[Haskell-cafe] Having a very annoying problem related with (I think) lazy IO

Ömer Sinan Ağacan omeragacan at gmail.com
Fri Dec 12 16:33:39 UTC 2014

Interesting, I think that worked. (I'll try on bigger examples and update again)

> Probably the loop is too fast, and it doesn't give other parts of your
program to take a lock.

Are there any ways to test for this? How do I know if this is really the case?

2014-12-12 18:20 GMT+02:00 Yuras Shumovich <shumovichy at gmail.com>:
> What if you replace the `putStrLn` with `yield` or `threadDelay 1`?
> Probably the loop is too fast, and it doesn't give other parts of your
> program to take a lock.
> On Fri, 2014-12-12 at 17:58 +0200, Ömer Sinan Ağacan wrote:
>> Hi all,
>> I'm having a horrible bug in my program: a print statement that
>> doesn't force any values(e.g. it just prints a static string)
>> effecting the behavior of the program.
>> I tried hard to produce a minimal example, but couldn't manage it. So
>> instead I'm just going to point to the source code of my program:
>> https://github.com/osa1/rho-torrent/blob/master/src/Rho/Listener.hs#L125
>> If I remove this print statement everything goes weird. I think when I
>> remove this statement some effects are not happening, or happening in
>> wrong times.
>> Here's how I'm testing the different behavior: This is a network
>> application and I'm creating an environment in my system and making
>> this program talk to another program.
>> I run my program 10 times without the print statement, and run 10 times with it.
>> Both groups produce same output in every time, but outputs of first
>> group and second group are different from each other. (apart from
>> extra prints, of course)
>> So this print statement effects some other things, and expected
>> behavior is the one happening with print statement.
>> I know the program is big for anyone to figure things in short amount
>> of time, I'm just looking for tips and pitfalls that I may be falling.
>> Does anyone have any ideas what I may be doing wrong? Note that the
>> program doesn't use any unsafe stuff.
>> Thanks in advance.
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