[Haskell-beginners] Global variables

[Michael Snoyman]

On Thu, Jun 30, 2011 at 8:21 AM, Ertugrul Soeylemez <es at ertes.de> wrote:
> Christopher Done <chrisdone at googlemail.com> wrote:
>
>> > I tend to use Control.Monad.Reader for stateful stuff like this. It
>> > is found in the mtl package
>> > http://hackage.haskell.org/package/mtl-2.0.1.0
>>
>> How would you use reader for a counter?
>
> Reader is a good choice for an interthread counter:
>
>    MonadIO m => ReaderT (TVar Int) m
>
> I think, then people are talking about global variables in Haskell, what
> they really want is such a thing.
>
>
> Greets,
> Ertugrul

Though in this case, you can go with the cheaper IORef, since
(presumably) the only operation is going to be incrementing the
counter. Just be sure to use atomicModifyIORef[1].

Michael

[1] http://hackage.haskell.org/packages/archive/base/4.3.1.0/doc/html/Data-IORef.html#v:atomicModifyIORef