Functional programming, though far from new, has gained much traction recently. Functional programming characteristics have started to appear in the PHP world, too. Microframeworks including Silex and Slim, middleware architectures (Stack) and even standards (PSR-7) rely on concepts such as lambdas, referential transparency and immutability, all of which come from functional programming. I’ll give you a crash course in Erlang, a pragmatic functional language to make you feel familiar with the functional paradigm. By comparing code samples between Erlang and PHP, you’ll find out how and why you should employ functional programming in your PHP applications. You’ll see that functional programming is nothing to be scared of. On the contrary, understanding its concepts broadens your programming horizon and provides you with valuable solutions to your problems.

1. Being functional in PHP
David de Boer

3. functional

4. functions

5. y = f(x)
f: X → Y

6. functional programming
functional thinking
functional communication
not languages

7. Why should you care?

10. f(x)
Others
PHP
C++
C
Java

11. – Robert C. Martin
“There is a freight train barreling
down the tracks towards us, with
multi-core emblazoned on it; and
you’d better be ready by the time
it gets here.”

12. λ
closures
__invoke
2001 2009 2011 2012
array_map
array_filter
callable
Slim
Silex
middlewaresSymfony2

13. 2013 2014 2015 2016
PSR-7
anonymous
classes
foreach
promises
futuresStackPHP
HTTP
middleware
proposal

14. “The limits of my language
mean the limits of my world.”
– Ludwig Wittgenstein

15. I’m David
/ddeboer
@ddeboer_nl

17. Erlang: The Movie

18. Erlang
Concurrent
Passing messages
Fault-tolerant

19. Let’s begin$ brew install erlang
$ erl
Erlang/OTP 18 [erts-7.2.1] [source] [64-bit] [smp:4:4] [async-threads:
10] [hipe] [kernel-poll:false] [dtrace]
Eshell V7.2.1 (abort with ^G)
1>
on OS X
REPL

20. ➊

21. <?php
$sum = 0;
for ($i = 1; $i <= 5; $i++) {
$sum = $sum + $i;
}
echo $sum;
// 15

22. -module(math).
-export([sum/1]).
sum(Number) ->
Sum = 0,
Numbers = lists:seq(1, Number),
lists:foreach(
fun(N) ->
Sum = Sum + N
end,
Numbers
),
Sum.
math:sum(5).
no return
keyword
** exception error: no match
of right hand side value 1

23. 1> X = 5.
5
2> X.
5
3> X = X * 2.
** exception error: no match of right hand side value 10
4> 5 = 10.
** exception error: no match of right hand side value 10
but isn’t
looks like
assignment

24. 1> lists:sum(lists:seq(1, 5)).
15

25. ➋

26. Imperative<?php
$sum = 0;
for ($i = 1; $i <= 5; $i++) {
$sum = $sum + $i;
}
iteration
keeps
changing

27. -module(math2).
-export([sum2/1]).
sum2(Number) ->
List = lists:seq(1, Number),
sum2(List, 0).
sum2([], Sum) -> Sum;
sum2([H|T], Sum) -> sum2(T, H + Sum).
3> math2:sum(5).
15
empty list
separate head from tail recurse
pattern
matches
generate list

28. Declarative 1<?php
// Declare a function!
function sum($x)
{
if ($x == 0) {
return $x;
}
return $x + sum($x - 1);
}
sum(5);
// still 15
$x never
changes
recursion

29. Declarative 2<?php
function sum2($number)
{
array_sum(range(1, $number));
}
echo sum2(5);
// yuuuup, 15 again
functionfunction
composition

30. Some history

31. Church Von Neumann
declarative imperative

33. A lot of our code is about the
hardware it runs on
But programmers should worry
about the conceptual problem domain

34. Recursion-module(math).
-export([fac/1]).
fac(0) -> 1;
fac(N) -> N * fac(N - 1).
1> math:fac(9).
362880
function
calls itself

