Control.Comonad

comonad-5.0.9: Comonads

Copyright | (C) 2008-2015 Edward Kmett (C) 2004 Dave Menendez
License | BSD-style (see the file LICENSE)
Maintainer | Edward Kmett <ekmett@gmail.com>
Stability | provisional
Portability | portable
Safe Haskell | Safe
Language | Haskell2010

Control.Comonad

Contents

- Comonads
- Combining Comonads
- Cokleisli Arrows
- Functors

Description

Synopsis

- class Functor w => Comonad (w :: Type -> Type) where
  - extract :: w a -> a
  - duplicate :: w a -> w (w a)
  - extend :: (w a -> b) -> w a -> w b
- liftW :: Comonad w => (a -> b) -> w a -> w b
- wfix :: Comonad w => w (w a -> a) -> a
- cfix :: Comonad w => (w a -> a) -> w a
- kfix :: ComonadApply w => w (w a -> a) -> w a
- (=>=) :: Comonad w => (w a -> b) -> (w b -> c) -> w a -> c
- (=<=) :: Comonad w => (w b -> c) -> (w a -> b) -> w a -> c
- (<<=) :: Comonad w => (w a -> b) -> w a -> w b
- (=>>) :: Comonad w => w a -> (w a -> b) -> w b
- class Comonad w => ComonadApply (w :: Type -> Type) where
  - (<@>) :: w (a -> b) -> w a -> w b
  - (@>) :: w a -> w b -> w b
  - (<@) :: w a -> w b -> w a
- (<@@>) :: ComonadApply w => w a -> w (a -> b) -> w b
- liftW2 :: ComonadApply w => (a -> b -> c) -> w a -> w b -> w c
- liftW3 :: ComonadApply w => (a -> b -> c -> d) -> w a -> w b -> w c -> w d
- newtype Cokleisli (w :: k -> Type) (a :: k) b = Cokleisli {
  - runCokleisli :: w a -> b
  }
- class Functor (f :: Type -> Type) where
  - fmap :: (a -> b) -> f a -> f b
  - (<$) :: a -> f b -> f a
- (<$>) :: Functor f => (a -> b) -> f a -> f b
- ($>) :: Functor f => f a -> b -> f b

# Comonads

class Functor w => Comonad (w :: Type -> Type) where #

There are two ways to define a comonad:

I. Provide definitions for `extract` and `extend` satisfying these laws:

    extend extract      = id
    extract . extend f  = f
    extend f . extend g = extend (f . extend g)

In this case, you may simply set `fmap` = `liftW`.

These laws are directly analogous to the laws for monads and perhaps can be made clearer by viewing them as laws stating that Cokleisli composition must be associative, and has extract for a unit:

    f =>= extract   = f
    extract =>= f   = f
    (f =>= g) =>= h = f =>= (g =>= h)

II. Alternately, you may choose to provide definitions for `fmap`, `extract`, and `duplicate` satisfying these laws:

    extract . duplicate      = id
    fmap extract . duplicate = id
    duplicate . duplicate    = fmap duplicate . duplicate

In this case you may not rely on the ability to define `fmap` in terms of `liftW`.

You may of course, choose to define both `duplicate` and `extend`. In that case you must also satisfy these laws:

    extend f  = fmap f . duplicate
    duplicate = extend id
    fmap f    = extend (f . extract)

These are the default definitions of `extend` and `duplicate` and the definition of `liftW` respectively.

Minimal complete definition

extract, (duplicate | extend)

