[Haskell-beginners] Using IORef

Brandon Allbery allbery.b at gmail.com
Tue Jan 7 03:21:17 UTC 2014

On Mon, Jan 6, 2014 at 10:15 PM, Courtney Robinson <courtney at crlog.info>wrote:
> On Tue, Jan 7, 2014 at 3:03 AM, Brandon Allbery <allbery.b at gmail.com>wrote:
>> behind
> Oh I see, thanks for the info. really helpful.
> It brings about another question now.
> How is newIORef meant to be used so that I only have a single IORef?

The Haskell way is to carry such things implicitly in a State or Reader
monad; since the IORef itself doesn't change, and you need the IO monad
around anyway to actually use it, you would use a ReaderT IORef IO and then
use ask >>= liftIO . readIORef to get the value and similar to write or
modify it. (Commonly one uses a type or newtype+newtype deriving to make
one's own monad combining them.)

brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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