| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
Data.Functor.Contravariant.CPS
Synopsis
- class Contravariant k => ContravariantCPS r k | k -> r where
- class Contravariant (f :: Type -> Type) where
Contravariant continuation-passing style
class Contravariant k => ContravariantCPS r k | k -> r where Source #
Instances
| ContravariantCPS Bool Predicate Source # | |
| ContravariantCPS Bool Equivalence Source # | |
Defined in Data.Functor.Contravariant.CPS Methods (<#>) :: ((a' ~~ Bool) ~> a) -> Equivalence a -> Equivalence a' Source # | |
| ContravariantCPS Ordering Comparison Source # | |
Defined in Data.Functor.Contravariant.CPS Methods (<#>) :: ((a' ~~ Ordering) ~> a) -> Comparison a -> Comparison a' Source # | |
| ContravariantCPS r (Op r) Source # | |
| ContravariantCPS r ((!) r) Source # | |
class Contravariant (f :: Type -> Type) where #
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 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:
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