[Haskell-cafe] Re: [Haskell] IVar

Simon Marlow simonmarhaskell at gmail.com
Wed Dec 5 03:58:11 EST 2007

Jan-Willem Maessen wrote:

>> Consider this:
>> do
>>    x <- newIVar
>>    let y = readIVar x
>>    writeIVar x 3
>>    print y
>> (I wrote the let to better illustrate the problem, of course you can 
>> inline y if you want).  Now suppose the compiler decided to evaluate y 
>> before the writeIVar.  What's to prevent it doing that?  Nothing in 
>> the Haskell spec, only implementation convention.
> Nope, semantics.  If we have a cyclic dependency, we have to respect 
> it---it's just like thunk evaluation order in that respect.

Ah, so I was thinking of the following readIVar:

  readIVar = unsafePerformIO . readIVarIO

But clearly there's a better one.  Fair enough.


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