Methods

extract :: w a -> a #

    extract . fmap f = f . extract

duplicate :: w a -> w (w a) #

    duplicate = extend id
    fmap (fmap f) . duplicate = duplicate . fmap f

extend :: (w a -> b) -> w a -> w b #

    extend f = fmap f . duplicate

#### Instances

Instances details

Comonad Tree # |
Instance details Defined in Control.Comonad Methods extract :: Tree a -> a # duplicate :: Tree a -> Tree (Tree a) # extend :: (Tree a -> b) -> Tree a -> Tree b #
Comonad NonEmpty # |
Instance details Defined in Control.Comonad Methods extract :: NonEmpty a -> a # duplicate :: NonEmpty a -> NonEmpty (NonEmpty a) # extend :: (NonEmpty a -> b) -> NonEmpty a -> NonEmpty b #
Comonad Identity # |
Instance details Defined in Control.Comonad Methods extract :: Identity a -> a # duplicate :: Identity a -> Identity (Identity a) # extend :: (Identity a -> b) -> Identity a -> Identity b #
Comonad (Arg e) # |
Instance details Defined in Control.Comonad Methods extract :: Arg e a -> a # duplicate :: Arg e a -> Arg e (Arg e a) # extend :: (Arg e a -> b) -> Arg e a -> Arg e b #
Comonad ((,) e) # |
Instance details Defined in Control.Comonad Methods extract :: (e, a) -> a # duplicate :: (e, a) -> (e, (e, a)) # extend :: ((e, a) -> b) -> (e, a) -> (e, b) #
Comonad w => Comonad (EnvT e w) # |
Instance details Defined in Control.Comonad.Trans.Env Methods extract :: EnvT e w a -> a # duplicate :: EnvT e w a -> EnvT e w (EnvT e w a) # extend :: (EnvT e w a -> b) -> EnvT e w a -> EnvT e w b #
Comonad w => Comonad (StoreT s w) # |
Instance details Defined in Control.Comonad.Trans.Store Methods extract :: StoreT s w a -> a # duplicate :: StoreT s w a -> StoreT s w (StoreT s w a) # extend :: (StoreT s w a -> b) -> StoreT s w a -> StoreT s w b #
(Comonad w, Monoid m) => Comonad (TracedT m w) # |
Instance details Defined in Control.Comonad.Trans.Traced Methods extract :: TracedT m w a -> a # duplicate :: TracedT m w a -> TracedT m w (TracedT m w a) # extend :: (TracedT m w a -> b) -> TracedT m w a -> TracedT m w b #
Comonad (Tagged s) # |
Instance details Defined in Control.Comonad Methods extract :: Tagged s a -> a # duplicate :: Tagged s a -> Tagged s (Tagged s a) # extend :: (Tagged s a -> b) -> Tagged s a -> Tagged s b #
Comonad w => Comonad (IdentityT w) # |
Instance details Defined in Control.Comonad Methods extract :: IdentityT w a -> a # duplicate :: IdentityT w a -> IdentityT w (IdentityT w a) # extend :: (IdentityT w a -> b) -> IdentityT w a -> IdentityT w b #
(Comonad f, Comonad g) => Comonad (Sum f g) # |
Instance details Defined in Control.Comonad Methods extract :: Sum f g a -> a # duplicate :: Sum f g a -> Sum f g (Sum f g a) # extend :: (Sum f g a -> b) -> Sum f g a -> Sum f g b #
Monoid m => Comonad ((->) m) # |
Instance details Defined in Control.Comonad Methods extract :: (m -> a) -> a # duplicate :: (m -> a) -> m -> (m -> a) # extend :: ((m -> a) -> b) -> (m -> a) -> m -> b #

liftW :: Comonad w => (a -> b) -> w a -> w b #

A suitable default definition for `fmap` for a `Comonad`. Promotes a function to a comonad.

You can only safely use `liftW` to define `fmap` if your `Comonad` defines `extend`, not just `duplicate`, since defining `extend` in terms of duplicate uses `fmap`!

    fmap f = liftW f = extend (f . extract)

wfix :: Comonad w => w (w a -> a) -> a #

Comonadic fixed point à la David Menendez

cfix :: Comonad w => (w a -> a) -> w a #

Comonadic fixed point à la Dominic Orchard

kfix :: ComonadApply w => w (w a -> a) -> w a #

Comonadic fixed point à la Kenneth Foner:

This is the `evaluate` function from his "Getting a Quick Fix on Comonads" talk.

(=>=) :: Comonad w => (w a -> b) -> (w b -> c) -> w a -> c infixr 1 #

Left-to-right `Cokleisli` composition

(=<=) :: Comonad w => (w b -> c) -> (w a -> b) -> w a -> c infixr 1 #

Right-to-left `Cokleisli` composition

(<<=) :: Comonad w => (w a -> b) -> w a -> w b infixr 1 #

`extend` in operator form

(=>>) :: Comonad w => w a -> (w a -> b) -> w b infixl 1 #

`extend` with the arguments swapped. Dual to `>>=` for a `Monad`.

# Combining Comonads

class Comonad w => ComonadApply (w :: Type -> Type) where #

`ComonadApply` is to `Comonad` like `Applicative` is to `Monad`.

Mathematically, it is a strong lax symmetric semi-monoidal comonad on the category `Hask` of Haskell types. That it to say that `w` is a strong lax symmetric semi-monoidal functor on Hask, where both `extract` and `duplicate` are symmetric monoidal natural transformations.

Laws:

    (.) <$> u <@> v <@> w = u <@> (v <@> w)
    extract (p <@> q) = extract p (extract q)
    duplicate (p <@> q) = (<@>) <$> duplicate p <@> duplicate q

If our type is both a `ComonadApply` and `Applicative` we further require

    (<*>) = (<@>)

Finally, if you choose to define (`<@`) and (`@>`), the results of your definitions should match the following laws:

