base-4.3.1.0: Basic libraries

Portability | portable
Stability | provisional
Maintainer | libraries@haskell.org

System.IO.Error

Contents

- I/O errors
  - Classifying I/O errors
  - Attributes of I/O errors
- Types of I/O error
  - `IOErrorType` predicates
- Throwing and catching I/O errors

Description

Standard IO Errors.

Synopsis

- type IOError = IOException
- userError :: String -> IOError
- mkIOError :: IOErrorType -> String -> Maybe Handle -> Maybe FilePath -> IOError
- annotateIOError :: IOError -> String -> Maybe Handle -> Maybe FilePath -> IOError
- isAlreadyExistsError :: IOError -> Bool
- isDoesNotExistError :: IOError -> Bool
- isAlreadyInUseError :: IOError -> Bool
- isFullError :: IOError -> Bool
- isEOFError :: IOError -> Bool
- isIllegalOperation :: IOError -> Bool
- isPermissionError :: IOError -> Bool
- isUserError :: IOError -> Bool
- ioeGetErrorType :: IOError -> IOErrorType
- ioeGetLocation :: IOError -> String
- ioeGetErrorString :: IOError -> String
- ioeGetHandle :: IOError -> Maybe Handle
- ioeGetFileName :: IOError -> Maybe FilePath
- ioeSetErrorType :: IOError -> IOErrorType -> IOError
- ioeSetErrorString :: IOError -> String -> IOError
- ioeSetLocation :: IOError -> String -> IOError
- ioeSetHandle :: IOError -> Handle -> IOError
- ioeSetFileName :: IOError -> FilePath -> IOError
- data IOErrorType
- alreadyExistsErrorType :: IOErrorType
- doesNotExistErrorType :: IOErrorType
- alreadyInUseErrorType :: IOErrorType
- fullErrorType :: IOErrorType
- eofErrorType :: IOErrorType
- illegalOperationErrorType :: IOErrorType
- permissionErrorType :: IOErrorType
- userErrorType :: IOErrorType
- isAlreadyExistsErrorType :: IOErrorType -> Bool
- isDoesNotExistErrorType :: IOErrorType -> Bool
- isAlreadyInUseErrorType :: IOErrorType -> Bool
- isFullErrorType :: IOErrorType -> Bool
- isEOFErrorType :: IOErrorType -> Bool
- isIllegalOperationErrorType :: IOErrorType -> Bool
- isPermissionErrorType :: IOErrorType -> Bool
- isUserErrorType :: IOErrorType -> Bool
- ioError :: IOError -> IO a
- catch :: IO a -> (IOError -> IO a) -> IO a
- try :: IO a -> IO (Either IOError a)
- modifyIOError :: (IOError -> IOError) -> IO a -> IO a

# I/O errors

type IOError = IOException

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 -> IOError

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 -> IOError

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 -> IOError

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 -> Bool

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

isDoesNotExistError :: IOError -> Bool

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

isAlreadyInUseError :: IOError -> Bool

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 -> Bool

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

isEOFError :: IOError -> Bool

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

isIllegalOperation :: IOError -> Bool

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 -> Bool

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

isUserError :: IOError -> Bool

A programmer-defined error value constructed using `userError`.

## Attributes of I/O errors

ioeGetErrorType :: IOError -> IOErrorType

ioeGetLocation :: IOError -> String

ioeGetErrorString :: IOError -> String

ioeGetHandle :: IOError -> Maybe Handle

ioeGetFileName :: IOError -> Maybe FilePath

ioeSetErrorType :: IOError -> IOErrorType -> IOError

ioeSetErrorString :: IOError -> String -> IOError

ioeSetLocation :: IOError -> String -> IOError

ioeSetHandle :: IOError -> Handle -> IOError

ioeSetFileName :: IOError -> FilePath -> IOError

# Types of I/O error

data IOErrorType

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

Instances

Eq IOErrorType |
Show IOErrorType |

alreadyExistsErrorType :: IOErrorType

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

doesNotExistErrorType :: IOErrorType

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

alreadyInUseErrorType :: IOErrorType

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

fullErrorType :: IOErrorType

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

eofErrorType :: IOErrorType

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

illegalOperationErrorType :: IOErrorType

I/O error where the operation is not possible.

permissionErrorType :: IOErrorType

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

userErrorType :: IOErrorType

I/O error that is programmer-defined.

## `IOErrorType` predicates

isAlreadyExistsErrorType :: IOErrorType -> Bool

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

isDoesNotExistErrorType :: IOErrorType -> Bool

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

isAlreadyInUseErrorType :: IOErrorType -> Bool

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

isFullErrorType :: IOErrorType -> Bool

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

isEOFErrorType :: IOErrorType -> Bool

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

isIllegalOperationErrorType :: IOErrorType -> Bool

I/O error where the operation is not possible.

isPermissionErrorType :: IOErrorType -> Bool

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

isUserErrorType :: IOErrorType -> Bool

I/O error that is programmer-defined.

# Throwing and catching I/O errors

ioError :: IOError -> IO a

Raise an `IOError` in the `IO` monad.

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

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

     f = catch 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.

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

The construct `try` `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.

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

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