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GHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27

Modulepipes-safe-2.3.5Haskell2010

Pipes.Safe

This module provides an orphan MonadMask instance for Proxy of the form:

instance (MonadMask m, MonadIO m) => MonadMask (Proxy a' a b' b m) where

Which is needed to implement the instance for MonadSafe for Proxy.

This module also provides generalized versions of some MonadCatch operations so that you can also protect against premature termination of connected components. For example, if you protect a readFile computation using bracket from this module:

-- readFile.hs
import Pipes
import qualified Pipes.Prelude as P
import Pipes.Safe
import qualified System.IO as IO
import Prelude hiding (readFile)

readFile :: FilePath -> Producer' String (SafeT IO) ()
readFile file = bracket
    (do h <- IO.openFile file IO.ReadMode
        putStrLn $ "{" ++ file ++ " open}"
        return h )
    (\h -> do
        IO.hClose h
        putStrLn $ "{" ++ file ++ " closed}" )
    P.fromHandle

... then this generalized bracket will guard against both exceptions and premature termination of other pipes:

Example1 expression
runSafeT $ runEffect $ readFile "readFile.hs" >-> P.take 4 >-> P.stdoutLn{readFile.hs open}-- readFile.hsimport Pipesimport qualified Pipes.Prelude as Pimport Pipes.Safe{readFile.hs closed}

Note that the MonadCatch instance for Proxy provides weaker versions of mask and uninterruptibleMask that do not completely prevent asynchronous exceptions. Instead, they provide a weaker guarantee that asynchronous exceptions will only occur during awaits or yields and nowhere else. For example, if you write:

mask_ $ do
    x <- await
    lift $ print x
    lift $ print x

... then you may receive an asynchronous exception during the await, but you will not receive an asynchronous exception during or in between the two print statements. This weaker guarantee suffices to provide asynchronous exception safety.

  • 7 types
  • 6 classes
  • 22 values
  • Packagepipes-safe-2.3.5
  • Exports35
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceSafe.hs

SafeT

3 declarations
newtypenewtype SafeT (m :: Type -> Type) r
#

SafeT is a monad transformer that extends the base monad with the ability to register and release finalizers.

All unreleased finalizers are called at the end of the SafeT block, even in the event of exceptions.

Constructors

  • SafeT (ReaderT (Env m) m r)

    Constructor exported in case it's necessary for integrating SafeT with other libraries. For example, implementing mtl-like MonadSomething instances will often require access to the SafeT constructor.

    Warning: Using the Env outside the corresponding SafeT scope will result in undefined behavior.

Instances23MonadTrans, MonadError, MonadState, MonadWriter, MonadBase, MonadBaseControl, …

MonadSafe

2 declarations
classclass (MonadCatch m, MonadMask m, MonadIO m, MonadIO (Base m)) => MonadSafe (m :: Type -> Type) where
#

MonadSafe lets you register and release finalizers that execute in a Base monad

Associated types

  • type family Base (m :: Type -> Type) :: Type -> Type

    The monad used to run resource management actions, corresponding to the monad directly beneath SafeT

Methods

Instances11MonadSafe, …

Utilities

7 declarations

These utilities let you supply a finalizer that runs in the Base monad (i.e. the monad directly beneath SafeT). If you don't need to use the full power of the Base monad and you only need to use to use IO, then just wrap the finalizer in liftIO, like this:

myAction `finally` (liftIO myFinalizer)

This will lead to a simple inferred type with a single MonadSafe constraint:

(MonadSafe m) => ...

For examples of this, see the utilities in Pipes.Safe.Prelude.

If you omit the liftIO, the compiler will infer the following constraint instead:

(MonadSafe m, Base m ~ IO) => ...

This means that this function would require IO directly beneath the SafeT monad transformer, which might not be what you want.

valueonException :: MonadSafe m => m a -> Base m b -> m a
#

Analogous to onException from Control.Monad.Catch, except this also protects against premature termination

(`onException` io) is a monad morphism.

valuefinally :: MonadSafe m => m a -> Base m b -> m a
#

Analogous to finally from Control.Monad.Catch, except this also protects against premature termination

valuebracket :: MonadSafe m => Base m a -> (a -> Base m b) -> (a -> m c) -> m c
#

Analogous to bracket from Control.Monad.Catch, except this also protects against premature termination

valuebracket_ :: MonadSafe m => Base m a -> Base m b -> m c -> m c
#

Analogous to bracket_ from Control.Monad.Catch, except this also protects against premature termination

Internals

1 declaration
newtypenewtype Env (m :: Type -> Type)
#

Internal SafeT read-write environment. Exported only so that it can be passed around unmodified by users of the SafeT constructor.

