[Haskell-cafe] One-shot? (was: Global variables and stuff)
k.schupke at imperial.ac.uk
Sat Nov 13 05:23:47 EST 2004
Well lets say:
userInit <- oneShot realInit
where realInit defines an MVar used for state storage that is used in
module A to implement
an accumulator. Now module B does some maths using the accumulator, and
module C does
some maths using the accumulator. If Main uses functions defined in both
B and C then they
will both be trying to use the _same_ MVar to store their state in -
which will result in the wrong answer. The following is a contrived
example, If arith and geom were in the same module, this would be an
error on the programmers part. But consider if A were in the standard
libraries, and B and C were two orthogonal extensions by different
authors, do we really want the situation where they break each other.
Note: this does not apply to declarations like (i=4) as this is true for
all time. The problem is essentially that the declaration in the
example is mutable. If mutable-declarations are not exportable, you can
reasonably say it is the module authors job to make sure all uses of the
MVar are consistent.
mVarA <- newMVar 1
acc :: Int -> IO ()
acc i = writeMVar mVarA (readMVar mVarA + i)
val :: IO Int
val = readMVar mVarA
arith :: IO [Int]
arith = do
i <- val
j <- arith
geom :: IO [Int]
geom = do
i <- val
j <- geom
main = do
a <- arith
g <- geom
putStrLn $ show (take 100 a)
putStrLn $ show (take 100 g)
Adrian Hey wrote:
>On Saturday 13 Nov 2004 9:15 am, Keean Schupke wrote:
>>>I'm not sure I understand what problem you think there is. Are the inits
>>>you're talking about module inits? If so, I don't think there's a problem,
>>>for several reasons.
>>>The idea under discussion is that a top level (x <- newThing) should
>>>be lazy, (no action at all occurs until value of x is demanded). IOW,
>>>it's exactly the same as the current unsafePerformIO hack, but not unsafe
>>>because the compiler knows the semantics. So there is no implied "module
>>Okay - I can see that with lazy semantics this might not be a problem...
>>What happens with
>>the second problem: That where module B uses A internally and C uses A
>>I write a new module that tries to use B & C together... This
>>potentially breaks B & C. I think
>>you need the extra restriction that the top level '<-' bindings must not
>>be exported. So where
>>does that leave us.
>>Top level inits are safe (I think) iff:
>> - They are lazy (the definition only happens when required)
>> - They contain only a subset of IO actions - namely those concerned
>> with name creation within Haskell that don't actually do any IO.
>> - They are not exportable from the module that contains them.
>>I think that covers it... have I forgotten anything?
>One of us has :-) Not sure who though.
>I thought I'd covered the second problem you're alluding to already.
>But if you think there's still a problem you'd better elaborate a little
>more. Certainly I see no reason why top level TWI's cannot be exported
>from a module. We don't have this constraint with the unsafePerformIO
>For instance, if I had
> userInit <- oneShot realInit
>is there any reason why userInit can't be safely exported and used
>in many different modules? The whole idea was that it should be.
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