[Haskell] threading mutable state through callbacks

[Jules Bean]

On 12 Oct 2004, at 20:25, Adrian Hey wrote:

> On Tuesday 12 Oct 2004 6:28 pm, Jules Bean wrote:
>> On 12 Oct 2004, at 14:08, Adrian Hey wrote:
>>>> x <- someAction
>>>> y <- someAction(x)
>>>
>>> I would say keep things as they currently are with the 
>>> unsafePerformIO
>>> solution, I.E. Order unspecified, the action that creates a 
>>> particular
>>> top level thing is executed only once, when the value of thing is
>>> demanded (perhaps not at all).
>>
>> Also consider the case of
>>
>> z = someAction(y)
>>
>> Here z is a value outside the IO monad, calculated using a function
>> outside the IO monad, based on a value (y) which also lies outside the
>> IO monad... What sane semantics will explain when the actions which 
>> led
>> to the value y should be taken?
>
> I've never really understood what people mean by things being "inside" 
> and
> "outside" the IO monad :-(
>
> Assuming from Vincenzos original example that ..
>
> someAction :: SomeType -> IO Thing
>
> then in your example..
>
> z :: IO Thing

Yes, I wrote it wrong. I meant

z = someFunction(y)

'Inside the IO Monad' means having some type IO a. Outside it means 
having a type not of this form. At least, that's what I meant. 
[Vincent's explanation is another way of looking at it]

> This still leaves the uncertainty of exactly when the creation occurs,
> but this is a minor issue IMO. We have the same problem with finalisers
> and any concurrent code. Either it just doesn't matter, or if it does
> then it's the programmers responsibility to use existing mechanisms
> (such as seq) to control things.
>

It does matter for general IO operations at the top level. Order 
doesn't matter in in any essential way for the particular case of 
newIORef, no.

> What I want is non-hack mechanism for creation of arbitrarily complex
> "things with identity" at the top level. I guess you could arrange that
> all such things were constructed at run time before executing main, but
> I don't see any real advantage in that.
>

Now it's my turn not to understand you. What is a "thing with identity"?

Jules