Job Vranish
I've often found myself wanting a function like this. It would make certain kinds of knot-tying/cycle detection _much_ easier.
Is there any reason why this function can't/shouldn't exist?
This makes me think of the more widely-known 'parallel OR' operator, which evaluates its arguments in parallel and returns whichever finishes evaluating first. This operator cannot be implemented in Lambda Calculus, but it can in Haskell (via threads). Unfortunately Googling for 'haskell "parallel or"' brings up sentence fragments ('...parallel or concurrent...') rather than a parallel or implementation. You couldn't use parallel or as-is, since your "fallback" value will probably evaluate faster than your "real" value most of the time, but it may give you hints. Cheers, Chris