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Why We Need Tail Calls? Senthil Porunan

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Page 1: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Why We Need Tail Calls? Senthil Porunan

Page 2: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Why We Need Tail Calls ?

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Page 3: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

3

History

Tail Call Optimization

Trampoline

Support

Agenda

Page 4: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

HISTORY

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Page 5: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

5

Edsger W. Dijkstra

(1930-2002)

Page 6: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Quality of programmers is a decreasing function

of the density of goto statements in the

programs they produce.

6

─Edsger W. Dijkstra

I became convinced that go to statement should

be abolished from all higher level programming

languages.

Page 7: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

7

Scheme language

• 1970s

• Tail call Optimization

Page 8: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

C and ML were both finished in

1973. ML had first-class functions,

GC, type inference, algebraic data

types, pattern matching, tail-calls

and exceptions.

8

─ Gary Bernhardt

Page 9: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Tail Call Optimization

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Page 10: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Execution Context without tail-call optimization

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10

GC GC GC GC GC

fact(5,1)

fact(4,5)

fact(5,1)

fact(3,20)

fact(4,5)

fact(5,1) fact(5,1)

fact(4,5)

fact(3,20)

fact(2,60)

GC

fact(5,1)

fact(4,5)

fact(3,20)

fact(2,60)

fact(1,120)

function fact(n, a) {

if (n <= 0) {

return a;

}

return fact(n-1, n *a);

}

Page 11: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

11

Without TCO

• Stack growing

• Call stack size

exceeded

Page 12: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Tail Call Optimization in ES6

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12

GC GC GC

fact(5,1) fact(4,5)

GC

fact(3,20)

GC

fact(2,60)

GC

fact(1,120)

function fact(n, a) {

if (n <= 0) {

return a;

}

return fact(n-1, n *a);

}

Page 13: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

13

With TCO

• No Stack Overflow • Constant Space

Page 14: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

14

Limitations

Page 15: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

15

function factorial(x) {

if (x <= 0) {

return 1;

}

else {

return x * factorial(x-1);

}

}

Stack will grow

function factorial(n) {

return factRec(n, 1);

}

function factRec(x, acc) {

if (x <= 1) {

return acc;

}

else {

return factRec(x-1,

x*acc);

}

}

Constant Stack

Page 16: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

16

Why we need tail call optimization ?

Constant Space

Page 17: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

17

Trampoline

Page 18: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

18

let { trampoline, done, cont } = require('trampoline-js') ;

const _fact = (acc, n) =>

n <= 0 ?

done(acc) :

cont(_fact, acc * n, n - 1)

const fact = trampoline(_fact, 1);

fact(16) //20922789888000

Page 19: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Support

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19

Page 20: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

20

Javascript – ES6

Elixir

Perl

Tcl

Common-Lisp

Language Support

Page 21: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Browser Support

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Page 22: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

22

Page 23: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

23

“ Programs must be written for people to read, and only incidentally for machines to execute. ”

- Harold Abelson, SICP

Page 24: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Thank You

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24

Page 25: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

Q & A

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Page 26: Why We Need Tail Calls? · 2017-07-19 · C and ML were both finished in 1973. ML had first-class functions, GC, type inference, algebraic data types, pattern matching, tail-calls

www.modsummit.com

www.developersummit.com

http://www.modsummit.com/
http://www.developersummit.com/

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