Safe Haskell	None
Language	Haskell2010

Foundation.Monad

Synopsis

class Monad m => MonadIO (m :: Type -> Type) where
- liftIO :: IO a -> m a
class Monad m => MonadFailure (m :: Type -> Type) where
- type Failure (m :: Type -> Type)
- mFail :: Failure m -> m ()
class Monad m => MonadThrow (m :: Type -> Type) where
- throw :: Exception e => e -> m a
class MonadThrow m => MonadCatch (m :: Type -> Type) where
- catch :: Exception e => m a -> (e -> m a) -> m a
class MonadCatch m => MonadBracket (m :: Type -> Type) where
- generalBracket :: m a -> (a -> b -> m ignored1) -> (a -> SomeException -> m ignored2) -> (a -> m b) -> m b
class MonadTrans (trans :: (Type -> Type) -> Type -> Type) where
- lift :: Monad m => m a -> trans m a
newtype Identity a = Identity {
- runIdentity :: a
}
replicateM :: Applicative m => CountOf a -> m a -> m [a]

Documentation

class Monad m => MonadIO (m :: Type -> Type) where #

Monads in which IO computations may be embedded. Any monad built by applying a sequence of monad transformers to the IO monad will be an instance of this class.

Instances should satisfy the following laws, which state that liftIO is a transformer of monads:

```
liftIO . return = return
```

liftIO (m >>= f) = liftIO m >>= (liftIO . f)

Methods

liftIO :: IO a -> m a #

Lift a computation from the IO monad. This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations (i.e. IO is the base monad for the stack).

Example

Expand

import Control.Monad.Trans.State -- from the "transformers" library

printState :: Show s => StateT s IO ()
printState = do
  state <- get
  liftIO $ print state

Had we omitted liftIO, we would have ended up with this error:

• Couldn't match type ‘IO’ with ‘StateT s IO’
 Expected type: StateT s IO ()
   Actual type: IO ()

The important part here is the mismatch between StateT s IO () and IO ().

Luckily, we know of a function that takes an IO a and returns an (m a): liftIO, enabling us to run the program and see the expected results:

> evalStateT printState "hello"
"hello"

> evalStateT printState 3
3

Instances

Instances details

MonadIO IO	Since: base-4.9.0.0
Instance details Defined in Control.Monad.IO.Class Methods liftIO :: IO a -> IO a #
MonadIO m => MonadIO (ResourceT m) Source #
Instance details Defined in Foundation.Conduit.Internal Methods liftIO :: IO a -> ResourceT m a #
MonadIO m => MonadIO (ExceptT e m) Source #
Instance details Defined in Foundation.Monad.Except Methods liftIO :: IO a -> ExceptT e m a #
MonadIO m => MonadIO (ReaderT r m) Source #
Instance details Defined in Foundation.Monad.Reader Methods liftIO :: IO a -> ReaderT r m a #
(Functor m, MonadIO m) => MonadIO (StateT s m) Source #
Instance details Defined in Foundation.Monad.State Methods liftIO :: IO a -> StateT s m a #
MonadIO m => MonadIO (Conduit i o m) Source #
Instance details Defined in Foundation.Conduit.Internal Methods liftIO :: IO a -> Conduit i o m a #

class Monad m => MonadFailure (m :: Type -> Type) where Source #

Monad that can represent failure

Similar to MonadFail but with a parametrized Failure linked to the Monad

Associated Types

type Failure (m :: Type -> Type) Source #

The associated type with the MonadFailure, representing what failure can be encoded in this monad

Methods

mFail :: Failure m -> m () Source #

Raise a Failure through a monad.

Instances

Instances details

MonadFailure Maybe

Instance details

Defined in Basement.Monad

Associated Types

type Failure Maybe
Instance details Defined in Basement.Monad type Failure Maybe = ()

Methods

mFail :: Failure Maybe -> Maybe () Source #

MonadFailure (Either a)

Instance details

Defined in Basement.Monad

Associated Types

type Failure (Either a)
Instance details Defined in Basement.Monad type Failure (Either a) = a

Methods

mFail :: Failure (Either a) -> Either a () Source #

Monad m => MonadFailure (ExceptT e m) Source #

Instance details

Defined in Foundation.Monad.Except

Associated Types

type Failure (ExceptT e m)
Instance details Defined in Foundation.Monad.Except type Failure (ExceptT e m) = e

Methods

mFail :: Failure (ExceptT e m) -> ExceptT e m () Source #

MonadFailure m => MonadFailure (ReaderT r m) Source #

Instance details

Defined in Foundation.Monad.Reader

Associated Types

type Failure (ReaderT r m)
Instance details Defined in Foundation.Monad.Reader type Failure (ReaderT r m) = Failure m

Methods

mFail :: Failure (ReaderT r m) -> ReaderT r m () Source #

(Functor m, MonadFailure m) => MonadFailure (StateT s m) Source #

Instance details

Defined in Foundation.Monad.State

Associated Types

type Failure (StateT s m)
Instance details Defined in Foundation.Monad.State type Failure (StateT s m) = Failure m

Methods

mFail :: Failure (StateT s m) -> StateT s m () Source #

MonadFailure m => MonadFailure (Conduit i o m) Source #

Instance details

Defined in Foundation.Conduit.Internal

Associated Types

type Failure (Conduit i o m)
Instance details Defined in Foundation.Conduit.Internal type Failure (Conduit i o m) = Failure m

Methods

mFail :: Failure (Conduit i o m) -> Conduit i o m () Source #

Monad state => MonadFailure (Builder collection mutCollection step state err)

