theValueRef is a reference to a "box" that contains a totally separate, mutable value.

When you use newIORef to create theValueRef, it's *copying* theValue into the box. When you mutate theValueRef, you're mutating the value inside the box - theValue remains unchanged.

Cheers,

Adam

On 22 June 2012 11:30, Captain Freako <capn.freako@gmail.com> wrote:

Hi experts,

I fear I don't understand how to properly use Data.IORef.
I wrote the following code:

1 -- Testing Data.IORef
2 module Main where
3
4 import Data.IORef
5
6 bump :: IORef Int -> IO()
7 bump theRef = do
8     tmp <- readIORef theRef
9     let tmp2 = tmp + 1
10     writeIORef theRef tmp2
11
12 main = do
13     let theValue = 1
14     print theValue
15     theValueRef <- newIORef theValue
16     bump theValueRef
17     return theValue

and got this, in ghci:

*Main> :load test2.hs
[1 of 1] Compiling Main             ( test2.hs, interpreted )
Ok, modules loaded: Main.
*Main> main
1
1

I was expecting this:

*Main> :load test2.hs
[1 of 1] Compiling Main             ( test2.hs, interpreted )
Ok, modules loaded: Main.
*Main> main
1
2

Can anyone help me understand what I'm doing wrong?

Thanks!
-db

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