| Safe Haskell | Safe-Inferred |
|---|---|
| Language | Haskell2010 |
Foundation.Monad
Synopsis
- class Monad m => MonadIO (m :: Type -> Type) where
- class Monad m => MonadFailure (m :: Type -> Type) where
- class Monad m => MonadThrow m where
- class MonadThrow m => MonadCatch m where
- class MonadCatch m => MonadBracket m where
- generalBracket :: m a -> (a -> b -> m ignored1) -> (a -> SomeException -> m ignored2) -> (a -> m b) -> m b
- class MonadTrans trans where
- newtype Identity a = Identity {
- runIdentity :: a
- replicateM :: Applicative m => CountOf a -> m a -> m [a]
Documentation
class Monad m => MonadIO (m :: Type -> Type) where Source #
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:
Methods
liftIO :: IO a -> m a Source #
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
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
• 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(m a): liftIO
> evalStateT printState "hello" "hello" > evalStateT printState 3 3
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
Instances
| MonadFailure Maybe | |
| MonadFailure (Either a) | |
| Monad m => MonadFailure (ExceptT e m) Source # | |
| MonadFailure m => MonadFailure (ReaderT r m) Source # | |
| (Functor m, MonadFailure m) => MonadFailure (StateT s m) Source # | |
| MonadFailure m => MonadFailure (Conduit i o m) Source # | |
| Monad state => MonadFailure (Builder collection mutCollection step state err) | |
class Monad m => MonadThrow m 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
| MonadThrow IO Source # | |
| MonadThrow m => MonadThrow (ResourceT m) Source # | |
| MonadThrow m => MonadThrow (ReaderT r m) Source # | |
| (Functor m, MonadThrow m) => MonadThrow (StateT s m) Source # | |
| MonadThrow m => MonadThrow (Conduit i o m) Source # | |
class MonadThrow m => MonadCatch m where Source #
Monad that can catch exception
Instances
| MonadCatch IO Source # | |
| MonadCatch m => MonadCatch (ResourceT m) Source # | |
| MonadCatch m => MonadCatch (ReaderT r m) Source # | |
| (Functor m, MonadCatch m) => MonadCatch (StateT s m) Source # | |
| MonadCatch m => MonadCatch (Conduit i o m) Source # | |
class MonadCatch m => MonadBracket m 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
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
| MonadBracket IO Source # | |
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 # | |
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 # | |
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 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
| MonadTrans ResourceT Source # | |
| MonadTrans (ExceptT e) Source # | |
| MonadTrans (ReaderT r) Source # | |
| MonadTrans (StateT s) Source # | |
| MonadTrans (Conduit i o) Source # | |
Identity functor and monad. (a non-strict monad)
Since: base-4.8.0.0
Constructors
| Identity | |
Fields
| |
Instances
replicateM :: Applicative m => CountOf a -> m a -> m [a] Source #
replicateM n actn times,
gathering the results.