Warning: Using the Env outside the corresponding SafeT scope will result in undefined behavior.

Re-exports

22 declarations

Control.Monad.Catch re-exports all functions except for the ones that conflict with the generalized versions provided here (i.e. bracket, finally, etc.).

Control.Exception re-exports Exception and SomeException.

classclass (Typeable e, Show e) => Exception e where
#

Any type that you wish to throw or catch as an exception must be an instance of the Exception class. The simplest case is a new exception type directly below the root:

data MyException = ThisException | ThatException
    deriving Show

instance Exception MyException

The default method definitions in the Exception class do what we need in this case. You can now throw and catch ThisException and ThatException as exceptions:

*Main> throw ThisException `catch` \e -> putStrLn ("Caught " ++ show (e :: MyException))
Caught ThisException

In more complicated examples, you may wish to define a whole hierarchy of exceptions:

---------------------------------------------------------------------
-- Make the root exception type for all the exceptions in a compiler

data SomeCompilerException = forall e . Exception e => SomeCompilerException e

instance Show SomeCompilerException where
    show (SomeCompilerException e) = show e

instance Exception SomeCompilerException

compilerExceptionToException :: Exception e => e -> SomeException
compilerExceptionToException = toException . SomeCompilerException

compilerExceptionFromException :: Exception e => SomeException -> Maybe e
compilerExceptionFromException x = do
    SomeCompilerException a <- fromException x
    cast a

---------------------------------------------------------------------
-- Make a subhierarchy for exceptions in the frontend of the compiler

data SomeFrontendException = forall e . Exception e => SomeFrontendException e

instance Show SomeFrontendException where
    show (SomeFrontendException e) = show e

instance Exception SomeFrontendException where
    toException = compilerExceptionToException
    fromException = compilerExceptionFromException

frontendExceptionToException :: Exception e => e -> SomeException
frontendExceptionToException = toException . SomeFrontendException

frontendExceptionFromException :: Exception e => SomeException -> Maybe e
frontendExceptionFromException x = do
    SomeFrontendException a <- fromException x
    cast a

---------------------------------------------------------------------
-- Make an exception type for a particular frontend compiler exception

data MismatchedParentheses = MismatchedParentheses
    deriving Show

instance Exception MismatchedParentheses where
    toException   = frontendExceptionToException
    fromException = frontendExceptionFromException

We can now catch a MismatchedParentheses exception as MismatchedParentheses, SomeFrontendException or SomeCompilerException, but not other types, e.g. IOException:

*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: IOException))
*** Exception: MismatchedParentheses

Methods

Instances31Exception, …
datadata SomeException
#

The SomeException type is the root of the exception type hierarchy. When an exception of type e is thrown, behind the scenes it is encapsulated in a SomeException.

Instances2Show, Exception
classclass MonadThrow m => MonadCatch (m :: Type -> Type) where
#

A class for monads which allow exceptions to be caught, in particular exceptions which were thrown by throwM.

Instances should obey the following law:

catch (throwM e) f = f e

Note that the ability to catch an exception does not guarantee that we can deal with all possible exit points from a computation. Some monads, such as continuation-based stacks, allow for more than just a success/failure strategy, and therefore catch cannot be used by those monads to properly implement a function such as finally. For more information, see MonadMask.

Methods

  • catch :: (HasCallStack, Exception e) => m a -> (e -> m a) -> m a

    Provide a handler for exceptions thrown during execution of the first action. Note that type of the type of the argument to the handler will constrain which exceptions are caught. See Control.Exception's catch.

Instances17MonadCatch, …
classclass Monad m => MonadThrow (m :: Type -> Type) where
#

A class for monads in which exceptions may be thrown.

Instances should obey the following law:

throwM e >> x = throwM e

In other words, throwing an exception short-circuits the rest of the monadic computation.

Methods

  • throwM :: (HasCallStack, Exception e) => e -> m a

    Throw an exception. Note that this throws when this action is run in the monad m, not when it is applied. It is a generalization of Control.Exception's throwIO.

