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

Roman Cheplyaka roma at
Sat Dec 14 11:19:26 UTC 2013

I agree that this function would be useful for quick&dirty prototyping.

However, I'm not sure we should encourage its use. What looks to me like
a superior approach is to throw proper exceptions.

Here's an example in my own code:

The main advantage of this is that it's much easier to turn the 'main'
code into library code, which is not supposed to write to stderr
anymore. Exceptions carry important semantic information about what
actually happened, and can be caught and handled if needed. (The
ExitCode exception can also be caught, but it doesn't tell us anything,
and the output would have already gone to stderr.)

As a bonus, this approach forces you to separate (in code) message
strings from places where you die, which in my experience leads to much
cleaner code.

There's a caveat that the standard doesn't specify what happens to
uncaught exceptions, so we have to rely on the runtime doing the right
thing for us. Well, GHC's one does.


* Simon Hengel <sol at> [2013-12-14 11:56:11+0100]
> Hi!
> I propose to add
>     die :: String -> IO ()
>     die err = hPutStrLn stderr err >> exitFailure
> to System.Exit.
> Reasoning:
> (1) It's a frequently required functionality.  Almost every command-line
>     program has code similar to this.
> (2) The definition is relatively short, but in addition to the
>     definition, you need two import statements.
> (3) It's frequently done wrong (e.g. writing to stdout instead of
>     stderr, or not using exitFailure, or both).
> I haven't done any extensive research on Hackage, but I quickly looked
> at Haddock.  Here we have a definition of die [1], but we also print to
> stdout at a couple of place and then call exitFailure [2].
> Personally, I think it should be re-exported from Prelude.  But this may
> be controversial.  So let's have two separate votes:
> Add System.Exit.die:            +1
> Re-export it from Prelude:      +1
> (discussion until December, 28th)
> Cheers,
> Simon
> [1]
> [2]
> _______________________________________________
> Libraries mailing list
> Libraries at
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