> left-fold and right-fold are not the only two. If you're dealing with commutative and associative operators, you can group them up in a binary (or n-ary) hier

> left-fold and right-fold are not the only two. If you're dealing with commutative and associative operators, you can group them up in a binary (or n-ary) hierarchy for parallel execution as well Unify isn't a functional language, nor do I particularly care about execution speed at present -- I'm writing it to test a concept I call Viewability. Having said that, the next version of Unify may well be more functional than the current one, and if so it could havre a way of tagging operators as commutative or associative, so the compiler can produce efficient code. > map is obviously parallelized in a purely functional language It's obviously parallelisable. It can only actually be parallelised if you have more than one CPU. > My opinion is that fold, otoh, is better limited to use with commutative and associative operators for clarity and transparent parallelization. I think it makes more sense in those cases, yes. In fact you could make fold a method with multiple implementations depending on the type of function, e.g: /* version that works with any function */ (fold ((Function f) ar) ...) /* version that requires a function that is associative and commutative, and can do appropriate optimisations */ (fold ((AssComFunction f) ar) ...)