Introduction to Pragmatic Functional PHP

Functional programming is a paradigm with origins in lambda calculus and the 1930's. Stack Overflow's developer surveys from both 2017 and 2018 show that the languages optimized for functional programming are low in popularity. However, the same survey recognizes that developers actually working in Elixir, Clojure, F#, and Haskell, actually love their languages much more than PHP or C developers.

So functional programming is unpopular but fun!

The good news is that it can be applied in a popular language like PHP. Whilst the bad news is that PHP might not be really optimized for functional programming, we already have pretty nice libraries of functional primitives for PHP. Accepting functional programming style promises to help PHP developers develop a bit cleaner or more interesting code. More importantly, it will make it a whole lot easier to pick up a purely functional language (Hello, Haskell!) or make use of serverless architectures such as AWS Lambda.

Transcript

1. Branislav Bujisic - Platform.sh Introduction to Pragmatic Functional PHP
2. –Saunders Mac Line, Categories for the Working Mathematician “All told, a monad in X is just a monoid in the category of endofunctors of X, with product X replaced by composition of endofunctors and unit set by the identity endofunctor.”
3. Continuous Deployment Cloud Hosting Drupal 7 / Drupal 8 / Front End Product Engineer
4. Bad Accent Included
8. Fathers of computing were playing in their laboratories... The goal: 
 a computation model free of human decision making
9. Alan Mathison Turing (1912-1954)
10. Alonzo Church (1903-1995)
11. x result A Function calculation
12. λx.x <?php // Identity function. function ($x) { return $x; } x x
13. λx.λy.x2+y2 <?php // Sum squares function. function($x, $y) { return $x*$x + $y*$y; } x x2+y2 x y
14. (λx.x)y ͢ y <?php $identity = function($x) { return $x; } // Expression $y applied to
 // the identity function $identity($y); // returns $y
15. (λx.x)(λx.λy.x2+y2) <?php $identity = function($x) { return $x; } $sum_squares = function($x, $y) { return $x * $x + $y * $y } return $identity( $sum_squares($x, $y); ); x x2+y2 x y x
16. EVERYTHING is a function, even a boolean value! TRUE := λa.λb.a FALSE := λa.λb.b AND := λp.λq.pqp OR := λp.λq.ppq
 NOT := λp.λa.λb.pba
17. TRUE AND FALSE = ? " TRUE FALSE TRUE " FALSE (λp.λq.pqp) TRUE FALSE p q " (λa.λb.a) FALSE TRUE a b
18. EVERYTHING is a function, even a natural number! 0 := λf.λx.x function zero($f, $x) { return $x;
 } 1 := λf.λx.fx function one($f, $x) { return $f($x);
 } 2 := λf.λx.f(fx) function two($f, $x) { return $f($f($x));
 }
19. 1+2 = ? " (λf.λx.f(f(fx)) (λf.λx.fx)(λf.λx.f(fx))
21. John Von Neumann (1903-1957)
 Neumann János
22. The Von Neumann Model Input Device Output Device Memory Central Processing Unit Control Unit Arithmetic / Logic Unit ebp-0x1 ebp-0x2 ... 00001010 00000101 ebp-0x4 ebp-0x3 mov eax, [ebp-0x1] jmp ebp-0x0 {
23. The Machine Language mov eax, [ebp-0x1] mov ebx, [ebp-0x2] loopTop: cmp eax, ebx jle loopDone dec eax jmp loopTop loopDone: ebp esp FFFD ebp-0x1 ebp-0x2 ... 10 5 ebp-0x4 ebp-0x3 FFF7 ... ...
24. Higher-Level Programming Languages 10 I = 10 20 J = 5 30 IF I <= J GOTO 60 40 I = I - 1 50 GOTO 30 60 ... mov eax, [ebp-0x1] mov ebx, [ebp-0x2] loopTop: cmp eax, ebx jle loopDone dec eax jmp loopTop loopDone: compiler
25. Higher-Level Programming Languages mov eax, [ebp-0x1] mov ebx, [ebp-0x2] loopTop: cmp eax, ebx jle loopDone dec eax jmp loopTop loopDone: for ($i = 5; $i <= 10; $i++) { ... } compiler
26. Higher-Level Programming Languages mov eax, [ebp-0x1] mov ebx, [ebp-0x2] loopTop: cmp eax, ebx jle loopDone dec eax jmp loopTop loopDone: compiler MyFancyClass - i: int + myFancyLoop
27. Meanwhile, the academia contuned playing with the lambda calculus...
28. Functional Programming Languages
29. Once set, a variable must not be changed during the runtime. Immutability Basic Concepts of Functional Programming class Immutable
 {
 private $var;
 