    a @> b = const id <$> a <@> b
    a <@ b = const <$> a <@> b

Minimal complete definition

Nothing

Methods

(<@>) :: w (a -> b) -> w a -> w b infixl 4 #

default (<@>) :: Applicative w => w (a -> b) -> w a -> w b #

(@>) :: w a -> w b -> w b infixl 4 #

(<@) :: w a -> w b -> w a infixl 4 #

#### Instances

Instances details

ComonadApply Tree # |
Instance details Defined in Control.Comonad Methods (<@>) :: Tree (a -> b) -> Tree a -> Tree b # (@>) :: Tree a -> Tree b -> Tree b # (<@) :: Tree a -> Tree b -> Tree a #
ComonadApply NonEmpty # |
Instance details Defined in Control.Comonad Methods (<@>) :: NonEmpty (a -> b) -> NonEmpty a -> NonEmpty b # (@>) :: NonEmpty a -> NonEmpty b -> NonEmpty b # (<@) :: NonEmpty a -> NonEmpty b -> NonEmpty a #
ComonadApply Identity # |
Instance details Defined in Control.Comonad Methods (<@>) :: Identity (a -> b) -> Identity a -> Identity b # (@>) :: Identity a -> Identity b -> Identity b # (<@) :: Identity a -> Identity b -> Identity a #
Semigroup m => ComonadApply ((,) m) # |
Instance details Defined in Control.Comonad Methods (<@>) :: (m, a -> b) -> (m, a) -> (m, b) # (@>) :: (m, a) -> (m, b) -> (m, b) # (<@) :: (m, a) -> (m, b) -> (m, a) #
(Semigroup e, ComonadApply w) => ComonadApply (EnvT e w) # |
Instance details Defined in Control.Comonad.Trans.Env Methods (<@>) :: EnvT e w (a -> b) -> EnvT e w a -> EnvT e w b # (@>) :: EnvT e w a -> EnvT e w b -> EnvT e w b # (<@) :: EnvT e w a -> EnvT e w b -> EnvT e w a #
(ComonadApply w, Semigroup s) => ComonadApply (StoreT s w) # |
Instance details Defined in Control.Comonad.Trans.Store Methods (<@>) :: StoreT s w (a -> b) -> StoreT s w a -> StoreT s w b # (@>) :: StoreT s w a -> StoreT s w b -> StoreT s w b # (<@) :: StoreT s w a -> StoreT s w b -> StoreT s w a #
(ComonadApply w, Monoid m) => ComonadApply (TracedT m w) # |
Instance details Defined in Control.Comonad.Trans.Traced Methods (<@>) :: TracedT m w (a -> b) -> TracedT m w a -> TracedT m w b # (@>) :: TracedT m w a -> TracedT m w b -> TracedT m w b # (<@) :: TracedT m w a -> TracedT m w b -> TracedT m w a #
ComonadApply w => ComonadApply (IdentityT w) # |
Instance details Defined in Control.Comonad Methods (<@>) :: IdentityT w (a -> b) -> IdentityT w a -> IdentityT w b # (@>) :: IdentityT w a -> IdentityT w b -> IdentityT w b # (<@) :: IdentityT w a -> IdentityT w b -> IdentityT w a #
Monoid m => ComonadApply ((->) m) # |
Instance details Defined in Control.Comonad Methods (<@>) :: (m -> (a -> b)) -> (m -> a) -> m -> b # (@>) :: (m -> a) -> (m -> b) -> m -> b # (<@) :: (m -> a) -> (m -> b) -> m -> a #

(<@@>) :: ComonadApply w => w a -> w (a -> b) -> w b infixl 4 #

A variant of `<@>` with the arguments reversed.

liftW2 :: ComonadApply w => (a -> b -> c) -> w a -> w b -> w c #

Lift a binary function into a `Comonad` with zipping

liftW3 :: ComonadApply w => (a -> b -> c -> d) -> w a -> w b -> w c -> w d #

Lift a ternary function into a `Comonad` with zipping

# Cokleisli Arrows

newtype Cokleisli (w :: k -> Type) (a :: k) b #

The `Cokleisli` `Arrow`s of a given `Comonad`

Constructors

Cokleisli |
Fields runCokleisli :: w a -> b

#### Instances

Instances details

Comonad w => Category (Cokleisli w :: Type -> Type -> Type) # |
Instance details Defined in Control.Comonad Methods id :: Cokleisli w a a # (.) :: Cokleisli w b c -> Cokleisli w a b -> Cokleisli w a c #
Comonad w => Arrow (Cokleisli w) # |
Instance details Defined in Control.Comonad Methods arr :: (b -> c) -> Cokleisli w b c # first :: Cokleisli w b c -> Cokleisli w (b, d) (c, d) # second :: Cokleisli w b c -> Cokleisli w (d, b) (d, c) # (***) :: Cokleisli w b c -> Cokleisli w b' c' -> Cokleisli w (b, b') (c, c') # (&&&) :: Cokleisli w b c -> Cokleisli w b c' -> Cokleisli w b (c, c') #
Comonad w => ArrowApply (Cokleisli w) # |
Instance details Defined in Control.Comonad Methods app :: Cokleisli w (Cokleisli w b c, b) c #
Comonad w => ArrowChoice (Cokleisli w) # |
Instance details Defined in Control.Comonad Methods left :: Cokleisli w b c -> Cokleisli w (Either b d) (Either c d) # right :: Cokleisli w b c -> Cokleisli w (Either d b) (Either d c) # (+++) :: Cokleisli w b c -> Cokleisli w b' c' -> Cokleisli w (Either b b') (Either c c') # (|||) :: Cokleisli w b d -> Cokleisli w c d -> Cokleisli w (Either b c) d #
ComonadApply w => ArrowLoop (Cokleisli w) # |
Instance details Defined in Control.Comonad Methods loop :: Cokleisli w (b, d) (c, d) -> Cokleisli w b c #
Applicative (Cokleisli w a) # |
Instance details Defined in Control.Comonad Methods pure :: a0 -> Cokleisli w a a0 # (<*>) :: Cokleisli w a (a0 -> b) -> Cokleisli w a a0 -> Cokleisli w a b # liftA2 :: (a0 -> b -> c) -> Cokleisli w a a0 -> Cokleisli w a b -> Cokleisli w a c # (*>) :: Cokleisli w a a0 -> Cokleisli w a b -> Cokleisli w a b # (<*) :: Cokleisli w a a0 -> Cokleisli w a b -> Cokleisli w a a0 #
Functor (Cokleisli w a) # |
Instance details Defined in Control.Comonad Methods fmap :: (a0 -> b) -> Cokleisli w a a0 -> Cokleisli w a b # (<$) :: a0 -> Cokleisli w a b -> Cokleisli w a a0 #
Monad (Cokleisli w a) # |
Instance details Defined in Control.Comonad Methods (>>=) :: Cokleisli w a a0 -> (a0 -> Cokleisli w a b) -> Cokleisli w a b # (>>) :: Cokleisli w a a0 -> Cokleisli w a b -> Cokleisli w a b # return :: a0 -> Cokleisli w a a0 #

