[Haskell-beginners] Using IORef

Oliver Charles ollie at ocharles.org.uk
Tue Jan 7 11:47:20 UTC 2014

On 07/01/14 03:21, Brandon Allbery wrote:
> On Mon, Jan 6, 2014 at 10:15 PM, Courtney Robinson
> <courtney at crlog.info <mailto:courtney at crlog.info>> wrote:
>     On Tue, Jan 7, 2014 at 3:03 AM, Brandon Allbery
>     <allbery.b at gmail.com <mailto: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.)

You don't have to go that far though, if you want a 'global' IORef then
you simply make it at the top of your program and then pass it around to
anyone who needs access it to:

main :: IO ()
main = do
  counter <- newIORef 0
  doThingsWithCounter counter

doThingsWithCounter :: IORef Int -> IO ()
doThingsWithCounter counter = |atomicModifyIORef counter $\x ->
  lety =x +1in(y,y)

|So this gives you a way to have global variables, but without the pain
that they can bring in other languages.

The Reader monad stuff that Brandon suggests is a way to implicitly have
this IORef passed around everywhere, which can be useful if you have
deeply nested calls where only the children really need access to the
IORef - it saves you having to add a new parameter everywhere.

- ocharles
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