Being functional in PHP (DPC 2016)

Being functional in PHP
David de Boer

functional programming
functional thinking
functional communication
not languages

– 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.”

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

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

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

I’m David
/ddeboer
@ddeboer_nl

Erlang
Concurrent
Passing messages
Fault-tolerant

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

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

-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

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

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

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

-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

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

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

Church Von Neumann
declarative imperative

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

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

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

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

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

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

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

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

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???

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

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

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

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

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

$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

$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

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

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

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

Middleware<?php
use PsrHttpMessageRequestInterface as Request;
$middlewares->add($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

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

Single responsibility
and
interface segregation

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

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

Single responsibility
taken to its
logical conclusion

You may not need
all those patterns

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

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

https://joind.in/talk/8dfe1
@ddeboer_nl
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Being functional in PHP (DPC 2016)

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