New feature in ECMAScript 2015 Yusuke Miyazaki 2016/7/3 Gee-Fes Kyoto @MTRL KYOTO Proper Tail Calls

ECMAScript

ECMAScript 2015 (ES6) • ECMA-262 Edition 6, released in June 2015 • Features: arrow functions / classes / promise / let / const / template literals / generators / proper tail calls etc… • You can try some of them with recent implementations or compilers (e.g. Babel, Traceur)

ECMAScript 2016 (ES7) • ECMA-262 Edition 7, released in June 2016
 !!!!!!!!!""""""""" • Minor update from ECMAScript 2015 • exponentiation operator ** • Array.prototype.includes

Factorial

Factorial • Definition • factorial(n) = n! = 1 * 2 * 3 * … * n • Recursive Definition • factorial(0) = 1 • factorial(n) = n * factorial(n - 1)

Implementation • Implement factorial using recursive definition • It’s natural & functional!! "use strict"; function fact(n) { if (n == 0) { return acc; } return n * fact(n - 1); // Not tail call } fact(100); // 100!

Tail Call Implementation • Rewrite previous code using tail call • Function call is performed in return statement "use strict"; function fact(n, acc) { if (n == 0) { return acc; } return fact(n - 1, n * acc); // Tail call } fact(100, 1); // 100!

Let’s try it n Result Time [msec] 1,000 Infinity 0.36 10,000 Infinity 1.65 100,000 Maximum call stack size exceeded. 14655.18 049&M$BQJUBO4BGBSJ
 .BD#PPL1SP &BSMZ $PSFJ6 ???

Call Stack

Call Stack • Call stack is used when function is called • Includes arguments, return address, etc… • Size is limited
 → We reached that limit when we call fact(100,000) ộ n = 97 n = 98 n = 99 n = 100 ộ

Proper Tail Calls

Proper Tail Calls (PTC) • If a tail call meets some conditions… • strict mode, normal function or arrow function, etc… • ECMAScript 2015 requires that a call in tail position will reuse the caller’s stack space!! • Differs from tail call optimization

Pseudo-translation "use strict"; function fact(n, acc) { while (true) { if (n == 0) { return acc; } acc = n * acc; n = n - 1; } } "use strict"; function fact(n, acc) { if (n == 0) { return acc; } return fact(n - 1, n * acc); }

Benefit of PTC • Unbounded number of consecutive tail calls • Reduce stack size / memory footprint • Performance improvements • Less processing when returning value • But maybe slower than for-loop → Better performance with beautiful code!

Let’s try it again! n Result Time [msec] 1,000 Infinity 0.41 10,000 Infinity 1.84 100,000 Infinity 13.56 10,000,000 Infinity 106.07 049&M$BQJUBO4BGBSJ5FDIOPMPHZ1SFWJFX 4VQQPSUT15$ 
 .BD#PPL1SP &BSMZ $PSFJ6

Conclusion • Proper Tail Call is one of the best feature in ES2015 • Natural, beautiful & high-performance code! • Let’s try it with following implementations • Safari Technology Preview • WebKit Nightly Build • XS6

Thank You for Listening! • Reference • http://www.ecma-international.org/ ecma-262/6.0/ • https://webkit.org/blog/6240/ecmascript-6- proper-tail-calls-in-webkit/ • Demo • https://ymyzk.github.io/ptc-example/