System.IO.Error

haskell2010-1.0.0.0: Compatibility with Haskell 2010

System.IO.Error

Contents

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

Synopsis

- type IOError = IOError
- 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
- ioeGetErrorString :: IOError -> String
- ioeGetHandle :: IOError -> Maybe Handle
- ioeGetFileName :: IOError -> Maybe FilePath
- data IOErrorType
- alreadyExistsErrorType :: IOErrorType
- doesNotExistErrorType :: IOErrorType
- alreadyInUseErrorType :: IOErrorType
- fullErrorType :: IOErrorType
- eofErrorType :: IOErrorType
- illegalOperationErrorType :: IOErrorType
- permissionErrorType :: IOErrorType
- userErrorType :: IOErrorType
- ioError :: IOError -> IO a
- catch :: IO a -> (IOError -> IO a) -> IO a
- try :: IO a -> IO (Either IOError a)

# I/O errors

type IOError = IOError

Errors of type `IOError` are used by the `IO` monad. This is an abstract type; the module System.IO.Error provides functions to interrogate and construct values of type `IOError`.

userError :: String -> IOError Source

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 Source

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 Source

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 Source

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

isDoesNotExistError :: IOError -> Bool Source

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

isAlreadyInUseError :: IOError -> Bool Source

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 Source

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

isEOFError :: IOError -> Bool Source

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

isIllegalOperation :: IOError -> Bool Source

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 Source

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 Source

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

## Attributes of I/O errors

ioeGetErrorString :: IOError -> String Source

ioeGetHandle :: IOError -> Maybe Handle Source

ioeGetFileName :: IOError -> Maybe FilePath Source

# Types of I/O error

data IOErrorType Source

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

Instances

Eq IOErrorType |
Show IOErrorType |

alreadyExistsErrorType :: IOErrorType Source

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

doesNotExistErrorType :: IOErrorType Source

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

alreadyInUseErrorType :: IOErrorType Source

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

fullErrorType :: IOErrorType Source

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

eofErrorType :: IOErrorType Source

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

illegalOperationErrorType :: IOErrorType Source

I/O error where the operation is not possible.

permissionErrorType :: IOErrorType Source

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

userErrorType :: IOErrorType Source

I/O error that is programmer-defined.

# Throwing and catching I/O errors

ioError :: IOError -> IO aSource

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.

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.