5 ms·

On (1) recursion elimination ... Tail-recursions are the exact equivalent of loops. We've lost sight of what "recursion" means. There's a difference between r

• lmm 12y ago
  Scala handles this in quite a nice way: the compiler will optimize tail-recursive functions without needing any special effort, but if you want to ensure a particular function is tail-recursive as a matter of correctness, you annotate it with @tailrec and the compiler will error out if it can't optimize that function.
• gohrt 12y ago
  > recursive algorithms, which need by definition a stack Your vocabulary is not standard. https://www.google.com/webhp?#q=recursive+algorithm https://www.google.com/webhp?#q=recursive+algorithm http://en.wikipedia.org/wiki/Recursion_(computer_science) http://en.wikipedia.org/wiki/Recursion_(computer_science) A recursive algorithm is one that computes a result by way of the same algorithm applied to a smaller input. (Smaller, or else the algorithm will diverge.) The distinction between "stack" and "parameter" is a minor one. A simple factorial function has an ever-growing (accumulating) parameter. Tail-recursive algorithms can be translated into loop algorithms. Thet are still recursive.
  • JadeNB 12y ago
    > (Smaller, or else the algorithm will diverge.) This is not true. The first example that comes to mind is the ridiculously over-powered one: conjecturally (https://en.wikipedia.org/wiki/Collatz_conjecture https://en.wikipedia.org/wiki/Collatz_conjecture), the definition f x | x `mod` 2 == 0 = f x/2 f x | x `mod` 2 == 1 = f $ 3*x - 1 f 1 = [ whatever ] is perfectly well defined on positive integers `x`, although many recursive calls actually apply `f` to larger values. (To be fair, you didn't actually say what 'smaller' meant. I think it's one of the standard term-rewriting theorems that, if given the freedom to define 'smaller' appropriately, then any total recursive function does make only smaller recursive calls.)
  • bad_user 12y ago
    It's very standard in computer science in the theory of formal languages. https://en.wikipedia.org/wiki/Pushdown_automaton https://en.wikipedia.org/wiki/Pushdown_automaton
• malisper 12y ago
  > Tail-recursions are the exact equivalent of loops. Not necessarily. In the common case of a procedure calling itself a loop does wind up being equivalent. OTOH with mutually recursive procedures (two or more procedures which call each other), there is no straightforward way to convert the tail calls into a loop.
  • mihai_ionic 12y ago
    Is there any reason why something like the following should not work? while (condition) { a(); if (!condition) break; b(); }
    • malisper 12y ago
      Your example assumes that you can inline the calls to a and b. If you can't (for example if one of them calls themselves) then your optimization won't work.
• more_original 12y ago
  > I'm also pretty sure that the compiler is extremely limited in what it can do here. And that is dangerous, as running in constant space is often a matter of correctness, not one of optimization. I would rather have the compiler err on me if it sees non-tail recursive calls, rather than try to be smart sometimes. I've sometimes wondered why OCaml has "let" and "let rec", but not "let tail rec". It seems that something like this would be good for documentation and avoinding errors.
• StefanKarpinski 12y ago
  This is really only essential in languages which provide recursion as a primitive and not iteration. Scheme is the prototypical example and was, I believe, the first to require tail call elimination in its standard for this very reason. In C it really doesn't matter that much because you can just write the iteration explicitly and then you know you won't blow out the stack since there is no recursive call in the first place. As a bonus, many programmers find it easier to understand.