    Should satisfy the law:

    throwM e >> f = throwM e
Instances22MonadThrow, …
classclass MonadCatch m => MonadMask (m :: Type -> Type) where
#

A class for monads which provide for the ability to account for all possible exit points from a computation, and to mask asynchronous exceptions. Continuation-based monads are invalid instances of this class.

Instances should ensure that, in the following code:

fg = f `finally` g

The action g is called regardless of what occurs within f, including async exceptions. Some monads allow f to abort the computation via other effects than throwing an exception. For simplicity, we will consider aborting and throwing an exception to be two forms of "throwing an error".

If f and g both throw an error, the error thrown by fg depends on which errors we're talking about. In a monad transformer stack, the deeper layers override the effects of the inner layers; for example, ExceptT e1 (Except e2) a represents a value of type Either e2 (Either e1 a), so throwing both an e1 and an e2 will result in Left e2. If f and g both throw an error from the same layer, instances should ensure that the error from g wins.

Effects other than throwing an error are also overridden by the deeper layers. For example, StateT s Maybe a represents a value of type s -> Maybe (a, s), so if an error thrown from f causes this function to return Nothing, any changes to the state which f also performed will be erased. As a result, g will see the state as it was before f. Once g completes, f's error will be rethrown, so g' state changes will be erased as well. This is the normal interaction between effects in a monad transformer stack.

By contrast, lifted-base's version of finally always discards all of g's non-IO effects, and g never sees any of f's non-IO effects, regardless of the layer ordering and regardless of whether f throws an error. This is not the result of interacting effects, but a consequence of MonadBaseControl's approach.

Methods

  • mask :: HasCallStack => ((forall a. m a -> m a) -> m b) -> m b

    Runs an action with asynchronous exceptions disabled. The action is provided a method for restoring the async. environment to what it was at the mask call. See Control.Exception's mask.

  • uninterruptibleMask :: HasCallStack => ((forall a. m a -> m a) -> m b) -> m b

    Like mask, but the masked computation is not interruptible (see Control.Exception's uninterruptibleMask. WARNING: Only use if you need to mask exceptions around an interruptible operation AND you can guarantee the interruptible operation will only block for a short period of time. Otherwise you render the program/thread unresponsive and/or unkillable.

  • generalBracket :: HasCallStack => m a -> (a -> ExitCase b -> m c) -> (a -> m b) -> m (b, c)

    A generalized version of bracket which uses ExitCase to distinguish the different exit cases, and returns the values of both the use and release actions. In practice, this extra information is rarely needed, so it is often more convenient to use one of the simpler functions which are defined in terms of this one, such as bracket, finally, onError, and bracketOnError.

    This function exists because in order to thread their effects through the execution of bracket, monad transformers need values to be threaded from use to release and from release to the output value.

    NOTE This method was added in version 0.9.0 of this library. Previously, implementation of functions like bracket and finally in this module were based on the mask and uninterruptibleMask functions only, disallowing some classes of tranformers from having MonadMask instances (notably multi-exit-point transformers like ExceptT). If you are a library author, you'll now need to provide an implementation for this method. The StateT implementation demonstrates most of the subtleties:

    generalBracket acquire release use = StateT $ \s0 -> do
  ((b, _s2), (c, s3)) <- generalBracket
    (runStateT acquire s0)
    (\(resource, s1) exitCase -> case exitCase of
      ExitCaseSuccess (b, s2) -> runStateT (release resource (ExitCaseSuccess b)) s2

      -- In the two other cases, the base monad overrides `use`'s state
      -- changes and the state reverts to `s1`.
      ExitCaseException e     -> runStateT (release resource (ExitCaseException e)) s1
      ExitCaseAbort           -> runStateT (release resource ExitCaseAbort) s1
    )
    (\(resource, s1) -> runStateT (use resource) s1)
  return ((b, c), s3)

    The StateT s m implementation of generalBracket delegates to the m implementation of generalBracket. The acquire, use, and release arguments given to StateT's implementation produce actions of type StateT s m a, StateT s m b, and StateT s m c. In order to run those actions in the base monad, we need to call runStateT, from which we obtain actions of type m (a, s), m (b, s), and m (c, s). Since each action produces the next state, it is important to feed the state produced by the previous action to the next action.