 public function __construct($var)
 {
 $this->var = $var;
 }
 
 public function get()
 {
 return $this->var;
 }
 } Pragm atic?
30. Not a requirement but a convenient way to prevent unplanned problems. Static Typing Basic Concepts of Functional Programming function sumOfInts(int ...$ints): int
 {
 return array_sum($ints);
 }
31. A pure function always returns the same result for the given set of parameters. sin() 3.14 0.00 1 0.84 2 0.91 Param Result 1 0.84 2 0.91 3.14 0.00 Referential Transparency Basic Concepts of Functional Programming
32. A pure function never changes a state outside of its scope. Eliminated Side-Effects Basic Concepts of Functional Programming user_save() $user $user
33. A pure function does not depend on external state and relies only on the parameters it was called with. No External Dependencies Basic Concepts of Functional Programming user_load($uid) $user
34. A higher-order function can accept another functions as arguments and return a function. Higher-Order Functions Basic Concepts of Functional Programming array_filter($users, $callback) $callback = function ($user) { return ($user->age >= 18); }; $callback = function ($user) { return ($user->gender == 'f')
 && ($user->age < 13); };
35. A function can be passed as argument to other functions, returned as value from other functions and assigned to variables. First Class Functions Basic Concepts of Functional Programming function area($r) {/*...*/} $area = function($r) {/*...*/} array_map($area, $radiuses);
36. But... Why?! • Testing • Caching • Complexity control • Performance*
37. Testing params result [5, 0] 25 [0, 5] 3 [5, 5] 28 [0, 0] 0 test($myObscureFunction, $table); $myFunction = function(int $a, int $b) { // I don't care about the content of
 // this function. If it is clean,
 // it will always return the same // response for the given params. }
38. Testing function test($function, $table) { foreach ($table as $row) { $params = $row['params']); $expected_result = $row['result'];
 
 $result = call_user_func_array($function, $params);
 if ($result !== $expected_result) { echo "Test failed for parameters "
 . implode(', ', $row['params']); } } }
39. Caching –Phil Carlton. Quoted by Martin Fowler in TwoHardThings "There are only two hard problems in Computer Science: cache invalidation and naming things.
40. Caching Memoization params result [5, 0] 25 [0, 5] 3 [5, 5] 28 [0, 0] 0 memoize($myObscureFunction, $parameters); $myFunction = function(int $a, int $b) { // I don't care about the content of
 // this function. If it is clean,
 // it will always return the same // response for the given params. }
41. Memoization:
 Nuts and Bolts function memoize(callable $func, $params = []) { return call_user_func_array($func, $params); } 
 $result = memoize($somethingTerriblyExpensive, [2, 5]); $somethingTerriblyExpensive = function($a, $b) { // ...
 } 
 static $storage = []; $key = get_class($func) . implode(':', $params); // OMG, BAD!! if (!isset($storage[$key])) { $storage[$key] = call_user_func_array($func, $params); } return $storage[$key];
42. Complexity Control Does not necessarily mean "less code to write"
43. –Neal Ford. Functional Thinking (Sebastopol: O'Reilly, 2014) "I’d rather spend my time at a higher level of abstraction, thinking about ways to solve complex business scenarios, not complicated plumbing problems."
44. Higher level of abstraction means: • Less plumbing problems • More focus on business problems • Easier debugging and refactoring • Less control over every aspect of the program • Performance tradeoffs
45. The Stack Push Pop FFFD FFFE FFFF FFFC FFFB FFFA Addr Value FFF8 FFF9 FFF7 FFF6 Stack Frame ebp esp Junk Junk
46. What else can be abstracted away?
47. Loops are not business logic $collection = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; $result = 0; foreach ($collection as $item) { if ($item % 2 == 0) { $result += $item * $item; } } print $result;
48. Abstract away the loop! $collection = new Collection([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]); print $collection ->filter(function ($x) { return $x % 2 == 0; }) ->map(function ($x) { return $x * $x; }) ->reduce(function($x, $y) { return $x + $y; }); use bbujisic\functional\Collection; composer require bbujisic/functional reduce filter map
49. Design patterns are not a business logic Therefore they should be abstracted away
50. Decorator Pattern Pizza 4.00 Mushrooms +1.00 Mozarella +2.00 Basil +0.50 Mushrooms getPrice() <i> Sellable getPrice() Pizza getPrice() <a> Topping __construct(Sellable)
 getPrice() Mozarella getPrice() Basil getPrice()
51. Decorator Pattern $pizza = new Pizza(); $mushroomPizza = new Mushrooms($pizza); $mushroomMozarellaPizza = new Mozarella($mushroomPizza); //... $mushroomMozarellaBasilPizza ->getPrice(); Mushrooms getPrice() <i> Sellable getPrice() Pizza getPrice() <a> Topping __construct(Sellable)
 getPrice() Mozarella getPrice() Basil getPrice() $badPizza = new Basil( new Mozarella( new Mushrooms ( new Pizza() ) ) );
52. Abstract away the Decorator!
 Use composition instead. $badPizza = compose($mushrooms, $mozarella, $basil); $mushrooms = function($price) {return $price + 1;};
 $mozarella = function($price) {return $price + 2;};
 $basil = function($price) {return $price + .5;}; $badPizzaPrice = $badPizza(4); // 4 == base price // Tripple Mozarella FTW
 $fixBadPizza = compose($badPizza, $mozarella, $mozarella);
53. It's not all lollipops and unicorns function identity($value) { return $value; } function compose(...$functions) { return array_reduce( $functions, function ($chain, $function) { return function ($input) use ($chain, $function) { return $function($chain($input)); }; }, 'identity' ); }
54. Primitives for functional programming in PHP { "require": { "lstrojny/functional-php": "~1.2" } }
55. Composition use function Functional\compose;
 
