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Today's Topics:
1. Re: Using IORef (Oliver Charles)
----------------------------------------------------------------------
Message: 1
Date: Tue, 07 Jan 2014 11:47:20 +0000
From: Oliver Charles <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Using IORef
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
On 07/01/14 03:21, Brandon Allbery wrote:
> On Mon, Jan 6, 2014 at 10:15 PM, Courtney Robinson
> <[email protected] <mailto:[email protected]>> wrote:
>
> On Tue, Jan 7, 2014 at 3:03 AM, Brandon Allbery
> <[email protected] <mailto:[email protected]>> 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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