Proper use of exit{Success,Failure}

9 Jun 2015

      Dear all,

I'm writing a command line program which may need to exit early based
on some condition. Because of the pipeline the program is a part of, I
want to be able to control the exit status when it does terminate. I
can successfully use 'error' to indicate failure, but also want to
exit early with success (or any other status). Here's what I've
managed:
...
import System.Exit (exitSuccess)
import Control.Monad (when)
...
main = do
... code to reasonably initialize condition, e.g.:
...
let condition = True
 when condition (putStrLn "Bailing out.")>>exitSuccess>>(return ())
... program continues

If I remove ">>(return ())" my code will not typecheck because
"exitSuccess :: IO a". But this looks ugly and smells sufficiently
like a kludge that I presume there is a better way to do it.

Any suggestions appreciated,
vale

Vale Cofer-Shabica

Sumit Sahrawat, Maths & Computing, IIT (BHU)

Alex Hammel

Vale Cofer-Shabica

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