# Functors

class Functor (f :: Type -> Type) where #

A type `f` is a Functor if it provides a function `fmap` which, given any types `a` and `b` lets you apply any function from `(a -> b)` to turn an `f a` into an `f b`, preserving the structure of `f`. Furthermore `f` needs to adhere to the following:

Identity
  ``fmap` `id` == `id``
Composition
  ``fmap` (f . g) == `fmap` f . `fmap` g`

Note, that the second law follows from the free theorem of the type `fmap` and the first law, so you need only check that the former condition holds. See these articles by School of Haskell or David Luposchainsky for an explanation.

Minimal complete definition

fmap

Methods

fmap :: (a -> b) -> f a -> f b #

`fmap` is used to apply a function of type `(a -> b)` to a value of type `f a`, where f is a functor, to produce a value of type `f b`. Note that for any type constructor with more than one parameter (e.g., `Either`), only the last type parameter can be modified with `fmap` (e.g., `b` in `Either a b`).

Some type constructors with two parameters or more have a ``Bifunctor`` instance that allows both the last and the penultimate parameters to be mapped over.

#### Examples

Expand

Convert from a ``Maybe` Int` to a `Maybe String` using `show`:

    >>> fmap show Nothing
    Nothing
    >>> fmap show (Just 3)
    Just "3"

Convert from an ``Either` Int Int` to an `Either Int String` using `show`:

    >>> fmap show (Left 17)
    Left 17
    >>> fmap show (Right 17)
    Right "17"

Double each element of a list:

    >>> fmap (*2) [1,2,3]
    [2,4,6]

Apply `even` to the second element of a pair:

    >>> fmap even (2,2)
    (2,True)

It may seem surprising that the function is only applied to the last element of the tuple compared to the list example above which applies it to every element in the list. To understand, remember that tuples are type constructors with multiple type parameters: a tuple of 3 elements `(a,b,c)` can also be written `(,,) a b c` and its `Functor` instance is defined for `Functor ((,,) a b)` (i.e., only the third parameter is free to be mapped over with `fmap`).

It explains why `fmap` can be used with tuples containing values of different types as in the following example:

    >>> fmap even ("hello", 1.0, 4)
    ("hello",1.0,True)

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

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

#### Examples

Expand

Perform a computation with `Maybe` and replace the result with a constant value if it is `Just`:

