base-4.3.1.0: Basic libraries

Portabilityportable
Stabilityprovisional
Maintainer[email protected]

Control.Monad.Instances

Description

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

Synopsis

Documentation

class Functor f whereSource

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, Data.Maybe.Maybe and System.IO.IO satisfy these laws.

Methods

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

Instances

Functor [] 
Functor IO 
Functor [::] 
Functor Maybe 
Functor ReadP 
Functor ReadPrec 
Functor STM 
Functor ZipList 
Functor Id 
Functor ((->) r) 
Functor (Either a) 
Functor ((,) a) 
Functor (ST s) 
Ix i => Functor (Array i) 
Monad m => Functor (WrappedMonad m) 
Functor (Const m) 
Functor (StateR s) 
Functor (StateL s) 
Functor (ST s) 
Arrow a => Functor (WrappedArrow a b) 

class Monad m whereSource

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.

Minimal complete definition: >>= and return.

Instances of Monad should satisfy the following laws:

 return a >>= k  ==  k a
 m >>= return  ==  m
 m >>= (\x -> k x >>= h)  ==  (m >>= k) >>= h

Instances of both Monad and Functor should additionally satisfy the law:

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

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

Methods

(>>=) :: forall a b. m a -> (a -> m b) -> m bSource

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 bSource

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

return :: a -> m aSource

Inject a value into the monadic type.

fail :: String -> m aSource

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