contramap f ≡ phantom

Operators

(>$<) :: Contravariant f => (a -> b) -> f b -> f a infixl 4 Source #

This is an infix alias for contramap.

(>$$<) :: Contravariant f => f b -> (a -> b) -> f a infixl 4 Source #

This is an infix version of contramap with the arguments flipped.

($<) :: Contravariant f => f b -> b -> f a infixl 4 Source #

This is >$ with its arguments flipped.

Predicates

newtype Predicate a Source #

Constructors

Predicate
Fields getPredicate :: a -> Bool

Instances

Instances details

Contravariant Predicate #	A `Predicate` is a `Contravariant` `Functor`, because `contramap` can apply its function argument to the input of the predicate. Without newtypes `contramap f` equals precomposing with `f` (= `(. f)`). contramap :: (a' -> a) -> (Predicate a -> Predicate a') contramap f (Predicate g) = Predicate (g . f)
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Predicate a -> Predicate a' Source # (>$) :: b -> Predicate b -> Predicate a Source #
Monoid (Predicate a) #	`mempty` on predicates always returns `True`. Without newtypes this equals `pure True`. mempty :: Predicate a mempty = _ -> True
Instance details Defined in Data.Functor.Contravariant Methods mempty :: Predicate a Source # mappend :: Predicate a -> Predicate a -> Predicate a Source # mconcat :: [Predicate a] -> Predicate a Source #
Semigroup (Predicate a) #	`(<>)` on predicates uses logical conjunction `(&&)` on the results. Without newtypes this equals `liftA2 (&&)`. (<>) :: Predicate a -> Predicate a -> Predicate a Predicate pred <> Predicate pred' = Predicate a -> pred a && pred' a
Instance details Defined in Data.Functor.Contravariant Methods (<>) :: Predicate a -> Predicate a -> Predicate a Source # sconcat :: NonEmpty (Predicate a) -> Predicate a Source # stimes :: Integral b => b -> Predicate a -> Predicate a Source #

Comparisons

newtype Comparison a Source #

Defines a total ordering on a type as per compare.

This condition is not checked by the types. You must ensure that the supplied values are valid total orderings yourself.

Constructors

Comparison
Fields getComparison :: a -> a -> Ordering

Instances

Instances details

Contravariant Comparison #	A `Comparison` is a `Contravariant` `Functor`, because `contramap` can apply its function argument to each input of the comparison function.
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Comparison a -> Comparison a' Source # (>$) :: b -> Comparison b -> Comparison a Source #
Monoid (Comparison a) #	`mempty` on comparisons always returns `EQ`. Without newtypes this equals `pure (pure EQ)`. mempty :: Comparison a mempty = Comparison _ _ -> EQ
Instance details Defined in Data.Functor.Contravariant Methods mempty :: Comparison a Source # mappend :: Comparison a -> Comparison a -> Comparison a Source # mconcat :: [Comparison a] -> Comparison a Source #
Semigroup (Comparison a) #	`(<>)` on comparisons combines results with `(<>) @Ordering`. Without newtypes this equals `liftA2 (liftA2 (<>))`. (<>) :: Comparison a -> Comparison a -> Comparison a Comparison cmp <> Comparison cmp' = Comparison a a' -> cmp a a' <> cmp a a'
Instance details Defined in Data.Functor.Contravariant Methods (<>) :: Comparison a -> Comparison a -> Comparison a Source # sconcat :: NonEmpty (Comparison a) -> Comparison a Source # stimes :: Integral b => b -> Comparison a -> Comparison a Source #

defaultComparison :: Ord a => Comparison a Source #

Compare using compare.

Equivalence Relations

newtype Equivalence a Source #

This data type represents an equivalence relation.

Equivalence relations are expected to satisfy three laws:

Reflexivity: getEquivalence f a a = True
Symmetry: getEquivalence f a b = getEquivalence f b a
Transitivity: If getEquivalence f a b and getEquivalence f b c are both True then so is getEquivalence f a c.

The types alone do not enforce these laws, so you'll have to check them yourself.

