Defines the exit codes that a program can return.
indicates successful termination;
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).
getArgs returns a list of the program's command
line arguments (not including the program name).
getProgName returns the name of the program as it was
However, this is hard-to-impossible to implement on some non-Unix
OSes, so instead, for maximum portability, we just return the leafname
of the program as invoked. Even then there are some differences
between platforms: on Windows, for example, a program invoked as foo
is probably really
FOO.EXE, and that is what
getProgName will return.
var returns the value
of the environment variable
This computation may fail with:
System.IO.Error.isDoesNotExistErrorif the environment variable does not exist.
system cmd returns the exit code produced when the
operating system runs the shell command
This computation may fail with
PermissionDenied: The process has insufficient privileges to perform the operation.
ResourceExhausted: Insufficient resources are available to perform the operation.
UnsupportedOperation: The implementation does not support system calls.
system passes the command to the Windows command
COMMAND.COM), hence Unixy shell tricks
will not work.
On program termination, the standard
stderr are flushed automatically; any other buffered
need to be flushed manually, otherwise the buffered data will be
A program that fails in any other way is treated as if it had
A program that terminates successfully without calling
explicitly is treated as it it had called
ExitCode is not an
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
(from Control.Exception) are also executed properly on
Note: in GHC,
exitWith should be called from the main program
thread in order to exit the process. When called from another
exitWith will throw an
ExitException as normal, but the
exception will not cause the process itself to exit.