[Haskell] Re: Global Variables and IO initializers
David Sabel
sabel at ki.informatik.uni-frankfurt.de
Sat Nov 6 08:25:12 EST 2004
The main reason is: Nobody asks for it.
I conjecture, a problem is:
if you use FUNDIO as a semantics for Haskell, you have to give up
referential transparency in the strong sense. FUNDIO-programs are
only referential transparent with respect to the defined contextual
equivalence.
David
Keean Schupke wrote:
> I hope this is not a stupid idea - but why not contribute the changes
> as patches back to the main GHC development?
>
> Keean.
>
> David Sabel wrote:
>
>> Inling isn't the only optimization, which can lead to a "wrong"
>> behavior,
>> "let floating out" and "common subexpression elimination" can also
>> change the behavior
>> of programs using unsafePerformIO.
>>
>> Our research group has developed the calculus FUNDIO as a semantic
>> basis:
>> It's a non-deterministic call-by-need lambda calculus with a
>> contextual equivalence.
>> Furthermore, with HasFuse there exists a modified implementation of the
>> Glasgow Haskell Compiler which compiles Haskell programs
>> using unsafePerformIO in a 'safe' way, i.e. deploys only those
>> optimizations
>> that have been proved correct w.r.t. FUNDIO.
>>
>> The technical report describing FUNDIO is available at
>> http://www.ki.informatik.uni-frankfurt.de/papers/schauss/FUNDIO.pdf
>>
>> More information about the related research project "DIAMOND":
>> http://www.ki.informatik.uni-frankfurt.de/research/diamond/en/
>>
>> Cheers,
>> David
>>
>> Keean Schupke wrote:
>>
>>> The problem I see here is how to proove the IO in safeIO is indeed
>>> safe. Perhaps "UnsafeIO" is a better name, as infact the IO is still
>>> unsafe - the compiler has to take special notice of this type and
>>> not inline its definitions.
>>>
>>> Your oneShot function has the same problem - if the compiler
>>> inlines the funtion you get two 'oneShot' functions.
>>>
>>> Keean.
>>>
>>> Adrian Hey wrote:
>>>
>>>> On Friday 05 Nov 2004 7:03 pm, MR K P SCHUPKE wrote:
>>>>
>>>>
>>>>> Could someone give an example of what these things are that need
>>>>> to be
>>>>> initialised and that are safe.
>>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Here's a utility I've concocted for dealing with partial ordering
>>>> constraints on initialisation of foreign libraries..
>>>>
>>>> oneShot :: IO a -> IO (IO a)
>>>> oneShot io = mdo mv <- newMVar $ do a <- io
>>>> let loop = do putMVar mv loop
>>>> return a
>>>> loop
>>>> return $ do act <- takeMVar mv
>>>> act
>>>>
>>>> The idea being that oneShot takes a real initialising action
>>>> as argument and returns a new action which will perform the
>>>> real initialisation at most once, no matter how many times it's
>>>> used.
>>>>
>>>> Suppose I want to use this to create a userInit (which is exported)
>>>> from a realInit (which isn't exported).
>>>>
>>>> Currently I have to write..
>>>>
>>>> userInit :: IO <whatever>
>>>> userInit = unsafePerformIO $ oneShot realInit
>>>>
>>>> but I think what I would really like is something like
>>>> this perhaps..
>>>>
>>>> -- For use from SafeIO monad
>>>> oneShotSafeIO :: IO a -> SafeIO (IO a)
>>>> <same definition>
>>>>
>>>> -- For use from IO monad
>>>> oneShotIO :: IO a -> IO (IO a)
>>>> oneShotIO io = liftSafeIO $ oneShotSafeIO io
>>>>
>>>> userInit :: IO <whatever>
>>>> userInit <- oneShotSafeIO realInit
>>>>
>>>> Though this could be simplified if SafeIO could be made a
>>>> sub-type of IO I guess (but I don't know a way to do this).
>>>>
>>>> Regards
>>>> --
>>>> Adrian Hey
>>>>
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>>>>
>>>>
>>>
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>>
>>
>>
>>
>
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