    >>> 'a' <$ Just 2
    Just 'a'
    >>> 'a' <$ Nothing
    Nothing

#### Instances

Instances details

Functor Complex | Since: base-4.9.0.0
Instance details Defined in Data.Complex Methods fmap :: (a -> b) -> Complex a -> Complex b # (<$) :: a -> Complex b -> Complex a #
Functor First | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods fmap :: (a -> b) -> First a -> First b # (<$) :: a -> First b -> First a #
Functor Last | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods fmap :: (a -> b) -> Last a -> Last b # (<$) :: a -> Last b -> Last a #
Functor Max | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods fmap :: (a -> b) -> Max a -> Max b # (<$) :: a -> Max b -> Max a #
Functor Min | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods fmap :: (a -> b) -> Min a -> Min b # (<$) :: a -> Min b -> Min a #
Functor ArgDescr | Since: base-4.7.0.0
Instance details Defined in System.Console.GetOpt Methods fmap :: (a -> b) -> ArgDescr a -> ArgDescr b # (<$) :: a -> ArgDescr b -> ArgDescr a #
Functor ArgOrder | Since: base-4.7.0.0
Instance details Defined in System.Console.GetOpt Methods fmap :: (a -> b) -> ArgOrder a -> ArgOrder b # (<$) :: a -> ArgOrder b -> ArgOrder a #
Functor OptDescr | Since: base-4.7.0.0
Instance details Defined in System.Console.GetOpt Methods fmap :: (a -> b) -> OptDescr a -> OptDescr b # (<$) :: a -> OptDescr b -> OptDescr a #
Functor SCC | Since: containers-0.5.4
Instance details Defined in Data.Graph Methods fmap :: (a -> b) -> SCC a -> SCC b # (<$) :: a -> SCC b -> SCC a #
Functor IntMap |
Instance details Defined in Data.IntMap.Internal Methods fmap :: (a -> b) -> IntMap a -> IntMap b # (<$) :: a -> IntMap b -> IntMap a #
Functor Digit |
Instance details Defined in Data.Sequence.Internal Methods fmap :: (a -> b) -> Digit a -> Digit b # (<$) :: a -> Digit b -> Digit a #
Functor Elem |
Instance details Defined in Data.Sequence.Internal Methods fmap :: (a -> b) -> Elem a -> Elem b # (<$) :: a -> Elem b -> Elem a #
Functor FingerTree |
Instance details Defined in Data.Sequence.Internal Methods fmap :: (a -> b) -> FingerTree a -> FingerTree b # (<$) :: a -> FingerTree b -> FingerTree a #
Functor Node |
Instance details Defined in Data.Sequence.Internal Methods fmap :: (a -> b) -> Node a -> Node b # (<$) :: a -> Node b -> Node a #
Functor Seq |
Instance details Defined in Data.Sequence.Internal Methods fmap :: (a -> b) -> Seq a -> Seq b # (<$) :: a -> Seq b -> Seq a #
Functor ViewL |
Instance details Defined in Data.Sequence.Internal Methods fmap :: (a -> b) -> ViewL a -> ViewL b # (<$) :: a -> ViewL b -> ViewL a #
Functor ViewR |
Instance details Defined in Data.Sequence.Internal Methods fmap :: (a -> b) -> ViewR a -> ViewR b # (<$) :: a -> ViewR b -> ViewR a #
Functor Tree |
Instance details Defined in Data.Tree Methods fmap :: (a -> b) -> Tree a -> Tree b # (<$) :: a -> Tree b -> Tree a #
Functor NonEmpty | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Base Methods fmap :: (a -> b) -> NonEmpty a -> NonEmpty b # (<$) :: a -> NonEmpty b -> NonEmpty a #
Functor Identity | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Data.Functor.Identity Methods fmap :: (a -> b) -> Identity a -> Identity b # (<$) :: a -> Identity b -> Identity a #
Functor First | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Data.Monoid Methods fmap :: (a -> b) -> First a -> First b # (<$) :: a -> First b -> First a #
Functor Last | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Data.Monoid Methods fmap :: (a -> b) -> Last a -> Last b # (<$) :: a -> Last b -> Last a #
Functor Dual | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Data.Semigroup.Internal Methods fmap :: (a -> b) -> Dual a -> Dual b # (<$) :: a -> Dual b -> Dual a #
Functor Product | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Data.Semigroup.Internal Methods fmap :: (a -> b) -> Product a -> Product b # (<$) :: a -> Product b -> Product a #
Functor Sum | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Data.Semigroup.Internal Methods fmap :: (a -> b) -> Sum a -> Sum b # (<$) :: a -> Sum b -> Sum a #
Functor Par1 | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> Par1 a -> Par1 b # (<$) :: a -> Par1 b -> Par1 a #
Functor IO | @since base-2.01
Instance details Defined in GHC.Internal.Base Methods fmap :: (a -> b) -> IO a -> IO b # (<$) :: a -> IO b -> IO a #
Functor AnnotDetails |
Instance details Defined in Text.PrettyPrint.Annotated.HughesPJ Methods fmap :: (a -> b) -> AnnotDetails a -> AnnotDetails b # (<$) :: a -> AnnotDetails b -> AnnotDetails a #
Functor Doc |
Instance details Defined in Text.PrettyPrint.Annotated.HughesPJ Methods fmap :: (a -> b) -> Doc a -> Doc b # (<$) :: a -> Doc b -> Doc a #
Functor Span |
Instance details Defined in Text.PrettyPrint.Annotated.HughesPJ Methods fmap :: (a -> b) -> Span a -> Span b # (<$) :: a -> Span b -> Span a #
Functor Q |
Instance details Defined in Language.Haskell.TH.Syntax Methods fmap :: (a -> b) -> Q a -> Q b # (<$) :: a -> Q b -> Q a #
Functor TyVarBndr |
Instance details Defined in Language.Haskell.TH.Syntax Methods fmap :: (a -> b) -> TyVarBndr a -> TyVarBndr b # (<$) :: a -> TyVarBndr b -> TyVarBndr a #
Functor Maybe | @since base-2.01
Instance details Defined in GHC.Internal.Base Methods fmap :: (a -> b) -> Maybe a -> Maybe b # (<$) :: a -> Maybe b -> Maybe a #
Functor Solo | @since base-4.15
Instance details Defined in GHC.Internal.Base Methods fmap :: (a -> b) -> Solo a -> Solo b # (<$) :: a -> Solo b -> Solo a #
Functor [] | @since base-2.01
Instance details Defined in GHC.Internal.Base Methods fmap :: (a -> b) -> [a] -> [b] # (<$) :: a -> [b] -> [a] #
Monad m => Functor (WrappedMonad m) | Since: base-2.1
Instance details Defined in Control.Applicative Methods fmap :: (a -> b) -> WrappedMonad m a -> WrappedMonad m b # (<$) :: a -> WrappedMonad m b -> WrappedMonad m a #
Functor (Arg a) | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods fmap :: (a0 -> b) -> Arg a a0 -> Arg a b # (<$) :: a0 -> Arg a b -> Arg a a0 #
Functor (SetM s) |
