42 System.IO.Error

Chapter 42
System.IO.Error

module System.IO.Error (
    IOError,  userError,  mkIOError,  annotateIOError,  isAlreadyExistsError,
    isDoesNotExistError,  isAlreadyInUseError,  isFullError,  isEOFError,
    isIllegalOperation,  isPermissionError,  isUserError,  ioeGetErrorString,
    ioeGetHandle,  ioeGetFileName,  IOErrorType,  alreadyExistsErrorType,
    doesNotExistErrorType,  alreadyInUseErrorType,  fullErrorType,
    eofErrorType,  illegalOperationErrorType,  permissionErrorType,
    userErrorType,  ioError,  catch,  try
  ) where

42.1 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: 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.

42.1.1 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.

42.1.2 Attributes of I/O errors

ioeGetErrorString :: IOError -> String
ioeGetHandle :: IOError -> Maybe Handle
ioeGetFileName :: IOError -> Maybe FilePath

42.2 Types of I/O error

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

instance Eq IOErrorType instance 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.

42.3 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 IOErrors 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 trycomp exposes IO errors which occur within a computation, and which are not fully handled.

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