Copyright	(C) 2008-2014 Edward Kmett
License	BSD-style (see the file LICENSE)
Maintainer	libraries@haskell.org
Stability	provisional
Portability	portable
Safe Haskell	Safe
Language	Haskell2010

Data.Bifunctor

Description

Since: base-4.8.0.0

Synopsis

class (forall a. Functor (p a)) => Bifunctor (p :: Type -> Type -> Type) where
- bimap :: (a -> b) -> (c -> d) -> p a c -> p b d
- first :: (a -> b) -> p a c -> p b c
- second :: (b -> c) -> p a b -> p a c

Documentation

class (forall a. Functor (p a)) => Bifunctor (p :: Type -> Type -> Type) where Source #

A bifunctor is a type constructor that takes two type arguments and is a functor in both arguments. That is, unlike with Functor, a type constructor such as Either does not need to be partially applied for a Bifunctor instance, and the methods in this class permit mapping functions over the Left value or the Right value, or both at the same time.

Formally, the class Bifunctor represents a bifunctor from Hask -> Hask.

Intuitively it is a bifunctor where both the first and second arguments are covariant.

The class definition of a Bifunctor p uses the QuantifiedConstraints language extension to quantify over the first type argument a in its context. The context requires that p a must be a Functor for all a. In other words a partially applied Bifunctor must be a Functor. This makes Functor a superclass of Bifunctor such that a function with a Bifunctor constraint may use fmap in its implementation. Functor has been a quantified superclass of Bifunctor since base-4.18.0.0.

You can define a Bifunctor by either defining bimap or by defining both first and second. The second method must agree with fmap:

second ≡ fmap

From this it follows that:

second id ≡ id

If you supply bimap, you should ensure that:

bimap id id ≡ id

If you supply first and second, ensure:

first id ≡ id
second id ≡ id

If you supply both, you should also ensure:

bimap f g ≡ first f . second g

These ensure by parametricity:

bimap  (f . g) (h . i) ≡ bimap f h . bimap g i
first  (f . g) ≡ first  f . first  g
second (f . g) ≡ second f . second g

Since: base-4.8.0.0

Minimal complete definition

bimap | first, second

Methods

bimap :: (a -> b) -> (c -> d) -> p a c -> p b d Source #

Map over both arguments at the same time.

bimap f g ≡ first f . second g

Examples

Expand

>>> bimap toUpper (+1) ('j', 3)
('J',4)

>>> bimap toUpper (+1) (Left 'j')
Left 'J'

>>> bimap toUpper (+1) (Right 3)
Right 4

first :: (a -> b) -> p a c -> p b c Source #

Map covariantly over the first argument.

first f ≡ bimap f id

Examples

Expand

>>> first toUpper ('j', 3)
('J',3)

>>> first toUpper (Left 'j')
Left 'J'

second :: (b -> c) -> p a b -> p a c Source #

Map covariantly over the second argument.

second ≡ bimap id

Examples

Expand

>>> second (+1) ('j', 3)
('j',4)

>>> second (+1) (Right 3)
Right 4

Instances

Instances details

Bifunctor Arg Source #	Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods bimap :: (a -> b) -> (c -> d) -> Arg a c -> Arg b d Source # first :: (a -> b) -> Arg a c -> Arg b c Source # second :: (b -> c) -> Arg a b -> Arg a c Source #
Bifunctor Either Source #	Since: base-4.8.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> Either a c -> Either b d Source # first :: (a -> b) -> Either a c -> Either b c Source # second :: (b -> c) -> Either a b -> Either a c Source #
Bifunctor (,) Source #	Class laws for tuples hold only up to laziness. Both `first` `id` and `second` `id` are lazier than `id` (and `fmap` `id`): `>>>` first id (undefined :: (Int, Word)) `seq` ()() `>>>` second id (undefined :: (Int, Word)) `seq` ()() `>>>` id (errorWithoutStackTrace "error!" :: (Int, Word)) `seq` ()*** Exception: error! Since: base-4.8.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> (a, c) -> (b, d) Source # first :: (a -> b) -> (a, c) -> (b, c) Source # second :: (b -> c) -> (a, b) -> (a, c) Source #
Bifunctor (Const :: Type -> Type -> Type) Source #	Since: base-4.8.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> Const a c -> Const b d Source # first :: (a -> b) -> Const a c -> Const b c Source # second :: (b -> c) -> Const a b -> Const a c Source #
Bifunctor ((,,) x1) Source #	Since: base-4.8.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> (x1, a, c) -> (x1, b, d) Source # first :: (a -> b) -> (x1, a, c) -> (x1, b, c) Source # second :: (b -> c) -> (x1, a, b) -> (x1, a, c) Source #
Bifunctor (K1 i :: Type -> Type -> Type) Source #	Since: base-4.9.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> K1 i a c -> K1 i b d Source # first :: (a -> b) -> K1 i a c -> K1 i b c Source # second :: (b -> c) -> K1 i a b -> K1 i a c Source #
Bifunctor ((,,,) x1 x2) Source #	Since: base-4.8.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> (x1, x2, a, c) -> (x1, x2, b, d) Source # first :: (a -> b) -> (x1, x2, a, c) -> (x1, x2, b, c) Source # second :: (b -> c) -> (x1, x2, a, b) -> (x1, x2, a, c) Source #
Bifunctor ((,,,,) x1 x2 x3) Source #	Since: base-4.8.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> (x1, x2, x3, a, c) -> (x1, x2, x3, b, d) Source # first :: (a -> b) -> (x1, x2, x3, a, c) -> (x1, x2, x3, b, c) Source # second :: (b -> c) -> (x1, x2, x3, a, b) -> (x1, x2, x3, a, c) Source #
Bifunctor ((,,,,,) x1 x2 x3 x4) Source #	Since: base-4.8.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> (x1, x2, x3, x4, a, c) -> (x1, x2, x3, x4, b, d) Source # first :: (a -> b) -> (x1, x2, x3, x4, a, c) -> (x1, x2, x3, x4, b, c) Source # second :: (b -> c) -> (x1, x2, x3, x4, a, b) -> (x1, x2, x3, x4, a, c) Source #
Bifunctor ((,,,,,,) x1 x2 x3 x4 x5) Source #	Since: base-4.8.0.0
Instance details Defined in Data.Bifunctor Methods bimap :: (a -> b) -> (c -> d) -> (x1, x2, x3, x4, x5, a, c) -> (x1, x2, x3, x4, x5, b, d) Source # first :: (a -> b) -> (x1, x2, x3, x4, x5, a, c) -> (x1, x2, x3, x4, x5, b, c) Source # second :: (b -> c) -> (x1, x2, x3, x4, x5, a, b) -> (x1, x2, x3, x4, x5, a, c) Source #