base-4.4.0.0: Basic libraries

Portabilityportable
Stabilityprovisional
Maintainer[email protected]

System.IO.Error

Contents

Description

Standard IO Errors.

Synopsis

I/O errors

type IOError = IOExceptionSource

The Haskell 98 type for exceptions in the IO monad. Any I/O operation may raise an IOError instead of returning a result. For a more general type of exception, including also those that arise in pure code, see Control.Exception.Exception.

In Haskell 98, this is an opaque type.

userError :: String -> IOErrorSource

Construct an IOError value with a string describing the error. The fail method of the IO instance of the Monad class raises a userError, thus:

 instance Monad IO where 
   ...
   fail s = ioError (userError s)

mkIOError :: IOErrorType -> String -> Maybe Handle -> Maybe FilePath -> IOErrorSource

Construct an IOError of the given type where the second argument describes the error location and the third and fourth argument contain the file handle and file path of the file involved in the error if applicable.

annotateIOError :: IOError -> String -> Maybe Handle -> Maybe FilePath -> IOErrorSource

Adds a location description and maybe a file path and file handle to an IOError. If any of the file handle or file path is not given the corresponding value in the IOError remains unaltered.

Classifying I/O errors

isAlreadyExistsError :: IOError -> BoolSource

An error indicating that an IO operation failed because one of its arguments already exists.

isDoesNotExistError :: IOError -> BoolSource

An error indicating that an IO operation failed because one of its arguments does not exist.

isAlreadyInUseError :: IOError -> BoolSource

An error indicating that an IO operation failed because one of its arguments is a single-use resource, which is already being used (for example, opening the same file twice for writing might give this error).

isFullError :: IOError -> BoolSource

An error indicating that an IO operation failed because the device is full.

isEOFError :: IOError -> BoolSource

An error indicating that an IO operation failed because the end of file has been reached.

isIllegalOperation :: IOError -> BoolSource

An error indicating that an IO operation failed because the operation was not possible. Any computation which returns an IO result may fail with isIllegalOperation. In some cases, an implementation will not be able to distinguish between the possible error causes. In this case it should fail with isIllegalOperation.

isPermissionError :: IOError -> BoolSource

An error indicating that an IO operation failed because the user does not have sufficient operating system privilege to perform that operation.

isUserError :: IOError -> BoolSource

A programmer-defined error value constructed using userError.

Attributes of I/O errors

Types of I/O error

data IOErrorType Source

An abstract type that contains a value for each variant of IOError.

alreadyExistsErrorType :: IOErrorTypeSource

I/O error where the operation failed because one of its arguments already exists.

doesNotExistErrorType :: IOErrorTypeSource

I/O error where the operation failed because one of its arguments does not exist.

alreadyInUseErrorType :: IOErrorTypeSource

I/O error where the operation failed because one of its arguments is a single-use resource, which is already being used.

fullErrorType :: IOErrorTypeSource

I/O error where the operation failed because the device is full.

eofErrorType :: IOErrorTypeSource

I/O error where the operation failed because the end of file has been reached.

illegalOperationErrorType :: IOErrorTypeSource

I/O error where the operation is not possible.

permissionErrorType :: IOErrorTypeSource

I/O error where the operation failed because the user does not have sufficient operating system privilege to perform that operation.

userErrorType :: IOErrorTypeSource

I/O error that is programmer-defined.

IOErrorType predicates

isAlreadyExistsErrorType :: IOErrorType -> BoolSource

I/O error where the operation failed because one of its arguments already exists.

isDoesNotExistErrorType :: IOErrorType -> BoolSource

I/O error where the operation failed because one of its arguments does not exist.

isAlreadyInUseErrorType :: IOErrorType -> BoolSource

I/O error where the operation failed because one of its arguments is a single-use resource, which is already being used.

isFullErrorType :: IOErrorType -> BoolSource

I/O error where the operation failed because the device is full.

isEOFErrorType :: IOErrorType -> BoolSource

I/O error where the operation failed because the end of file has been reached.

isIllegalOperationErrorType :: IOErrorType -> BoolSource

I/O error where the operation is not possible.

isPermissionErrorType :: IOErrorType -> BoolSource

I/O error where the operation failed because the user does not have sufficient operating system privilege to perform that operation.

isUserErrorType :: IOErrorType -> BoolSource

I/O error that is programmer-defined.

Throwing and catching I/O errors

ioError :: IOError -> IO aSource

Raise an IOError in the IO monad.

catchIOError :: IO a -> (IOError -> IO a) -> IO aSource

The catchIOError function establishes a handler that receives any IOError raised in the action protected by catchIOError. An IOError is caught by the most recent handler established by one of the exception handling functions. These handlers are not selective: all IOErrors are caught. Exception propagation must be explicitly provided in a handler by re-raising any unwanted exceptions. For example, in

 f = catchIOError g (\e -> if IO.isEOFError e then return [] else ioError e)

the function f returns [] when an end-of-file exception (cf. isEOFError) occurs in g; otherwise, the exception is propagated to the next outer handler.

When an exception propagates outside the main program, the Haskell system prints the associated IOError value and exits the program.

Non-I/O exceptions are not caught by this variant; to catch all exceptions, use Control.Exception.catch from Control.Exception.

catch :: IO a -> (IOError -> IO a) -> IO aSource

The catch function is deprecated. Please use the new exceptions variant, Control.Exception.catch from Control.Exception, instead.

tryIOError :: IO a -> IO (Either IOError a)Source

The construct tryIOError comp exposes IO errors which occur within a computation, and which are not fully handled.

Non-I/O exceptions are not caught by this variant; to catch all exceptions, use Control.Exception.try from Control.Exception.

try :: IO a -> IO (Either IOError a)Source

The try function is deprecated. Please use the new exceptions variant, Control.Exception.try from Control.Exception, instead.

modifyIOError :: (IOError -> IOError) -> IO a -> IO aSource

Catch any IOError that occurs in the computation and throw a modified version.