[gizmo686]

I don't think a lazy language can necessarily make that decision. Consider the simple implementation of ||:

  ||(f,g){
    if (eval(f)) then return true
    if (eval(g)) then return true
    return false
  }

A lazy language will not evaluate a function until it knows it needs it. In this implementation, there is no way of knowing that g will be needed until after computing f, so f will be computed first. Having said that, it is possible for an optimizing compiler to realize that the order doesn't matter and make a decision on which one to check first, however that is an optimization that the compiler would need to prove does not change the results.

This seems like a good example of why purity seems to be really beneficial to lazy evaluation.