[Haskell-cafe] readEitherChan

[Simon Marlow]

On 19 June 2004 03:06, Thomas Hallgren wrote:

> Tomasz Zielonka wrote:
> 
>> On Tue, Jun 15, 2004 at 06:29:59PM -0400, S. Alexander Jacobson
>> wrote: 
>> 
>> 
>>>  readEitherChan::Chan a -> Chan b -> Either a b
>>> 
>>>  eitherChan a b left right = readEitherChan a b >>= either left
>>> right 
>>> 
>>> 
>> ... I think it would be safer to create once new Chan (Either a b),
>> and then read from it. 
>> 
>>    createEitherChan :: Chan a -> Chan b -> IO (Chan (Either a b))
>> 
>> 
> Although createEitherChan might be the best solution for some
> application, it would be interesting to know what the recommended way
> to implement the function eitherChan above is, if that is what you
> really need? 
> 
> The eitherChan function corresponds to guarded nondeterministic
> choice, usually written (a?P)+(b?Q) process calculi, i.e., a rather
> fundamental concurrency operator.
> 
> Section 5 of [1] enumerates some reasons why guarded nondeterministic
> choice is not available as a primitive in Concurrent Haskell. Section
> 5.2 describes a function select for nondeterministic choice, but I
> don't see how to use that to implement readEitherChan (or
> takeEitherMVar)... 

It's a hard problem.  Trying to code up takeEitherMVar by hand always
seems to lead to solutions with race conditions - I can't see a way to
do it.

Perhaps takeEitherMVar should be provided as a primitive, but I'm
concerned that the same problems will crop up if we tried to implement
takeEitherMVar in a multi-threaded runtime (GHC doesn't currently have a
true multi-threaded runtime, but we'd like to do it someday).

So I think in order to do this you have to build another abstraction
around MVars.  I've attached some code I just hacked up to do just that;
it's untested (but compiles), and has a couple of known deficiencies,
but it might be along the right lines.

Cheers,
	Simon
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