Job Vranish wrote:
Is a function like the following possible?:
avoidCircularDataDependency :: a -> a -> a avoidCircularDataDependency a b = ?
For one, it breaks purity. If we have let x = avoidCircularDataDependency y True y = avoidCircularDataDependency x False then whether x and y are True or False depends on which of them is evaluated first. Note that x and y might be evaluated in parallel by different threads. Secondly, an implementation of avoidCircularDataDependencies in a threaded runtime system requires cycle detection on the heap, because running into a black hole does not reliably indicate a data dependency cycle. GHC currently only does this when things have already gone wrong; cycles are detected during garbage collection and the threads involved receive NonTermination exceptions as a result. To make avoidCircularDataDependencies useful, you'd need a more efficient way of detecting cycles, which looks like a hard problem to me.
I've often found myself wanting a function like this. It would make certain kinds of knot-tying/cycle detection _much_ easier.
Actually I'd be interested in seeing an example for this. Cheers, Bertram