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+%
+% (c) The GRASP/AQUA Project, Glasgow University, 1995
+%
+\section[LibSystem]{Haskell 1.3 System Interaction}
+
+\begin{code}
+module LibSystem where
+
+import PreludeGlaST
+import PreludeIOError
+import PreludeDialogueIO  ( unpackArgv, unpackProgName )
+
+data ExitCode = ExitSuccess 
+	      | ExitFailure Int
+{- mattson -} deriving (Eq, Ord, Text)
+
+\end{code}
+
+The $ExitCode$ type defines the exit codes that a program
+can return.  $ExitSuccess$ indicates successful termination;
+and $ExitFailure code$ indicates program failure
+with value {\em code}.  The exact interpretation of {\em code}
+is operating-system dependent.  In particular, some values of 
+{\em code} may be prohibited (e.g. 0 on a POSIX-compliant system).
+
+\begin{code}
+getArgs :: IO [String] 
+getArgs = return (unpackArgv ``prog_argv'' (``prog_argc''::Int))
+\end{code}
+
+Computation $getArgs$ returns a list of the program's command
+line arguments (not including the program name).
+
+\begin{code}
+getProgName :: IO String
+getProgName = return (unpackProgName ``prog_argv'')
+\end{code}
+
+Computation $getProgName$ returns the name of the program
+as it was invoked.
+
+\begin{code}
+getEnv :: String -> IO String
+getEnv name = 
+    _ccall_ getenv name				    `thenPrimIO` \ litstring ->
+    if litstring /= ``NULL'' then
+	return (_unpackPS (_packCString litstring)) -- cheaper than it looks
+    else
+	failWith (NoSuchThing ("environment variable: " ++ name))
+\end{code}
+
+Computation $getEnv var$ returns the value
+of the environment variable {\em var}.  
+
+This computation may fail with
+\begin{itemize}
+\item $NoSuchThing$
+The environment variable does not exist.
+\end{itemize}
+
+\begin{code}
+system :: String -> IO ExitCode
+system "" = failWith (InvalidArgument "null command")
+system cmd = 
+    _ccall_ systemCmd cmd			    `thenPrimIO` \ status ->
+    case status of
+        0  -> return ExitSuccess
+        -1 -> _constructError			    `thenPrimIO` \ ioError ->
+	      failWith ioError
+        n  -> return (ExitFailure n)
+\end{code}
+
+Computation $system cmd$ returns the exit code
+produced when the operating system processes the command {\em cmd}.
+
+This computation may fail with
+\begin{itemize}
+\item $PermissionDenied$
+The process has insufficient privileges to perform the operation.
+\item $ResourceExhausted$
+Insufficient resources are available to perform the operation.  
+\item $UnsupportedOperation$
+The implementation does not support system calls.
+\end{itemize}
+
+\begin{code}
+exitWith :: ExitCode -> IO a
+exitWith ExitSuccess = 
+    _ccall_ EXIT (0::Int)			    `thenPrimIO` \ () ->
+    failWith (OtherError13 "exit should not return")
+
+exitWith (ExitFailure n) 
+  | n == 0 = failWith (InvalidArgument "ExitFailure 0")
+  | otherwise = 
+    _ccall_ EXIT n				    `thenPrimIO` \ () ->
+    failWith (OtherError13 "exit should not return")
+\end{code}
+
+Computation $exitWith code$ terminates the
+program, returning {\em code} to the program's caller.
+Before it terminates, any open or semi-closed handles are first closed.
+
+