| Safe Haskell | Safe |
|---|---|
| Language | Haskell2010 |
Semigroupoids.Do
Description
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 Bifunctor
Examples
Convert from a Maybe IntMaybe String
using show:
>>>fmap show NothingNothing>>>fmap show (Just 3)Just "3"
Convert from an Either Int IntEither 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)
pure :: Applicative f => a -> f a #
Lift a value.