haskeline-0.7.1.2: A command-line interface for user input, written in Haskell.

Safe HaskellNone
LanguageHaskell98

System.Console.Haskeline.MonadException

Contents

Description

This module redefines some of the functions in Control.Exception to work for more general monads built on top of IO.

Synopsis

The MonadException class

class MonadIO m => MonadException m where

An instance of MonadException is generally made up of monad transformers layered on top of the IO monad.

The controlIO method enables us to "lift" a function that manages IO actions (such as bracket or catch) into a function that wraps arbitrary monadic actions.

Methods

controlIO :: (RunIO m -> IO (m a)) -> m a

Generalizations of Control.Exception

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

handle :: (MonadException m, Exception e) => (e -> m a) -> m a -> m a

catches :: MonadException m => m a -> [Handler m a] -> m a

data Handler m a

Constructors

forall e . Exception e => Handler (e -> m a) 

finally :: MonadException m => m a -> m b -> m a

throwIO :: (MonadIO m, Exception e) => e -> m a

throwTo :: (MonadIO m, Exception e) => ThreadId -> e -> m ()

bracket :: MonadException m => m a -> (a -> m b) -> (a -> m c) -> m c

Helpers for defining "wrapper" functions

liftIOOp :: MonadException m => ((a -> IO (m b)) -> IO (m c)) -> (a -> m b) -> m c

Lift a IO operation

wrap :: (a -> IO b) -> IO b

to a more general monadic operation

liftIOOp wrap :: MonadException m => (a -> m b) -> m b

For example:

 liftIOOp (withFile f m) :: MonadException m => (Handle -> m r) -> m r

 liftIOOp alloca :: (MonadException m, Storable a) => (Ptr a -> m b) -> m b

 liftIOOp (withForeignPtr fp) :: MonadException m => (Ptr a -> m b) -> m b

liftIOOp_ :: MonadException m => (IO (m a) -> IO (m a)) -> m a -> m a

Lift an IO operation

wrap :: IO a -> IO a

to a more general monadic operation

liftIOOp_ wrap :: MonadException m => m a -> m a

Internal implementation

newtype RunIO m

A RunIO function takes a monadic action m as input, and outputs an IO action which performs the underlying impure part of m and returns the ''pure'' part of m.

Note that (RunIO return) is an incorrect implementation, since it does not separate the pure and impure parts of the monadic action. This module defines implementations for several common monad transformers.

Constructors

RunIO (forall b. m b -> IO (m b)) 

Extensible Exceptions

class (Typeable * e, Show e) => Exception e Source

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, Typeable)



 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

     deriving Typeable



 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

     deriving Typeable



 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 (Typeable, 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

data SomeException :: * where Source

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.

Constructors

SomeException :: Exception e => e -> SomeException 

data IOException :: * Source

Exceptions that occur in the IO monad. An IOException records a more specific error type, a descriptive string and maybe the handle that was used when the error was flagged.