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

On 14 December 2013 22:19, Roman Cheplyaka wrote:

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.

I've written something similar to this die function specifically to deal with these exceptions (or to exit early out of main if I know nothing more can proceed) rather than relying upon GHC to deal with them. That said, I'm not sure if it's worth adding; +0.5 to having it in System.Exit.

Here's an example in my own code: https://github.com/haskell-suite/haskell-names/blob/master/hs-gen-iface/src/...

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.

Roman

* Simon Hengel [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] https://github.com/ghc/haddock/blob/8d4c94ca5a969a5ebbb791939fb0195dc672429e... [2] https://github.com/ghc/haddock/blob/c6faeae064668125721b0d5e60f067f90c538933... _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

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