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

[Vale Cofer-Shabica]

Thank you Alex, Sumit!

I did indeed have stray parentheses. I feel a bit silly for convincing
myself that something with my types was wrong when it was just an
error in grouping. Thanks for the clarification and the note about
void.

-vale



On Tue, Jun 9, 2015 at 11:38 AM, Alex Hammel <ahammel87 at gmail.com> wrote:
> 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 at 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 <vale.cofershabica at gmail.com>
>> 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
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>>
>>
>> --
>> Regards
>>
>> Sumit Sahrawat
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>
>
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