Re: [Haskell-beginners] Proper use of exit{Success,Failure}

9 Jun 2015

      exitSuccess :: IO a means that exitSuccess type-checks for any `a`. Your
program should typecheck without `void`. I suspect you've got a stray paren.

This prints "Bailing out." and exits zero for me:

module Main where

import System.Exit (exitSuccess)
import Control.Monad (when)

main = do
    let condition = True
    when condition (putStrLn "Bailing out." >> exitSuccess)
    putStrLn "Continuing normally..."

On Tue, 9 Jun 2015 at 08:27 Sumit Sahrawat, Maths & Computing, IIT (BHU) <
sumit.sahrawat.apm13@iitbhu.ac.in> wrote:
...
Try `Control.Monad.void`. It is equivalent of adding `>> return ()` to the
end of a block.
void $ do
 ...
On 9 June 2015 at 20:55, Vale Cofer-Shabica 
wrote:
...
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
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--
Regards
Sumit Sahrawat
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Re: [Haskell-beginners] Proper use of exit{Success,Failure}

Alex Hammel