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<title>The Haskell 1.3 Library Report: System functions</title>
<body bgcolor="#ffffff"> <i>The Haskell 1.4 Library Report</i><br> <a href="index.html">top</a> | <a href="directory.html">back</a> | <a href="time.html">next</a> | <a href="libindex.html">contents</a> <br><hr>
<p>
<a name="sect13"></a>
<h2>13<tt> </tt>System Functions</h2><p>
<table border=2 cellpadding=3>
<tr><td>
<tt><br>
module System ( <br>
ExitCode(ExitSuccess,ExitFailure),<br>
getArgs, getProgName, getEnv, system, exitWith ) where<br>
<br>
data ExitCode = ExitSuccess | ExitFailure Int <br>
deriving (Eq, Ord, Read, Show)<br>
<br>
getArgs :: IO [String]<br>
getProgName :: IO String<br>
getEnv :: String -> IO String<br>
system :: String -> IO ExitCode<br>
exitWith :: ExitCode -> IO a<br>
</tt></td></tr></table>
<p>
This library describes the interaction of the program with the
operating system. <p>
Any <tt>System</tt> operation could raise an <tt>isIllegalOperationError</tt>, as
described in Section <a href="io.html#IOError">11.1</a>; all other permissible errors are
described below. Note that, in particular, if an implementation does
not support an operation it must raise an <tt>isIllegalOperationError</tt>.<p>
The <tt>ExitCode</tt> type defines the exit codes that a program can return.
<tt>ExitSuccess</tt> indicates successful termination; and <tt>ExitFailure
</tt><I>code</I> indicates program failure with value <I>code</I>. The exact
interpretation of <I>code</I> is operating-system dependent. In
particular, some values of <I>code</I> may be prohibited (for instance, 0 on a
POSIX-compliant system).<p>
Computation <tt>getArgs</tt> returns a list of the program's command
line arguments (not including the program name).
Computation <tt>getProgName</tt> returns the name of the program
as it was invoked.
Computation <tt>getEnv</tt> <I>var</I> returns the value
of the environment variable <I>var</I>.
If variable <I>var</I> is undefined, the
<tt>isDoesNotExistError</tt> exception is raised.
Computation <tt>system</tt> <I>cmd</I> returns the exit code produced when the
operating system processes the command <I>cmd</I>.<p>
Computation <tt>exitWith</tt> <I>code</I> terminates the program, returning <I>code
</I>to the program's caller. Before the
program terminates, any open or semi-closed handles are first closed.
The caller may interpret the return code as it wishes, but the program
should return <tt>ExitSuccess</tt> to mean normal completion, and
<tt>ExitFailure </tt><I>n</I> to mean that the program encountered a problem from
which it could not recover. The value <tt>exitFailure</tt> is equal to
<tt>exitWith (ExitFailure </tt><I>exitfail</I><tt>)</tt>, where <I>exitfail</I> is
implementation-dependent. <tt>exitWith</tt> bypasses the error handling in
the I/O monad and cannot be intercepted by <tt>catch</tt>.<p>
If a program terminates as a result of calling <tt>error</tt> or
because its value is otherwise determined to be <I>_|_</I>, then it
is treated identically to the computation <tt>exitFailure</tt>. Otherwise, if any
program <I>p</I> terminates without calling <tt>exitWith</tt> explicitly, it is treated
identically to the computation
<tt><br>
(</tt><I>p</I><tt> >> exitWith ExitSuccess) `catch` \ _ -> exitFailure
<br>
<p>
<hr><i>The Haskell 1.4 Library Report</i><br><a href="index.html">top</a> | <a href="directory.html">back</a> | <a href="time.html">next</a> | <a href="libindex.html">contents</a> <br><font size=2>April 4, 1997</font>
</tt>
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