[Haskell-cafe] How can i safely change the value of specified key ?

[zaxis]

>>value <- readIORef aaa
>>writeIORef aaa (f value) 
then aaa will *point to* a new value. The original value will be Garbage
Collected, right ?
BTW, 
Is [(1,1),(2,2),(3,3)] been regarded as a hash ? If not, what is the best
way to change it to [(1,1),(2,2222),(3,3)] in function `f` ?


Bulat Ziganshin-2 wrote:
> 
> Hello zaxis,
> 
> Thursday, October 22, 2009, 11:28:14 AM, you wrote:
> 
>> aaa <- newIORef ([]::[(Int,Int)])
>> writeIORef aaa [(1,1),(2,2),(3,3)] 
> 
>> then if i want to change aaa to [(1,1),(2,222),(3,3)] , what's the best
>> way
>> ? 
>> re-write aaa is not permitted.
> 
> it's the only way. in Haskell, you have *immutable* values. aaa is a
> reference to immutable value. you can mutate reference so it will
> point to another immutable value but you cannot change this value.
> 
> there are two ways to make aaa==[(1,1),(2,222),(3,3)]. first, you can
> apply function to whole value:
> 
> value <- readIORef aaa
> writeIORef aaa (f value)
> 
> second, you may create list of IORefs, tuple of IORefs and so:
> 
> a <- newIORef (1,1)
> ...
> let aaa = [a,b,c]
> 
> now aaa is a immutable list of mutable IORefs. of course, you can
> create IORef pointing to list of IORefs too
> 
> 
> 
> -- 
> Best regards,
>  Bulat                            mailto:Bulat.Ziganshin at gmail.com
> 
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> 
> 

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