Safe Haskell	Safe
Language	Haskell98

Control.Monad.Compat

Synopsis

Documentation

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

(>>=) :: 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.

(>>) :: 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 P
Monad STM
Monad First
Monad Last
Monad ReadPrec
Monad ReadP
Monad Maybe
Monad ((->) r)
Monad (Either e)
Monad (ST s)
Monad (ST s)
Monad m => Monad (WrappedMonad m)
ArrowApply a => Monad (ArrowMonad a)
Monad (Proxy *)
Monad f => Monad (Alt * f)

class (Alternative m, Monad m) => MonadPlus m where

Monads that also support choice and failure.

Minimal complete definition

Nothing

Methods

mzero :: m a

the identity of mplus. It should also satisfy the equations

mzero >>= f  =  mzero
v >> mzero   =  mzero

mplus :: m a -> m a -> m a

an associative operation

Instances

MonadPlus []
MonadPlus P
MonadPlus STM
MonadPlus ReadPrec
MonadPlus ReadP
MonadPlus Maybe
(ArrowApply a, ArrowPlus a) => MonadPlus (ArrowMonad a)
MonadPlus f => MonadPlus (Alt * f)

forever :: Applicative f => f a -> f b

forever act repeats the action infinitely.

filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a]

This generalizes the list-based filter function.

mapAndUnzipM :: Applicative m => (a -> m (b, c)) -> [a] -> m ([b], [c])

The mapAndUnzipM function maps its first argument over a list, returning the result as a pair of lists. This function is mainly used with complicated data structures or a state-transforming monad.

zipWithM :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m [c]

The zipWithM function generalizes zipWith to arbitrary applicative functors.

zipWithM_ :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m ()

zipWithM_ is the extension of zipWithM which ignores the final result.

replicateM :: Applicative m => Int -> m a -> m [a]

replicateM n act performs the action n times, gathering the results.

replicateM_ :: Applicative m => Int -> m a -> m ()

Like replicateM, but discards the result.