semigroupoids-6.0.1: Semigroupoids: Category sans id

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`` 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)

(<*) :: Apply f => f a -> f b -> f a #

Since: 5.3.6

(*>) :: Apply f => f a -> f b -> f b #

Since: 5.3.6

(<*>) :: Apply f => f (a -> b) -> f a -> f b #

Since: 5.3.6

(>>) :: Bind m => m a -> m b -> m b #

Since: 5.3.6

(>>=) :: Bind m => m a -> (a -> m b) -> m b #

Since: 5.3.6

join :: Bind m => m (m a) -> m a #

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

Lift a value into the Structure.

#### Examples

Expand

    >>> pure 1 :: Maybe Int
    Just 1

    >>> pure 'z' :: [Char]
    "z"

    >>> pure (pure ":D") :: Maybe [String]
    Just [":D"]

return :: Monad m => a -> m a #

Inject a value into the monadic type. This function should not be different from its default implementation as `pure`. The justification for the existence of this function is merely historic.

fail :: Plus m => String -> m a #

# Important note

This ignores whatever `String` you give it. It is a bad idea to use `fail` as a form of labelled error; instead, it should only be defaulted to when a pattern match fails.

Since: 5.3.6