System.Exit

base-4.3.1.0: Basic libraries

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

System.Exit

Description

Exiting the program.

Synopsis

- data ExitCode
  - = ExitSuccess
  - | ExitFailure Int
- exitWith :: ExitCode -> IO a
- exitFailure :: IO a
- exitSuccess :: IO a

# Documentation

data ExitCode

Defines the exit codes that a program can return.

Constructors

ExitSuccess | indicates successful termination;
ExitFailure Int | indicates program failure with an exit code. The exact interpretation of the code is operating-system dependent. In particular, some values may be prohibited (e.g. 0 on a POSIX-compliant system).

Instances

Eq ExitCode |
Ord ExitCode |
Read ExitCode |
Show ExitCode |
Typeable ExitCode |
Exception ExitCode |

exitWith :: ExitCode -> IO a

Computation `exitWith` `code` throws `ExitCode` `code`. Normally this terminates the program, returning `code` to the program's caller.

On program termination, the standard `Handle`s `stdout` and `stderr` are flushed automatically; any other buffered `Handle`s need to be flushed manually, otherwise the buffered data will be discarded.

A program that fails in any other way is treated as if it had called `exitFailure`. A program that terminates successfully without calling `exitWith` explicitly is treated as it it had called `exitWith` `ExitSuccess`.

As an `ExitCode` is not an `IOError`, `exitWith` bypasses the error handling in the `IO` monad and cannot be intercepted by `catch` from the Prelude. However it is a `SomeException`, and can be caught using the functions of Control.Exception. This means that cleanup computations added with `Control.Exception.bracket` (from Control.Exception) are also executed properly on `exitWith`.

Note: in GHC, `exitWith` should be called from the main program thread in order to exit the process. When called from another thread, `exitWith` will throw an `ExitException` as normal, but the exception will not cause the process itself to exit.

exitFailure :: IO a

The computation `exitFailure` is equivalent to `exitWith` `(``ExitFailure` exitfail`)`, where exitfail is implementation-dependent.

exitSuccess :: IO a

The computation `exitSuccess` is equivalent to `exitWith` `ExitSuccess`, It terminates the program successfully.