The Haskell 98 Library Report: System functions

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## 23`` System Functions

`module System ( ExitCode(ExitSuccess,ExitFailure), getArgs, getProgName, getEnv, system, exitWith, exitFailure ) where data ExitCode = ExitSuccess | ExitFailure Int deriving (Eq, Ord, Read, Show) getArgs :: IO [String] getProgName :: IO String getEnv :: String -> IO String system :: String -> IO ExitCode exitWith :: ExitCode -> IO a exitFailure :: IO a`

This library describes the interaction of the program with the operating system.

Any `System` operation could raise an `isIllegalOperation`, as described in Section 21.1; all other permissible errors are described below. Note that, in particular, if an implementation does not support an operation it must raise an `isIllegalOperation`.

The `ExitCode` type defines the exit codes that a program can return. `ExitSuccess` indicates successful termination; and `ExitFailure` code indicates program failure with value code. The exact interpretation of code is operating-system dependent. In particular, some values of code may be prohibited (for instance, 0 on a POSIX-compliant system).

Computation `getArgs` returns a list of the program's command line arguments (not including the program name). Computation `getProgName` returns the name of the program as it was invoked. Computation `getEnv` var returns the value of the environment variable var. If variable var is undefined, the `isDoesNotExistError` exception is raised. Computation `system` cmd returns the exit code produced when the operating system processes the command cmd.

Computation `exitWith` code terminates the program, returning code 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 `ExitSuccess` to mean normal completion, and `ExitFailure` n to mean that the program encountered a problem from which it could not recover. The value `exitFailure` is equal to `exitWith (ExitFailure` exitfail`)`, where exitfail is implementation-dependent. `exitWith` bypasses the error handling in the I/O monad and cannot be intercepted by `catch`.

If a program terminates as a result of calling `error` or because its value is otherwise determined to be _|_, then it is treated identically to the computation `exitFailure`. Otherwise, if any program p terminates without calling `exitWith` explicitly, it is treated identically to the computation `
(`p`>> exitWith ExitSuccess) `catch` \ _ -> exitFailure
`

The Haskell 98 Report
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December 2002