[Haskell-cafe] Beginner: IORef constructor?

[TJ]

Thanks. I've been reading the docs and examples on State (in
Control.Monad.State), but I can't understand it at all. ticks and
plusOnes... All they seem to do is return their argument plus 1...

On 12/1/06, Bernie Pope <bjpop at csse.unimelb.edu.au> wrote:
>
> On 01/12/2006, at 6:08 PM, TJ wrote:
>
> > First of all, sorry if this is a really silly question, but I couldn't
> > figure it out from experimenting in GHCi and from the GHC libraries
> > documentation (or Google).
> >
> > Is there an IORef consturctor? Or is it just internal to the
> > Data.IORef module?
> >
> > I want a "global variable", so I did the following:
> >
> > ------
> > module VirtualWorld where
> >  import Data.IORef
> >  theWorld = IORef [] -- This will be writeIORef'ed with a populated
> > list as the user modifies the world.
> > -----
> >
> > It doesn't work. GHCi says that the IORef constructor is not in scope.
> > I did a ":module Data.IORef" and then "IORef []" and it still gives me
> > the same error.
> >
> > I'm using GHC 6.6 on Windows.
>
> Hi TJ,
>
> IORef is an abstract data type, so you cannot refer to its
> constructors directly.
>
> Instead you must use:
>
>     newIORef :: a -> IO (IORef a)
>
> which will create an IORef on your behalf. Note that the result is in
> the IO type,
> which limits what you can do with it.
>
> If you want a global variable then you can use something like:
>
>     import System.IO.Unsafe (unsafePerformIO)
>
>     global = unsafePerformIO (newIORef [])
>
> But this is often regarded as bad programming style (depends who you
> talk to). So you
> should probably avoid this unless it is really necessary (perhaps you
> could use a state
> monad instead?)
>
> Read the comments about unsafePerformIO on this page:
>
>     http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-
> IO-Unsafe.html
>
> especially the notes about NOINLINE and -fno-cse
>
> Cheers,
> Bernie.
>