[Haskell-cafe] Stupid newbie question of the day: why is newMVar in the IO monad?

[Ryan Yates]

Also note linked from the global variables page on the wiki is:
http://www.haskell.org/haskellwiki/Top_level_mutable_state

Important for the `unsafePerformIO` option is the NOINLINE pragma to ensure
that only one global variable exists.  In the STM case it is also important
to use `newTVarIO` rather then `unsafePerformIO $ atomically newTVar` which
does not work.

Ryan


On Thu, Apr 17, 2014 at 10:38 AM, Ben Foppa <benjamin.foppa at gmail.com>wrote:

> In my (limited) experience, there are two main solutions to this kind of
> problem:
>
>    - Dependency-injection, i.e. add the MVar as an explicit parameter to
>    every function you use it in. This is ideal, but it's often a little
>    cumbersome.
>    - unsafePerformIO, i.e. just initialize it globally. I've never really
>    had issues with this approach, if used sparingly and appropriately.
>
> This might also be relevant:
> http://www.haskell.org/haskellwiki/Global_variables
>
> Hope that helps!
>
>
> On Thu, Apr 17, 2014 at 10:34 AM, Brian Hurt <bhurt at spnz.org> wrote:
>
>> Thanks.  This helps.  I was right to mistrust the unsafePerformIO
>> "solution".  :-)  What Haskell was telling me is that I need to think about
>> the scope of the identifiers I'm allocating, and what guarantees I'm making.
>>
>>
>>
>>
>>
>> On Thu, Apr 17, 2014 at 10:26 AM, Danny Gratzer <danny.gratzer at gmail.com>wrote:
>>
>>> New `MVar` has to return a different memory location every time and this
>>> is noticeable, it's not referentially transparent.
>>>
>>> Consider what would happen if we made the transformation
>>>
>>>      let a = newMVar 0
>>>      let b = newMVar 0
>>>      putMVar a 1
>>>      readMVar b
>>>
>>> to
>>>
>>>     let a = newMVar 0
>>>          b = a
>>>      ...
>>>
>>> If newMVar was referentially transparent, we can automatically share any
>>> of it's calls with same arguments since they're supposed to return the same
>>> thing everytime. Since it's not referentially transparent, back into the IO
>>> monad it goes.
>>>
>>> Also if you do that toplevel counter trick, you want NoInline otherwise
>>> GHC might just inline it at each occurrence and you'll end up with separate
>>> counters.
>>>
>>> Cheers,
>>> Danny Gratzer
>>>
>>
>>
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>
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