35. Recursion fail-module(math).
-export([fac/1]).
fac(0) -> 1;
fac(N) -> N * fac(N - 1).
%%fac(9) = 9 * fac(9 - 1)
%% = 9 * 8 * fac(8 - 1)
%% = 9 * 8 * 7 * fac(7 - 1)
%% = 9 * 8 * 7 * 6 * fac(6 - 1)
%% = 9 * 8 * 7 * 6 * 5 * fac(5 -1)
%% = 9 * 8 * 7 * 6 * 5 * 4 * fac(4 - 1)
%% = 9 * 8 * 7 * 6 * 5 * 4 * 3 * fac(3 - 1)
%% = 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * fac(2 - 1)
%% = 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1 * fac(1 - 1)
%% = 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1 * 1
%% = 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1
%% = 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2
%% = 9 * 8 * 7 * 6 * 5 * 4 * 6
%% = 9 * 8 * 7 * 6 * 5 * 24
%% = 9 * 8 * 7 * 6 * 120
%% = 9 * 8 * 7 * 720
%% = 9 * 8 * 5040
%% = 9 * 40320
%% = 362880
10 terms in
memory

36. Tail recursion-module(math).
-export([tail_fac/1]).
tail_fac(N) -> tail_fac(N, 1).
tail_fac(0, Acc) -> Acc;
tail_fac(N, Acc) -> tail_fac(N - 1, N * Acc).
tail_fac(9) = tail_fac(9, 1).
tail_fac(9, 1) = tail_fac(9 - 1, 9 * 1).
tail_fac(8, 9) = tail_fac(8 - 1, 8 * 9).
tail_fac(7, 72) = tail_fac(7 - 1, 7 * 72).
tail_fac(6, 504) = tail_fac(6 - 1, 6 * 504).
tail_fac(5, 3024) = tail_fac(5 - 1, 5 * 3024).
…
tail_fac(0, 362880) = 362880
do calculation
before recursing

37. ➌

39. $object->setFoo(5);
$value = $object->getFoo();
no
return
value
no input
value

40. $object->setFoo(5);
$value = $object->getFoo();
$object->setFoo('Change of state!');
$value = $object->getFoo();
no
return
value
no input
value
same argument,
different return value

41. No side-effects
A pure function
does not rely on data outside itself
does not change data outside itself

42. Object orientation
Solve problems with objects
Objects have internal state
Modify state through methods

43. With side-effectsclass Todo
{
public $status = 'todo';
}
function finish(Todo $task)
{
$task->status = 'done';
}
$uhoh = new Todo();
$uhoh2 = $uhoh;
finish($uhoh);
echo $uhoh->status; // done
echo $uhoh2->status; // done???

44. No side-effectsclass Todo
{
public $status = 'todo';
}
function finish(Todo $task)
{
$copy = clone $task;
$copy->status = 'done';
return $copy;
}
$uhoh = new Todo();
$finished = finish($uhoh);
echo $finished->status; // done
echo $uhoh->status; // todo
cloning is
cheap

45. Some state
must change

46. Read/write database
Get user input
Current date and time
Random values (security)

47. Immutable objects
PSR-7 HTTP messages
Domain value objects
DateTimeImmutable
Service objects

48. Immutable infrastructure
declarative configuration (Puppet, Chef)
containers (Docker)

49. ➍

50. Higher-order functions
Functions are values
so they can be arguments
and return values

51. $names = ['Billy', 'Bob', 'Thornton'];
$anonymise = anonymise('sha256');
var_dump(array_map($anonymise, $names));
// array(3) {
// [0]=>
// string(64) "85eea4a0285dcb11cceb68f39df10d1aa132567dec49b980345142f09f4cb05e"
// [1]=>
// string(64) "cd9fb1e148ccd8442e5aa74904cc73bf6fb54d1d54d333bd596aa9bb4bb4e961"
// [2]=>
// string(64) "d7034215823c40c12ec0c7aaff96db94a0e3d9b176f68296eb9d4ca7195c958e"
// }
using PHP built-ins