Instance details Defined in Data.Graph Methods fmap :: (a -> b) -> SetM s a -> SetM s b # (<$) :: a -> SetM s b -> SetM s a #
Functor (Map k) |
Instance details Defined in Data.Map.Internal Methods fmap :: (a -> b) -> Map k a -> Map k b # (<$) :: a -> Map k b -> Map k a #
Functor (Array i) | @since base-2.01
Instance details Defined in GHC.Internal.Arr Methods fmap :: (a -> b) -> Array i a -> Array i b # (<$) :: a -> Array i b -> Array i a #
Arrow a => Functor (ArrowMonad a) | @since base-4.6.0.0
Instance details Defined in GHC.Internal.Control.Arrow Methods fmap :: (a0 -> b) -> ArrowMonad a a0 -> ArrowMonad a b # (<$) :: a0 -> ArrowMonad a b -> ArrowMonad a a0 #
Functor (Either a) | @since base-3.0
Instance details Defined in GHC.Internal.Data.Either Methods fmap :: (a0 -> b) -> Either a a0 -> Either a b # (<$) :: a0 -> Either a b -> Either a a0 #
Functor (U1 :: Type -> Type) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> U1 a -> U1 b # (<$) :: a -> U1 b -> U1 a #
Functor (V1 :: Type -> Type) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> V1 a -> V1 b # (<$) :: a -> V1 b -> V1 a #
Functor f => Functor (Lift f) |
Instance details Defined in Control.Applicative.Lift Methods fmap :: (a -> b) -> Lift f a -> Lift f b # (<$) :: a -> Lift f b -> Lift f a #
Functor m => Functor (MaybeT m) |
Instance details Defined in Control.Monad.Trans.Maybe Methods fmap :: (a -> b) -> MaybeT m a -> MaybeT m b # (<$) :: a -> MaybeT m b -> MaybeT m a #
Functor ((,) a) | @since base-2.01
Instance details Defined in GHC.Internal.Base Methods fmap :: (a0 -> b) -> (a, a0) -> (a, b) # (<$) :: a0 -> (a, b) -> (a, a0) #
Arrow a => Functor (WrappedArrow a b) | Since: base-2.1
Instance details Defined in Control.Applicative Methods fmap :: (a0 -> b0) -> WrappedArrow a b a0 -> WrappedArrow a b b0 # (<$) :: a0 -> WrappedArrow a b b0 -> WrappedArrow a b a0 #
Functor w => Functor (EnvT e w) # |
Instance details Defined in Control.Comonad.Trans.Env Methods fmap :: (a -> b) -> EnvT e w a -> EnvT e w b # (<$) :: a -> EnvT e w b -> EnvT e w a #
Functor w => Functor (StoreT s w) # |
Instance details Defined in Control.Comonad.Trans.Store Methods fmap :: (a -> b) -> StoreT s w a -> StoreT s w b # (<$) :: a -> StoreT s w b -> StoreT s w a #
Functor w => Functor (TracedT m w) # |
Instance details Defined in Control.Comonad.Trans.Traced Methods fmap :: (a -> b) -> TracedT m w a -> TracedT m w b # (<$) :: a -> TracedT m w b -> TracedT m w a #
(Applicative f, Monad f) => Functor (WhenMissing f x) | Since: containers-0.5.9
Instance details Defined in Data.IntMap.Internal Methods fmap :: (a -> b) -> WhenMissing f x a -> WhenMissing f x b # (<$) :: a -> WhenMissing f x b -> WhenMissing f x a #
Functor m => Functor (Kleisli m a) | @since base-4.14.0.0
Instance details Defined in GHC.Internal.Control.Arrow Methods fmap :: (a0 -> b) -> Kleisli m a a0 -> Kleisli m a b # (<$) :: a0 -> Kleisli m a b -> Kleisli m a a0 #
Functor f => Functor (Ap f) | @since base-4.12.0.0
Instance details Defined in GHC.Internal.Data.Monoid Methods fmap :: (a -> b) -> Ap f a -> Ap f b # (<$) :: a -> Ap f b -> Ap f a #
Functor f => Functor (Alt f) | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Data.Semigroup.Internal Methods fmap :: (a -> b) -> Alt f a -> Alt f b # (<$) :: a -> Alt f b -> Alt f a #
(Generic1 f, Functor (Rep1 f)) => Functor (Generically1 f) | @since base-4.17.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> Generically1 f a -> Generically1 f b # (<$) :: a -> Generically1 f b -> Generically1 f a #
Functor f => Functor (Rec1 f) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> Rec1 f a -> Rec1 f b # (<$) :: a -> Rec1 f b -> Rec1 f a #
Functor (URec (Ptr ()) :: Type -> Type) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> URec (Ptr ()) a -> URec (Ptr ()) b # (<$) :: a -> URec (Ptr ()) b -> URec (Ptr ()) a #
Functor (URec Char :: Type -> Type) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> URec Char a -> URec Char b # (<$) :: a -> URec Char b -> URec Char a #
Functor (URec Double :: Type -> Type) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> URec Double a -> URec Double b # (<$) :: a -> URec Double b -> URec Double a #
Functor (URec Float :: Type -> Type) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> URec Float a -> URec Float b # (<$) :: a -> URec Float b -> URec Float a #
Functor (URec Int :: Type -> Type) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> URec Int a -> URec Int b # (<$) :: a -> URec Int b -> URec Int a #
Functor (URec Word :: Type -> Type) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> URec Word a -> URec Word b # (<$) :: a -> URec Word b -> URec Word a #
Functor f => Functor (Indexing f) |
Instance details Defined in WithIndex Methods fmap :: (a -> b) -> Indexing f a -> Indexing f b # (<$) :: a -> Indexing f b -> Indexing f a #
Functor (Tagged s) |
Instance details Defined in Data.Tagged Methods fmap :: (a -> b) -> Tagged s a -> Tagged s b # (<$) :: a -> Tagged s b -> Tagged s a #
Functor f => Functor (Backwards f) | Derived instance.
Instance details Defined in Control.Applicative.Backwards Methods fmap :: (a -> b) -> Backwards f a -> Backwards f b # (<$) :: a -> Backwards f b -> Backwards f a #
Functor m => Functor (AccumT w m) |
Instance details Defined in Control.Monad.Trans.Accum Methods fmap :: (a -> b) -> AccumT w m a -> AccumT w m b # (<$) :: a -> AccumT w m b -> AccumT w m a #
Functor m => Functor (ExceptT e m) |
Instance details Defined in Control.Monad.Trans.Except Methods fmap :: (a -> b) -> ExceptT e m a -> ExceptT e m b # (<$) :: a -> ExceptT e m b -> ExceptT e m a #
Functor m => Functor (IdentityT m) |
Instance details Defined in Control.Monad.Trans.Identity Methods fmap :: (a -> b) -> IdentityT m a -> IdentityT m b # (<$) :: a -> IdentityT m b -> IdentityT m a #
Functor m => Functor (ReaderT r m) |
Instance details Defined in Control.Monad.Trans.Reader Methods fmap :: (a -> b) -> ReaderT r m a -> ReaderT r m b # (<$) :: a -> ReaderT r m b -> ReaderT r m a #
Functor m => Functor (SelectT r m) |
