Safe Haskell | Safe |
---|---|

Language | Haskell2010 |

This module re-exports operators from Data.Functor.Apply and
Data.Functor.Bind, but under the same
names as their `Applicative`

and `Monad`

counterparts. This makes it convenient
to use do-notation on a type that is a `Bind`

but not a monad (or an `Apply`

but not an `Applicative`

with `ApplicativeDo`

), either using the
`QualifiedDo`

extension or the more traditional `RebindableSyntax`

.

{-# LANGUAGE ApplicativeDo #-} {-# LANGUAGE QualifiedDo #-} foo :: Apply f => f a -> f b -> f (a, b) foo as bs = Semi.do a <- as b <- bs pure (a, b) bar :: Bind m => (a -> b -> m c) -> m a -> m b -> m c bar f as bs = Semi.do a <- as b <- bs f a b

## Synopsis

- fmap :: Functor f => (a -> b) -> f a -> f b
- (<*) :: Apply f => f a -> f b -> f a
- (*>) :: Apply f => f a -> f b -> f b
- (<*>) :: Apply f => f (a -> b) -> f a -> f b
- (>>) :: Bind m => m a -> m b -> m b
- (>>=) :: Bind m => m a -> (a -> m b) -> m b
- join :: Bind m => m (m a) -> m a
- pure :: Applicative f => a -> f a
- return :: Monad m => a -> m a
- fail :: Plus m => String -> m a

# Documentation

fmap :: Functor f => (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

instance that allows
both the last and the penultimate parameters to be mapped over.`Bifunctor`

#### Examples

Convert from a

to a `Maybe`

Int`Maybe String`

using `show`

:

`>>>`

Nothing`fmap show Nothing`

`>>>`

Just "3"`fmap show (Just 3)`

Convert from an

to an
`Either`

Int Int`Either Int String`

using `show`

:

`>>>`

Left 17`fmap show (Left 17)`

`>>>`

Right "17"`fmap show (Right 17)`

Double each element of a list:

`>>>`

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

Apply `even`

to the second element of a pair:

`>>>`

(2,True)`fmap even (2,2)`

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:

`>>>`

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

pure :: Applicative f => a -> f a #

Lift a value.