 $betterPizza = compose( $pizzaDough, $mozarella, $mozarella, $mushrooms, $basil, $tomatoes );
56. Memoization use function Functional\memoize;
 
 $results = memoize( $expensiveFunctionYouWantToCache, [$param1, $param2, $param3] );
57. Operations on traversable objects and arrays use function Functional\every; use function Functional\select; use function Functional\invoke;
 
 if (every($group, function($user) {$user->age > 18}) {
 print "You may enter the bar!"; } $kids = select($group, function($user) {
 $user->age < 12;
 }); invoke($kids, 'goToBed', []); // ...
58. Performance
59. time 0s 0.4s 0.8s 1.2s 1.6s map filter reduce 0.86 1.54 1.27 0.48 1.12 0.72 0.23 0.54 0.58 imperative functional lstrojny/functional-php Tested on OS X, Apache 2.4 and PHP 7.1 10 000 000 operations
60. It is safe to parallelize stateless processes
61. Synchronous monthly invoicing Payment Pending Order Email the user Charge Credit Card Completed Order 1 000 times in a single night,
 1 minute per job (optimistic)
 Total: 16h 40m Issue the invoice
62. Synchronous monthly invoicing Payment Pending Order Email the user Charge Credit Card Completed Order 1 000 times in a single night,
 1 minute per job (optimistic)
 Total: 16h 40m Issue the invoice Invoicing Queue
63. Synchronous monthly invoicing Payment Pending Order Email the user Charge Credit Card Completed Order 1 000 times in a single night,
 1 minute per job (optimistic)
 Total: 16h 40m Invoicing Queue Issue the invoice Issue the invoice Issue the invoice
64. Upload a function Request Response
65. Serverless • Compute Service On Demand • Pay infrastructure only during the execution of functions • AWS Lambda (Java, Python, Node.js, C#)
 Google Cloud Functions (Node.js)
 MS Azure Functions (C#, Node.js)
 IBM Cloud Functions (Node.js, Swift, Java, PHP, Python, any compiled language in Docker) • Execute functions on event or on http request
66. PHP is not a functional language
67. PHP monads?
68. Use functional style of programming together with the object-oriented code
69. And if you are a full-stack developer...
70. FTW const collection = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; const result = collection .filter(i => i % 2 === 0) .map(i => i * i) .reduce((x, y) => x + y, 0); console.log(result);
71. Wanna talk about functional programming, platform.sh or even monads? Branislav Bujisic
 branislav@platform.sh @bbujisic Thanks!