Constructors

Equivalence
Fields getEquivalence :: a -> a -> Bool

Instances

Instances details

Contravariant Equivalence #	Equivalence relations are `Contravariant`, because you can apply the contramapped function to each input to the equivalence relation.
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Equivalence a -> Equivalence a' Source # (>$) :: b -> Equivalence b -> Equivalence a Source #
Monoid (Equivalence a) #	`mempty` on equivalences always returns `True`. Without newtypes this equals `pure (pure True)`. mempty :: Equivalence a mempty = Equivalence _ _ -> True
Instance details Defined in Data.Functor.Contravariant Methods mempty :: Equivalence a Source # mappend :: Equivalence a -> Equivalence a -> Equivalence a Source # mconcat :: [Equivalence a] -> Equivalence a Source #
Semigroup (Equivalence a) #	`(<>)` on equivalences uses logical conjunction `(&&)` on the results. Without newtypes this equals `liftA2 (liftA2 (&&))`. (<>) :: Equivalence a -> Equivalence a -> Equivalence a Equivalence equiv <> Equivalence equiv' = Equivalence a b -> equiv a b && equiv a b
Instance details Defined in Data.Functor.Contravariant Methods (<>) :: Equivalence a -> Equivalence a -> Equivalence a Source # sconcat :: NonEmpty (Equivalence a) -> Equivalence a Source # stimes :: Integral b => b -> Equivalence a -> Equivalence a Source #

defaultEquivalence :: Eq a => Equivalence a Source #

Check for equivalence with ==.

Note: The instances for Double and Float violate reflexivity for NaN.

comparisonEquivalence :: Comparison a -> Equivalence a Source #

Dual arrows

newtype Op a b Source #

Dual function arrows.

Constructors

Op
Fields getOp :: b -> a

Instances

Instances details

Category Op #
Instance details Defined in Data.Functor.Contravariant Methods id :: forall (a :: k). Op a a Source # (.) :: forall (b :: k) (c :: k) (a :: k). Op b c -> Op a b -> Op a c Source #
Contravariant (Op a) #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a0) -> Op a a0 -> Op a a' Source # (>$) :: b -> Op a b -> Op a a0 Source #
Monoid a => Monoid (Op a b) #	`mempty @(Op a b)` without newtypes is `mempty @(b->a)` = `_ -> mempty`. mempty :: Op a b mempty = Op _ -> mempty
Instance details Defined in Data.Functor.Contravariant Methods mempty :: Op a b Source # mappend :: Op a b -> Op a b -> Op a b Source # mconcat :: [Op a b] -> Op a b Source #
Semigroup a => Semigroup (Op a b) #	`(<>) @(Op a b)` without newtypes is `(<>) @(b->a)` = `liftA2 (<>)`. This lifts the `Semigroup` operation `(<>)` over the output of `a`. (<>) :: Op a b -> Op a b -> Op a b Op f <> Op g = Op a -> f a <> g a
Instance details Defined in Data.Functor.Contravariant Methods (<>) :: Op a b -> Op a b -> Op a b Source # sconcat :: NonEmpty (Op a b) -> Op a b Source # stimes :: Integral b0 => b0 -> Op a b -> Op a b Source #
Floating a => Floating (Op a b) #
Instance details Defined in Data.Functor.Contravariant Methods pi :: Op a b Source # exp :: Op a b -> Op a b Source # log :: Op a b -> Op a b Source # sqrt :: Op a b -> Op a b Source # (**) :: Op a b -> Op a b -> Op a b Source # logBase :: Op a b -> Op a b -> Op a b Source # sin :: Op a b -> Op a b Source # cos :: Op a b -> Op a b Source # tan :: Op a b -> Op a b Source # asin :: Op a b -> Op a b Source # acos :: Op a b -> Op a b Source # atan :: Op a b -> Op a b Source # sinh :: Op a b -> Op a b Source # cosh :: Op a b -> Op a b Source # tanh :: Op a b -> Op a b Source # asinh :: Op a b -> Op a b Source # acosh :: Op a b -> Op a b Source # atanh :: Op a b -> Op a b Source # log1p :: Op a b -> Op a b Source # expm1 :: Op a b -> Op a b Source # log1pexp :: Op a b -> Op a b Source # log1mexp :: Op a b -> Op a b Source #
Num a => Num (Op a b) #
Instance details Defined in Data.Functor.Contravariant Methods (+) :: Op a b -> Op a b -> Op a b Source # (-) :: Op a b -> Op a b -> Op a b Source # (*) :: Op a b -> Op a b -> Op a b Source # negate :: Op a b -> Op a b Source # abs :: Op a b -> Op a b Source # signum :: Op a b -> Op a b Source # fromInteger :: Integer -> Op a b Source #
Fractional a => Fractional (Op a b) #
Instance details Defined in Data.Functor.Contravariant Methods (/) :: Op a b -> Op a b -> Op a b Source # recip :: Op a b -> Op a b Source # fromRational :: Rational -> Op a b Source #