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Today's Topics:
1. Proper use of exit{Success,Failure} (Vale Cofer-Shabica)
2. Re: Proper use of exit{Success,Failure}
(Sumit Sahrawat, Maths & Computing, IIT (BHU))
3. Re: Proper use of exit{Success,Failure} (Alex Hammel)
4. Re: Proper use of exit{Success,Failure} (Vale Cofer-Shabica)
----------------------------------------------------------------------
Message: 1
Date: Tue, 9 Jun 2015 11:25:43 -0400
From: Vale Cofer-Shabica <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Proper use of exit{Success,Failure}
Message-ID:
<caazfv4tk6zvyj3bnh4hdualm6z++qyummg_h7rlddip_p53...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
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
------------------------------
Message: 2
Date: Tue, 9 Jun 2015 20:57:52 +0530
From: "Sumit Sahrawat, Maths & Computing, IIT (BHU)"
<[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Proper use of exit{Success,Failure}
Message-ID:
<CAJbEW8P3h9YXceFuA54dpvWP+aG7CsRG-4rqEJpN=uqmy6u...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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 <[email protected]>
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
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
--
Regards
Sumit Sahrawat
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Message: 3
Date: Tue, 09 Jun 2015 15:38:15 +0000
From: Alex Hammel <[email protected]>
To: [email protected], The Haskell-Beginners Mailing
List - Discussion of primarily beginner-level topics related to
Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Proper use of exit{Success,Failure}
Message-ID:
<CA+_xFeoZ76QrZncXe-bd88KEmSRtsoZ7+STOLNVsvh=d2y2...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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) <
[email protected]> 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 <[email protected]>
> 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
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
>
>
> --
> Regards
>
> Sumit Sahrawat
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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------------------------------
Message: 4
Date: Tue, 9 Jun 2015 12:19:40 -0400
From: Vale Cofer-Shabica <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Proper use of exit{Success,Failure}
Message-ID:
<CAAzfV4SVEtaq+ce3udhxg1wTxSWXx45=68opz0q56oadx+8...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
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 <[email protected]> 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)
> <[email protected]> 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 <[email protected]>
>> 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
>>> [email protected]
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>>
>>
>> --
>> Regards
>>
>> Sumit Sahrawat
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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