Instance details Defined in Control.Monad.Trans.Select Methods fmap :: (a -> b) -> SelectT r m a -> SelectT r m b # (<$) :: a -> SelectT r m b -> SelectT r m a #
Functor m => Functor (StateT s m) |
Instance details Defined in Control.Monad.Trans.State.Lazy Methods fmap :: (a -> b) -> StateT s m a -> StateT s m b # (<$) :: a -> StateT s m b -> StateT s m a #
Functor m => Functor (StateT s m) |
Instance details Defined in Control.Monad.Trans.State.Strict Methods fmap :: (a -> b) -> StateT s m a -> StateT s m b # (<$) :: a -> StateT s m b -> StateT s m a #
Functor m => Functor (WriterT w m) |
Instance details Defined in Control.Monad.Trans.Writer.CPS Methods fmap :: (a -> b) -> WriterT w m a -> WriterT w m b # (<$) :: a -> WriterT w m b -> WriterT w m a #
Functor m => Functor (WriterT w m) |
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods fmap :: (a -> b) -> WriterT w m a -> WriterT w m b # (<$) :: a -> WriterT w m b -> WriterT w m a #
Functor m => Functor (WriterT w m) |
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods fmap :: (a -> b) -> WriterT w m a -> WriterT w m b # (<$) :: a -> WriterT w m b -> WriterT w m a #
Functor (Constant a :: Type -> Type) |
Instance details Defined in Data.Functor.Constant Methods fmap :: (a0 -> b) -> Constant a a0 -> Constant a b # (<$) :: a0 -> Constant a b -> Constant a a0 #
Functor f => Functor (Reverse f) | Derived instance.
Instance details Defined in Data.Functor.Reverse Methods fmap :: (a -> b) -> Reverse f a -> Reverse f b # (<$) :: a -> Reverse f b -> Reverse f a #
Functor ((,,) a b) | @since base-4.14.0.0
Instance details Defined in GHC.Internal.Base Methods fmap :: (a0 -> b0) -> (a, b, a0) -> (a, b, b0) # (<$) :: a0 -> (a, b, b0) -> (a, b, a0) #
(Functor f, Functor g) => Functor (Product f g) | Since: base-4.9.0.0
Instance details Defined in Data.Functor.Product Methods fmap :: (a -> b) -> Product f g a -> Product f g b # (<$) :: a -> Product f g b -> Product f g a #
(Functor f, Functor g) => Functor (Sum f g) | Since: base-4.9.0.0
Instance details Defined in Data.Functor.Sum Methods fmap :: (a -> b) -> Sum f g a -> Sum f g b # (<$) :: a -> Sum f g b -> Sum f g a #
Functor (Cokleisli w a) # |
Instance details Defined in Control.Comonad Methods fmap :: (a0 -> b) -> Cokleisli w a a0 -> Cokleisli w a b # (<$) :: a0 -> Cokleisli w a b -> Cokleisli w a a0 #
Functor f => Functor (WhenMatched f x y) | Since: containers-0.5.9
Instance details Defined in Data.IntMap.Internal Methods fmap :: (a -> b) -> WhenMatched f x y a -> WhenMatched f x y b # (<$) :: a -> WhenMatched f x y b -> WhenMatched f x y a #
(Applicative f, Monad f) => Functor (WhenMissing f k x) | Since: containers-0.5.9
Instance details Defined in Data.Map.Internal Methods fmap :: (a -> b) -> WhenMissing f k x a -> WhenMissing f k x b # (<$) :: a -> WhenMissing f k x b -> WhenMissing f k x a #
(Functor f, Functor g) => Functor (f :*: g) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> (f :*: g) a -> (f :*: g) b # (<$) :: a -> (f :*: g) b -> (f :*: g) a #
(Functor f, Functor g) => Functor (f :+: g) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> (f :+: g) a -> (f :+: g) b # (<$) :: a -> (f :+: g) b -> (f :+: g) a #
Functor (K1 i c :: Type -> Type) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> K1 i c a -> K1 i c b # (<$) :: a -> K1 i c b -> K1 i c a #
Functor (ContT r m) |
Instance details Defined in Control.Monad.Trans.Cont Methods fmap :: (a -> b) -> ContT r m a -> ContT r m b # (<$) :: a -> ContT r m b -> ContT r m a #
Functor ((,,,) a b c) | @since base-4.14.0.0
Instance details Defined in GHC.Internal.Base Methods fmap :: (a0 -> b0) -> (a, b, c, a0) -> (a, b, c, b0) # (<$) :: a0 -> (a, b, c, b0) -> (a, b, c, a0) #
Functor ((->) r) | @since base-2.01
Instance details Defined in GHC.Internal.Base Methods fmap :: (a -> b) -> (r -> a) -> r -> b # (<$) :: a -> (r -> b) -> r -> a #
(Functor f, Functor g) => Functor (Compose f g) | Since: base-4.9.0.0
Instance details Defined in Data.Functor.Compose Methods fmap :: (a -> b) -> Compose f g a -> Compose f g b # (<$) :: a -> Compose f g b -> Compose f g a #
Functor f => Functor (WhenMatched f k x y) | Since: containers-0.5.9
Instance details Defined in Data.Map.Internal Methods fmap :: (a -> b) -> WhenMatched f k x y a -> WhenMatched f k x y b # (<$) :: a -> WhenMatched f k x y b -> WhenMatched f k x y a #
(Functor f, Functor g) => Functor (f :.: g) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> (f :.: g) a -> (f :.: g) b # (<$) :: a -> (f :.: g) b -> (f :.: g) a #
Functor f => Functor (M1 i c f) | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Generics Methods fmap :: (a -> b) -> M1 i c f a -> M1 i c f b # (<$) :: a -> M1 i c f b -> M1 i c f a #
Functor m => Functor (RWST r w s m) |
Instance details Defined in Control.Monad.Trans.RWS.CPS Methods fmap :: (a -> b) -> RWST r w s m a -> RWST r w s m b # (<$) :: a -> RWST r w s m b -> RWST r w s m a #
Functor m => Functor (RWST r w s m) |
Instance details Defined in Control.Monad.Trans.RWS.Lazy Methods fmap :: (a -> b) -> RWST r w s m a -> RWST r w s m b # (<$) :: a -> RWST r w s m b -> RWST r w s m a #
Functor m => Functor (RWST r w s m) |
Instance details Defined in Control.Monad.Trans.RWS.Strict Methods fmap :: (a -> b) -> RWST r w s m a -> RWST r w s m b # (<$) :: a -> RWST r w s m b -> RWST r w s m a #
Functor ((,,,,) a b c d) | @since base-4.18.0.0
Instance details Defined in GHC.Internal.Base Methods fmap :: (a0 -> b0) -> (a, b, c, d, a0) -> (a, b, c, d, b0) # (<$) :: a0 -> (a, b, c, d, b0) -> (a, b, c, d, a0) #
Functor ((,,,,,) a b c d e) | @since base-4.18.0.0
Instance details Defined in GHC.Internal.Base Methods fmap :: (a0 -> b0) -> (a, b, c, d, e, a0) -> (a, b, c, d, e, b0) # (<$) :: a0 -> (a, b, c, d, e, b0) -> (a, b, c, d, e, a0) #
Functor ((,,,,,,) a b c d e f) | @since base-4.18.0.0
Instance details Defined in GHC.Internal.Base Methods fmap :: (a0 -> b0) -> (a, b, c, d, e, f, a0) -> (a, b, c, d, e, f, b0) # (<$) :: a0 -> (a, b, c, d, e, f, b0) -> (a, b, c, d, e, f, a0) #

