Why Tailcalls

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Are tailcalls fated to come in second place on every feature priority list?

Let's gather the use cases and consider the implementation.

(Note: This page is about "hard tail calls" as defined in the Rose blog. Soft TCO is already in many compilers, but does not have a strong effect on software architecture.)

Note from Guy Steele

Contents

use cases

multi-core task distribution

(Doug Lea) chaining task execution; without tail calls you blow the stack needlessly

languages with guaranteed TCO

These are languages with functional patterns, including Scheme, Scala, F#. Seph also aims to give this guarantee.

robust support for design patterns

Comment from Matthias Felleisen:

One common misunderstanding about TCO is due to the word 'optimization'. It is indeed a space optimization (don't use more space than goto, as Guy said in the 1970s) but a language should implement TCO in support of PROPER DESIGN. To wit, go through the OO design pattern books and inspect all the little patterns. Pick some -- say interpreter or composite -- and design your Java program accordingly. Then run a stress test and weep. Java blows up even if all the method calls are tail-recursive because it doesn't support TCO. Now do the same in PLT Scheme's class system and smile. It works -- for all inputs. See my ECOOP keynote (2004) for some more details.

Pages 51-59 in the keynote are especially relevant.

See also this discussion in the Fortress Community website:


guaranteed disposal of stack-frame

Clojure needs workarounds to avoid floating garbage

 public static int count(Object o) {
   if (o instanceof Counted)
     return ((Counted) o).count();
   return countFrom(Util.ret1(o, o = null));
 }
 static public Object ret1(Object ret, Object nil) { return ret; }

Reduce call stack size !

(RĂ©mi) Avoid to show language implementation internals by collapsing runtime stack frame.

external links

Proposal
http://blogs.sun.com/jrose/entry/tail_calls_in_the_vm
Thesis
http://www.ssw.uni-linz.ac.at/Research/Papers/Schwaighofer09Master/
mlvm Wiki
http://wikis.sun.com/display/mlvm/TailCalls
mlvm Code
http://hg.openjdk.java.net/mlvm/mlvm/hotspot/file/tip/tailc.patch
Fortress discussion
http://projectfortress.sun.com/Projects/Community/blog/ObjectOrientedTailRecursion
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