base-4.8.2.0: Basic libraries

Copyright | (c) The University of Glasgow 2001
License | BSD-style (see the file libraries/base/LICENSE)
Maintainer | libraries@haskell.org
Stability | provisional
Portability | portable
Safe Haskell | Safe
Language | Haskell2010

Control.Monad.Instances

Description

Deprecated: This module now contains no instances and will be removed in the future

This module is DEPRECATED and will be removed in the future!

`Functor` and `Monad` instances for `(->) r` and `Functor` instances for `(,) a` and ``Either` a`.

Synopsis

- class Functor f where
  - fmap :: (a -> b) -> f a -> f b
  - (<$) :: a -> f b -> f a
- class Applicative m => Monad m where
  - (>>=) :: forall a b. m a -> (a -> m b) -> m b
  - (>>) :: forall a b. m a -> m b -> m b
  - return :: a -> m a
  - fail :: String -> m a

# Documentation

class Functor f where

The `Functor` class is used for types that can be mapped over. Instances of `Functor` should satisfy the following laws:

    fmap id  ==  id
    fmap (f . g)  ==  fmap f . fmap g

The instances of `Functor` for lists, `Maybe` and `IO` satisfy these laws.

Minimal complete definition

fmap

Methods

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

(<$) :: 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.

Instances

Functor [] |
Functor IO |
Functor Maybe |
Functor ReadP |
Functor ReadPrec |
Functor Last |
Functor First |
Functor STM |
Functor Handler |
Functor ZipList |
Functor Identity |
Functor ArgDescr |
Functor OptDescr |
Functor ArgOrder |
Functor ((->) r) |
Functor (Either a) |
Functor ((,) a) |
Functor (ST s) |
Functor (Proxy *) |
Arrow a => Functor (ArrowMonad a) |
Monad m => Functor (WrappedMonad m) |
Functor (Const m) |
Functor (ST s) |
Functor f => Functor (Alt * f) |
Arrow a => Functor (WrappedArrow a b) |

class Applicative m => Monad m where

The `Monad` class defines the basic operations over a monad, a concept from a branch of mathematics known as category theory. From the perspective of a Haskell programmer, however, it is best to think of a monad as an abstract datatype of actions. Haskell's `do` expressions provide a convenient syntax for writing monadic expressions.

Instances of `Monad` should satisfy the following laws:

-     return a >>= k  =  k a

-     m >>= return  =  m

-     m >>= (x -> k x >>= h)  =  (m >>= k) >>= h

Furthermore, the `Monad` and `Applicative` operations should relate as follows:

-     pure = return

-     (<*>) = ap

The above laws imply:

-     fmap f xs  =  xs >>= return . f

-     (>>) = (*>)

and that `pure` and (`<*>`) satisfy the applicative functor laws.

The instances of `Monad` for lists, `Maybe` and `IO` defined in the Prelude satisfy these laws.

Minimal complete definition

(>>=)

Methods

(>>=) :: forall a b. m a -> (a -> m b) -> m b infixl 1

Sequentially compose two actions, passing any value produced by the first as an argument to the second.

(>>) :: forall a b. m a -> m b -> m b infixl 1

Sequentially compose two actions, discarding any value produced by the first, like sequencing operators (such as the semicolon) in imperative languages.

return :: a -> m a

Inject a value into the monadic type.

fail :: String -> m a

Fail with a message. This operation is not part of the mathematical definition of a monad, but is invoked on pattern-match failure in a `do` expression.

Instances

Monad [] |
Monad IO |
Monad Maybe |
Monad ReadP |
Monad ReadPrec |
Monad Last |
Monad First |
Monad STM |
Monad Identity |
Monad ((->) r) |
Monad (Either e) |
Monad (ST s) |
Monad (Proxy *) |
ArrowApply a => Monad (ArrowMonad a) |
Monad m => Monad (WrappedMonad m) |
Monad (ST s) |
Monad f => Monad (Alt * f) |