Proposal: die to System.Exit (and/or Prelude)

Herbert Valerio Riedel hvr at
Mon Dec 16 09:53:21 UTC 2013

Hello Simon,

On 2013-12-14 at 11:56:11 +0100, Simon Hengel wrote:
> I propose to add
>     die :: String -> IO ()
>     die err = hPutStrLn stderr err >> exitFailure
> to System.Exit.

Why not simply use the existing `fail :: String -> IO a` method instead?

The differences & similiarities I see wrt `fail`:

 - `die` throws an `ExitCode` exception, whereas `fail` throws an
   `IOError` exception

 - Both result in the message written to stderr and a non-zero exit code

 - As both use exceptions, `die` does guarantee termination (as it can
   be caught)

 - `die` writes to stderr at invocation time, whereas `fail` attaches
   the message to the exception, which is then output by the top
   exception handler.

   Due to that, catching a `die`-caused exception will print the
   termination message even though the process-exit was
   cancelled. Whereas attaching the message to the exception allows for
   more flexibility as well as for atomicity wrt message-output and
 - `fail` is available from Prelude (and part of Haskell98/2010),
   whereas `System.Exit.die` would require an import of "System.Exit"
   (with the small risk of breaking code that expects only
   Haskell2010-known entities exported by "System.Exit")

Therefore I'm not convinced (yet), that the suggested `die`
implementation has sensible semantics in the context of exceptions, and
that it offers enough benefits over the already existing `fail` method.


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