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Comment by jimktrains2 | Hacker News Reader

Stack limits are pretty large. It's a natural way to express many problems such as graphs (and their subsets trees).

When people talk of "Tail Call Optimization" (TCO) they are talking about a way to recurse without leaving things on the stack (tail-call meaning that the recuse call is the last thing in the function so the state of the function doesn't need pushed on the stack, it can be discarded.

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> Stack limits are pretty large. It's a natural way to express many problems such as graphs (and their subsets trees).

Coincidentally, I had a stack overflow while working on some trees a few months ago, it ate all my ram within about 2 seconds. Although my stack was sizeable, my program was blind and unaware of the stack limit.

Other languages probably do this too, but some Scheme implementations don't have a stack limit, or no stack at all. So you can define certain functions (map being a notable one) using their natural recursion and not worry about exhausting the stack with large inputs.

That's because of TCO, not because of a stack limit or its absence. A non-tail-call version of map would still blow up when you run out of memory, whether the implementation uses a stack like C, or keeps the stack on the heap (as I understand some scheme implementations do, particularly for dealing with continuations).

Yes, of course you can exhaust the heap, but there are some tricks that can be done to make it not much of an issue. For instance, "Guile arranges to return the unused part of the stack to the operating system, but without causing the stack to shrink." [0]

So, thanks to that, we can define map the clear and easy way:

    (define (map f l)
      (if (pair? l)
          (cons (f (car l))
                (map f (cdr l)))
          '()))

Instead of the more wasteful, less readable way:

    (define (map f l)
      (let lp ((l l) (out '()))
        (if (pair? l)
            (lp (cdr l) (cons (f (car l)) out))
            (reverse out))))

[0] https://gnu.org/software/guile/docs/master/guile.html/Stack-...

Further down in that document, and in your quoted portion, it's clear that this doesn't prevent stack growth to the limit of the system's memory. It only mitigates the issue. They double the size of the stack (by default) when they hit the current stack limit. When the GC happens they take the still unused portion of the stack memory and turn it back over to the OS by freeing it. If the recursion is still too large for the overall system memory it'll still hit a limit (system memory), they've just succeeded in delaying it for non-tail recursive functions.

That said, it is a nice way to reduce the chance or delay the occurrence of hitting the stack limit (whether it's on the heap or an explicit stack that's still what's happening).

Well yeah, if you can make a procedure less than O(n) in space by using tail recursion, you should. But in the case of map, you might as well use the stack, since you have to reverse the list in the tail recursive version anyway.

Oh, I definitely see the elegance of expression programs or concepts recursively.

Is TCO used in C++? I've only ever seen it mentioned in reference to Scheme and Haskell. It's a compiler optimization, so it would be transparent to the programmer, right?

C++ optimizes only with -O2 or /O2 passed to the compiler (for GCC and MSVC++, not sure about Clang).

You can't count on it in the general case even with gcc (even when taking care to heed the restrictions like not passing pointers to data on the stack, and putting all functions into the same compilation unit). I did tests a couple years ago where it would compile up to about a handful of mutually recursive functions with TCO, then when you added one more, it would disable TCO. I guess because some algorithm will stop analyzing the call graph past a certain mark and then disallow TCO from being applied.

If it isn't on the standard, anything goes.

There are much more compilers out there than those three.

TCO is available in plenty of other languages as well. Scala, Clojure, Erlang, and Elixir support it out of the box. Ruby has a VM setting for it, and there was a proposal to add it to JavaScript. I'm sure there are plenty of others.

Thanks, that's good to know.

Just to be picky.... Clojure doesn't have TCO, it has 'recur' as a substitute instead.

To add a finer point, Clojure doesn't have TCO per se because the JVM doesn't have tco. Recur optimizes the special case of "tail-recursive self-call and not the generalized tail call."[0] That is to say when the tail of A calls A, but not when A calls B which calls A.

[0] "The Joy of Clojure," Fogus and Houser

Interestingly, the 64bit CLR JIT does TCO while the 32 bit variety does not.

That's not accurate. In fact, in the .NET 2.0 timeframe the opposite was the case (the 64-bit JIT would fail to honor the IL tail. prefix in many cases, though it would also make tail calls in other cases even when the prefix wasn't applied)[1]. But as of .NET 4.0, both platforms honor the tail. prefix except in a very limited set of circumstances[2] (e.g. calls from untrusted code to trusted code).

As far as I know, F# is the only reasonably popular .NET language to emit the tail. prefix, though[3].

[1] http://blogs.msdn.com/b/davbr/archive/2007/06/20/tail-call-j...

[2] http://blogs.msdn.com/b/clrcodegeneration/archive/2009/05/11...

[3] http://blogs.msdn.com/b/fsharpteam/archive/2011/07/08/tail-c...

Per ch4s3's comment on Clojure, Scala also does not have TCO, at least, last I checked. The JVM doesn't allow it. It has a specialized handling of functions that call themselves, which it can optimize, but A that calls B that calls A that etc is not (nor, in the more general case where any function that ends with a function is removed from the stack).

Yes it does.

That is an implementation detail. It doesn't matter if it is does at compiler or whatever runtime is targeted.

This is called lowering in compiler design speak.

Umm...yes, it does matter.

Closures? Can't be optimized. Polymorphic method? Can't be optimized (even if every implementation is tail recursive).

This is more than an implementation detail, it actively influences the way you write your code.

And Scala -doesn't- optimize A calls B calls A etc cases. It requires the developer to explicitly make use of scala.util.control.TailCalls. See http://www.scala-lang.org/api/2.11.2/index.html#scala.util.c... and associated white paper.

What Scala gives you is that if A calls A (calls A calls A), and is tail recursive, and A can not be overridden, it will optimize it.

It's implementation-dependent; a particular compiler may do it, or may not. Scheme, OTOH, has it written in the spec--you WILL do tail call optimization, so programmers can count on it.

Most C++ compilers perform TCO when convenient, but the C++ language doesn't guarantee that TCO will be done, so it's unwise to write code that depends on it. In practice, every compiler decides when to do TCO in its own way. Also, it's useful to be able to run code with optimization disabled. Also, it's easy to accidentally have a variable that needs a destructor call which makes TCO impossible. This is all in contrast to Scheme, which does give you a guarantee, and which makes it easy.

TCO isn't really transparent to the programmer. Some recursive calls cannot be optimized, so the programmer has to know how to write recursive functions that can be.

The problem with TCO in C++ is destructors; unless you're using a very limited set of types, they all but guarantee you're going to have code to execute after returning from what would otherwise be a tail call.

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