    In the ExitCaseSuccess case, the state starts at s0, flows through acquire to become s1, flows through use to become s2, and finally flows through release to become s3. In the other two cases, release does not receive the value s2, so its action cannot see the state changes performed by use. This is fine, because in those two cases, an error was thrown in the base monad, so as per the usual interaction between effects in a monad transformer stack, those state changes get reverted. So we start from s1 instead.

    Finally, the m implementation of generalBracket returns the pairs (b, s) and (c, s). For monad transformers other than StateT, this will be some other type representing the effects and values performed and returned by the use and release actions. The effect part of the use result, in this case _s2, usually needs to be discarded, since those effects have already been incorporated in the release action.

    The only effect which is intentionally not incorporated in the release action is the effect of throwing an error. In that case, the error must be re-thrown. One subtlety which is easy to miss is that in the case in which use and release both throw an error, the error from release should take priority. Here is an implementation for ExceptT which demonstrates how to do this.

    generalBracket acquire release use = ExceptT $ do
  (eb, ec) <- generalBracket
    (runExceptT acquire)
    (\eresource exitCase -> case eresource of
      Left e -> return (Left e) -- nothing to release, `acquire` didn't succeed
      Right resource -> case exitCase of
        ExitCaseSuccess (Right b) -> runExceptT (release resource (ExitCaseSuccess b))
        ExitCaseException e       -> runExceptT (release resource (ExitCaseException e))
        _                         -> runExceptT (release resource ExitCaseAbort))
    (either (return . Left) (runExceptT . use))
  return $ do
    -- The order in which we perform those two `Either` effects determines
    -- which error will win if they are both `Left`s. We want the error from
    -- `release` to win.
    c <- ec
    b <- eb
    return (b, c)
Instances15MonadMask, …
valuecatchAll
  1. :: (HasCallStack, MonadCatch m)
  2. => m a
  3. -> SomeException -> m a
  4. -> m a
#

Catches all exceptions, and somewhat defeats the purpose of the extensible exception system. Use sparingly.

NOTE This catches all exceptions, but if the monad supports other ways of aborting the computation, those other kinds of errors will not be caught.

classclass (Typeable e, Show e) => Exception e where
#

Any type that you wish to throw or catch as an exception must be an instance of the Exception class. The simplest case is a new exception type directly below the root:

data MyException = ThisException | ThatException
    deriving Show

instance Exception MyException

The default method definitions in the Exception class do what we need in this case. You can now throw and catch ThisException and ThatException as exceptions:

*Main> throw ThisException `catch` \e -> putStrLn ("Caught " ++ show (e :: MyException))
Caught ThisException

In more complicated examples, you may wish to define a whole hierarchy of exceptions:

---------------------------------------------------------------------
-- Make the root exception type for all the exceptions in a compiler

data SomeCompilerException = forall e . Exception e => SomeCompilerException e

instance Show SomeCompilerException where
    show (SomeCompilerException e) = show e

instance Exception SomeCompilerException

compilerExceptionToException :: Exception e => e -> SomeException
compilerExceptionToException = toException . SomeCompilerException

compilerExceptionFromException :: Exception e => SomeException -> Maybe e
compilerExceptionFromException x = do
    SomeCompilerException a <- fromException x
    cast a

---------------------------------------------------------------------
-- Make a subhierarchy for exceptions in the frontend of the compiler

data SomeFrontendException = forall e . Exception e => SomeFrontendException e

instance Show SomeFrontendException where
    show (SomeFrontendException e) = show e

instance Exception SomeFrontendException where
    toException = compilerExceptionToException
    fromException = compilerExceptionFromException

frontendExceptionToException :: Exception e => e -> SomeException
frontendExceptionToException = toException . SomeFrontendException

frontendExceptionFromException :: Exception e => SomeException -> Maybe e
frontendExceptionFromException x = do
    SomeFrontendException a <- fromException x
    cast a

---------------------------------------------------------------------
-- Make an exception type for a particular frontend compiler exception

data MismatchedParentheses = MismatchedParentheses
    deriving Show

instance Exception MismatchedParentheses where
    toException   = frontendExceptionToException
    fromException = frontendExceptionFromException

We can now catch a MismatchedParentheses exception as MismatchedParentheses, SomeFrontendException or SomeCompilerException, but not other types, e.g. IOException:

*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: IOException))
*** Exception: MismatchedParentheses

Methods

Instances31Exception, …

Orphan instances

1 instance