52. $names = ['Billy', 'Bob', 'Thornton'];
$anonymise = anonymise('sha256');
var_dump(array_map($anonymise, $names));
// array(3) {
// [0]=>
// string(64) "85eea4a0285dcb11cceb68f39df10d1aa132567dec49b980345142f09f4cb05e"
// [1]=>
// string(64) "cd9fb1e148ccd8442e5aa74904cc73bf6fb54d1d54d333bd596aa9bb4bb4e961"
// [2]=>
// string(64) "d7034215823c40c12ec0c7aaff96db94a0e3d9b176f68296eb9d4ca7195c958e"
// }
function anonymise($algorithm)
{
return function ($value) use ($algorithm) {
return hash($algorithm, $value);
};
}
higher-order function
closure
using PHP built-ins
function as value
function as argument

53. Middleware<?php
use PsrHttpMessageRequestInterface;
function add_header($header, $value)
{
return function (callable $handler) use ($header, $value) {
return function (
RequestInterface $request,
array $options
) use ($handler, $header, $value) {
$request = $request->withHeader($header, $value);
return $handler($request, $options);
};
};
}
$myStack = (new MiddlewareStack())
->push(add_header('Silly-Header', 'and its value'));
call next
in stack
immutable
higher-order function

54. Middleware<?php
use PsrHttpMessageServerRequestInterface as Request;
use PsrHttpMessageResponseInterface as Response;
$app = new SlimApp();
$app->add(function (Request $request, Response $response, callable $next) {
$response->getBody()->write('Hey there, ');
$response = $next($request, $response);
$response->getBody()->write('up?');
return $response;
});
$app->get('/', function ($request, $response, $args) {
$response->getBody()->write('what’s');
return $response;
});
$app->run();
// Hey there, what’s up?
stream is not
immutable
before
after

55. Typehints?

56. Middleware<?php
use PsrHttpMessageRequestInterface as Request;
use PsrHttpMessageResponseInterface as Response;
$middlewares->add($callback);
class Middlewares
{
public function add(
callable($request, Response $response, callable $next):Response
) {
}
}
declaration of
callable arguments

57. Middleware<?php
use PsrHttpMessageRequestInterface as Request;
use PsrHttpMessageResponseInterface as Response;
$middlewares->add($callback);
class Action implements Middleware
{
public function __invoke(Request $request, Response $response, callable $next)
{
// do something before
$next($request, $response);
// do something after
}
}
wrap in class

58. kapolos/pramda

59. Middleware<?php
$planets = [
[
"name" => "Earth",
"order" => 3,
"has" => ["moon", "oreos"],
"contact" => [
"name" => "Bob Spongebob",
"email" => "bob@spongebob.earth"
]
],
[
"name" => "Mars",
"order" => 4,
"has" => ["aliens", "rover"],
"contact" => [
"name" => "Marvin Martian",
"email" => "marvin@theparanoidandroid.mars"
]
],
// ...
];
$nameOfContact = P::compose(P::prop('name'), P::prop('contact'));
$getContactNames = P::map($nameOfContact);
// Application
$contacts = $getContactNames($planets);
P::toArray($contacts);
//=> ["Bob Spongebob", "Marvin Martian", ...]
returns a
generator

60. ➎

61. Composition over
inheritance

63. Single responsibility
and
interface segregation

64. Service objects<?php
namespace SymfonyComponentSecurityCore;
interface SecurityContextInterface
{
public function getToken();
public function isGranted($attributes, $object = null);
}

65. Service objects<?php
namespace SymfonyComponentSecurityCoreAuthenticationTokenStorage;
interface TokenStorageInterface
{
public function getToken();
}
namespace SymfonyComponentSecurityCoreAuthorization;
interface AuthorizationCheckerInterface
{
public function isGranted($attributes, $object = null);
}
single function

66. Single responsibility
taken to its
logical conclusion

67. You may not need
all those patterns

68. Wrap up
➊ No assignment
➋ Declarative
➌ Side-effects
➍ Higher-order functions
➎ Patterns

69. Take-aways
Reduce and isolate side-effects
Create immutable value objects
Be declarative

70. More?

71. https://joind.in/talk/8dfe1
@ddeboer_nl
Questions?