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

An infix synonym for `fmap`.

The name of this operator is an allusion to `$`. Note the similarities between their types:

     ($)  ::              (a -> b) ->   a ->   b
    (<$>) :: Functor f => (a -> b) -> f a -> f b

Whereas `$` is function application, `<$>` is function application lifted over a `Functor`.

#### Examples

Expand

Convert from a ``Maybe` `Int`` to a ``Maybe` `String`` using `show`:

    >>> show <$> Nothing
    Nothing

    >>> show <$> Just 3
    Just "3"

Convert from an ``Either` `Int` `Int`` to an ``Either` `Int`` `String` using `show`:

    >>> show <$> Left 17
    Left 17

    >>> show <$> Right 17
    Right "17"

Double each element of a list:

    >>> (*2) <$> [1,2,3]
    [2,4,6]

Apply `even` to the second element of a pair:

    >>> even <$> (2,2)
    (2,True)

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

Flipped version of `<$`.

@since base-4.7.0.0

#### Examples

Expand

Replace the contents of a ``Maybe` `Int`` with a constant `String`:

    >>> Nothing $> "foo"
    Nothing

    >>> Just 90210 $> "foo"
    Just "foo"

Replace the contents of an ``Either` `Int` `Int`` with a constant `String`, resulting in an ``Either` `Int` `String``:

    >>> Left 8675309 $> "foo"
    Left 8675309

    >>> Right 8675309 $> "foo"
    Right "foo"

Replace each element of a list with a constant `String`:

    >>> [1,2,3] $> "foo"
    ["foo","foo","foo"]

Replace the second element of a pair with a constant `String`:

    >>> (1,2) $> "foo"
    (1,"foo")