A Humane Introduction to Functional Programming

A gentle look at what functional programming is.

Transcript

1. A Humane Introduction to Functional Programming 9th May, 2014-ScotlandJS #sjsJohn
2. A Humane Introduction to Functional Programming 9th May, 2014-ScotlandJS #sjsJohn
3. John Sutherland sneeu.com #sjsJohn
4. A Humane Introduction to Functional Programming 9th May, 2014-ScotlandJS #sjsJohn
5. Functional Programming “… treats computation as the evaluation of mathematical functions and avoids state and mutable data #sjsJohn
6. Humane “Characterised by tenderness, compassion, and sympathy for people and animals, especially for the suffering or distressed #sjsJohn
7. “We’ll need to be strict with the 20 minute duration #sjsJohn -Peter Aitken
8. Functional Programming Matters #sjsJohn Why
9. Functional Programming Offers #sjsJohn What
10. Warning! Contrived examples ahead #sjsJohn
11. Higher-Order functions #sjsJohn Functions which take a function as an argument, return a function, or both.
12. #sjsJohn var makeAdder = function (x) { return function (n) { return x + n; }; }; ! _.map([2, 3, 5], makeAdder(7)); // [9, 10, 12] 1 2 3 4 5 6 7
13. #sjsJohn var map = function (list, func) { var i, result = []; ! for (i = 0; i < list.length; i++) { result.push(func(list[i])); } ! return result; }; 1 2 3 4 5 6 7 8 9
14. Purity #sjsJohn A pure function always evaluates to the same result given the same arguments.
15. #sjsJohn var x = 3; ! var impure = function (y) { return x + y; }; ! expect(impure(4)).toBe(7); 1 2 3 4 5 6 7
16. Immutability #sjsJohn An immutable object is an object whose value cannot be changed after it is created.
17. #sjsJohn var map = function (list, func) { var i, result = []; ! for (i = 0; i < list.length; i++) { result.push(func(list[i])); } ! return result; }; 1 2 3 4 5 6 7 8 9
18. #sjsJohn var map = function (list, func) { if (list.length == 0) { return []; } else { return [func(list[0])].concat( map(list.slice(1), func) ); } }; 1 2 3 4 5 6 7 8 9
19. “If a pure function mutates some local data in order to produce an immutable return value, is that ok? #sjsJohn -Rich Hickey
20. Application #sjsJohn
21. Project Euler Problem No. 2 #sjsJohn By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.
22. Fibonacci sequence #sjsJohn Fn = Fn−1 + Fn−2
23. 1, 1, 2, 3, 5, 8, 13 … #sjsJohn
24. var euler2 = function () { var a = 1, b = 1, result = 0, next = a + b; ! while (next <= 4000000) { if (next % 2 == 0) result += next; a = b; b = next; next = a + b; } ! return result; }; 1 2 3 4 5 6 7 8 9 10 11 12 13 #sjsJohn
25. var euler2 = function () { var a = 1, b = 1, result = 0, next = a + b; ! while (next <= 4000000) { if (next % 2 == 0) result += next; a = b; b = next; next = a + b; } ! return result; }; 1 2 3 4 5 6 7 8 9 10 11 12 13 #sjsJohn
26. var fib = function (n) { if (n < 2) return 1; return fib(n - 1) + fib(n - 2); }; 1 2 3 4 #sjsJohn
27. var fibSequence = function (max) { var i, seq = []; ! for (i = 0; fib(i) <= max; i++) { seq.push(fib(i)); } ! return seq; }; 1 2 3 4 5 6 7 8 9 #sjsJohn
28. var fibSequence = function (pred) { var i, seq = []; ! for (i = 0; pred(fib(i), i); i++) { seq.push(fib(i)); } ! return seq; }; 1 2 3 4 5 6 7 8 9 #sjsJohn
29. var twentyTerms = function (f, i) { return i < 20; }; ! var maxTerm = function (max) { return function (f, i) { return f <= max; }; }; 1 2 3 4 5 6 7 8 9 #sjsJohn
30. var fibs = fibSequence(maxTerm(4000000)); 1 #sjsJohn
31. var isEven = function (n) { return n % 2 == 0; }; ! var fibs = _.filter( fibSequence(maxTerm(4000000)), isEven ); 1 2 3 4 5 6 7 8 #sjsJohn
32. The Sum of a List #sjsJohn … is the first item plus the sum of the rest.
33. var add = function (a, b) { return a + b; }; ! var sum = function (list) { return _.reduce(list, add, 0); }; 1 2 3 4 5 6 7 #sjsJohn
34. _.reduce([1, 2, 3, 4, 5], add, 0); ! add(0, 1) // 1 add(1, 2) // 3 add(3, 3) // 6 add(6, 4) // 10 add(10, 5) // 15 #sjsJohn 1 2 3 4 5 6 7
35. Drum-roll please #sjsJohn
36. sum( _.filter( fibSequence( maxTerm(4000000) ), function (n) { return n % 2 == 0; } ) ); 1 2 3 4 5 6 7 8 #sjsJohn
37. var fib = function (n) { if (n < 2) return 1; return fib(n - 1) + fib(n - 2); }; 1 2 3 4 #sjsJohn
38. var fib = _.memoize(function (n) { if (n < 2) return 1; return fib(n - 1) + fib(n - 2); }); 1 2 3 4 #sjsJohn
39. var memoize = function (func) { var cache = {}; ! return function () { if (!_.has(cache, arguments)) { cache[arguments] = func.apply(this, arguments); } return cache[arguments]; }; }; 1 2 3 4 5 6 7 8 9 10 11 #sjsJohn
40. Review & an Improvement #sjsJohn
41. var fibs = function* () { var i = 0; ! while (true) { yield fib(i); i++; } }; 1 2 3 4 5 6 7 8 #sjsJohn
42. var fibGenerator = fibs(); ! fibGenerator.next(); // 1 fibGenerator.next(); // 1 fibGenerator.next(); // 2 fibGenerator.next(); // 3 fibGenerator.next(); // 5 fibGenerator.next(); // 8 1 2 3 4 5 6 7 8 #sjsJohn
43. Further … #sjsJohn Adventures in Functional Programming - Jim Weirich Enemy of the State - Phil Roberts Functional JavaScript - Michael Fogus
45. Thank you!