Instance details

Defined in Basement.MutableBuilder

Associated Types

type Failure (Builder collection mutCollection step state err)
Instance details Defined in Basement.MutableBuilder type Failure (Builder collection mutCollection step state err) = err

Methods

mFail :: Failure (Builder collection mutCollection step state err) -> Builder collection mutCollection step state err () Source #

class Monad m => MonadThrow (m :: Type -> Type) where Source #

Monad that can throw exception

Methods

throw :: Exception e => e -> m a Source #

Throw immediatity an exception. Only a MonadCatch monad will be able to catch the exception using catch

Instances

Instances details

MonadThrow IO Source #
Instance details Defined in Foundation.Monad.Exception Methods throw :: Exception e => e -> IO a Source #
MonadThrow m => MonadThrow (ResourceT m) Source #
Instance details Defined in Foundation.Conduit.Internal Methods throw :: Exception e => e -> ResourceT m a Source #
MonadThrow m => MonadThrow (ReaderT r m) Source #
Instance details Defined in Foundation.Monad.Reader Methods throw :: Exception e => e -> ReaderT r m a Source #
(Functor m, MonadThrow m) => MonadThrow (StateT s m) Source #
Instance details Defined in Foundation.Monad.State Methods throw :: Exception e => e -> StateT s m a Source #
MonadThrow m => MonadThrow (Conduit i o m) Source #
Instance details Defined in Foundation.Conduit.Internal Methods throw :: Exception e => e -> Conduit i o m a Source #

class MonadThrow m => MonadCatch (m :: Type -> Type) where Source #

Monad that can catch exception

Methods

catch :: Exception e => m a -> (e -> m a) -> m a Source #

Instances

Instances details

MonadCatch IO Source #
Instance details Defined in Foundation.Monad.Exception Methods catch :: Exception e => IO a -> (e -> IO a) -> IO a Source #
MonadCatch m => MonadCatch (ResourceT m) Source #
Instance details Defined in Foundation.Conduit.Internal Methods catch :: Exception e => ResourceT m a -> (e -> ResourceT m a) -> ResourceT m a Source #
MonadCatch m => MonadCatch (ReaderT r m) Source #
Instance details Defined in Foundation.Monad.Reader Methods catch :: Exception e => ReaderT r m a -> (e -> ReaderT r m a) -> ReaderT r m a Source #
(Functor m, MonadCatch m) => MonadCatch (StateT s m) Source #
Instance details Defined in Foundation.Monad.State Methods catch :: Exception e => StateT s m a -> (e -> StateT s m a) -> StateT s m a Source #
MonadCatch m => MonadCatch (Conduit i o m) Source #
Instance details Defined in Foundation.Conduit.Internal Methods catch :: Exception e => Conduit i o m a -> (e -> Conduit i o m a) -> Conduit i o m a Source #

class MonadCatch m => MonadBracket (m :: Type -> Type) where Source #

Monad that can ensure cleanup actions are performed even in the case of exceptions, both synchronous and asynchronous. This usually excludes continuation-based monads.

Methods

generalBracket Source #

Arguments

:: m a	acquire some resource
-> (a -> b -> m ignored1)	cleanup, no exception thrown
-> (a -> SomeException -> m ignored2)	cleanup, some exception thrown. The exception will be rethrown
-> (a -> m b)	inner action to perform with the resource
-> m b

A generalized version of the standard bracket function which allows distinguishing different exit cases.

Instances

Instances details

MonadBracket IO Source #
Instance details Defined in Foundation.Monad.Exception Methods generalBracket :: IO a -> (a -> b -> IO ignored1) -> (a -> SomeException -> IO ignored2) -> (a -> IO b) -> IO b Source #
MonadBracket m => MonadBracket (ResourceT m) Source #
Instance details Defined in Foundation.Conduit.Internal Methods generalBracket :: ResourceT m a -> (a -> b -> ResourceT m ignored1) -> (a -> SomeException -> ResourceT m ignored2) -> (a -> ResourceT m b) -> ResourceT m b Source #
MonadBracket m => MonadBracket (ReaderT r m) Source #
Instance details Defined in Foundation.Monad.Reader Methods generalBracket :: ReaderT r m a -> (a -> b -> ReaderT r m ignored1) -> (a -> SomeException -> ReaderT r m ignored2) -> (a -> ReaderT r m b) -> ReaderT r m b Source #

class MonadTrans (trans :: (Type -> Type) -> Type -> Type) where Source #

Basic Transformer class

Methods

lift :: Monad m => m a -> trans m a Source #

Lift a computation from an inner monad to the current transformer monad

Instances

Instances details

MonadTrans ResourceT Source #
Instance details Defined in Foundation.Conduit.Internal Methods lift :: Monad m => m a -> ResourceT m a Source #
MonadTrans (ExceptT e) Source #
Instance details Defined in Foundation.Monad.Except Methods lift :: Monad m => m a -> ExceptT e m a Source #
MonadTrans (ReaderT r) Source #
Instance details Defined in Foundation.Monad.Reader Methods lift :: Monad m => m a -> ReaderT r m a Source #
MonadTrans (StateT s) Source #
Instance details Defined in Foundation.Monad.State Methods lift :: Monad m => m a -> StateT s m a Source #
MonadTrans (Conduit i o) Source #
Instance details Defined in Foundation.Conduit.Internal Methods lift :: Monad m => m a -> Conduit i o m a Source #

newtype Identity a #

Identity functor and monad. (a non-strict monad)

Since: base-4.8.0.0

Constructors

Identity
Fields runIdentity :: a

Instances

Instances details

MonadFix Identity

Since: base-4.8.0.0

Instance details

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