4 ms·

Tail Recursion

• anew 14y ago
  I've recently enjoyed the explanation of tail recursion on "learn you some Erlang": http://learnyousomeerlang.com/recursion http://learnyousomeerlang.com/recursion
• sdevlin 14y ago
  Decent explanation, but I have a couple quibbles. First, the term "tail recursion" is overly narrow. This transformation should apply to all tail calls, not just recursive ones. This is useful, for example, in families of mutually recursive functions. Second, this transformation is not really an optimization, as it changes the behavior of some programs (by preventing stack overflow or memory allocation errors). It's a documented feature of languages (or specific implementations of languages) that users rely on to write programs in a particular style. I prefer the term "tail call elimination" to describe these transformations.
  • kvb 14y ago
    Of course, some languages only support elimination of tail calls when they're recursive. I believe that Scala suffers from this (since the JVM doesn't support tail call elimination).
    • sdevlin 14y ago
      Good point, I'm not very familiar with Scala. I think Clojure may also have this limitation.
• kvb 14y ago
  People interested in how tail call elimination is performed by the compilers of other languages may be interested in a post I wrote on the F# team blog some time ago: http://blogs.msdn.com/b/fsharpteam/archive/2011/07/08/tail-calls-in-fsharp.aspx http://blogs.msdn.com/b/fsharpteam/archive/2011/07/08/tail-c....
  • sdevlin 14y ago
    Nice article. This makes me wonder if Visual Studio's F# mode could help detect common errors, e.g. highlight recursive calls not made in tail position. On the other hand, this could get annoying as such calls are often not a real problem in practice.
    • kvb 14y ago
      It would be possible to do this, but as you note it would potentially be annoying in practice. "tailcall" is a reserved (but unused) word in F#, so the compiler could be extended to issue an error if the user prefixed a non-tail call with that word. This "opt in" approach would presumably be less invasive while allowing people to indicate cases where the semantics are actually important to them. Having said that, as far as I'm aware there are no current plans to implement such a feature (and I haven't seen users clamoring for it, either).
• tbirdz 14y ago
  Straight out of SICP :)