max($list, $function) in php

max.php

<?php

// test data

class Foo {
    private $a;
    private $b;

    function __construct($a, $b)
    {
        $this->a = $a;
        $this->b = $b;
    }

    public function getA() { return $this->a; }
    public function getB() { return $this->b; }
}

$list = [
    $foo1 = new Foo(2, "bar"),
    $foo2 = new Foo(5, "baz"),
    $foo3 = new Foo(1, "bam"),
];


// simple

$isGreater = function($carry, $item) {
    return is_null($carry) || $item->getA() > $carry->getA() ? $item : $carry;
};

$maxOfList = array_reduce($list, $isGreater);
assert($maxOfList === $foo2);


// reusable max() function

$max = function($list, callable $extract) {
    $compare = function($carry, $item) use($extract) {
        return is_null($carry) || $extract($item) > $extract($carry) ? $item : $carry;
    };
    return array_reduce($list, $compare);
};

$extractA = function($x) { return $x->getA();};
assert($max($list, $extractA) === $foo2);

@turanct true, it just seems like a lot of extra work/code when only basic OOP is necessary. It's also reinventing array_reduce(), in a way: both max_by() and array_reduce() would have the same signature.

As @turanct suggests, it's perfectly possible to combine FP techniques with DDD principles. DDD is not tied to OOP at all. For example, using Lambdalicious http://verraes.net/2014/11/higher-order-programming/ you would create functions like this:

<?php
$findOldestIn = max_by( __, method(@getAge, [], __));

$oldestPupil = $findOldestIn($listOfPupils);

I think you'll agree this covers the domain + ubiquitous language part.
Ihe example that you gave is not encapsulated. Foo::maxA is a global function, that happens to reside in a class. That's not the same as encapsulation. In my example, $findOldestIn has local scope, and can only be accessed by other scopes when you explicitly hand over the function to them. That's a powerful way of information hiding.

it just seems like a lot of extra work/code when only basic OOP is necessary.

max_by only needs to defined once, somewhere globally, as it is a very generic and low level function. The definition of $findOldestIn is extremely compact. You can of course combine FP and OOP and define it inside a class:

<?php
class Pupil
{
    public static function findOldestIn($listOfPupils) {
        return max_by( $listOfPupils, method(@getAge, [], __));

(note: At the moment, max_by is not implemented in Lambdalicious, but it will sooner or later)

Foo::maxA may not be encapsulated, but the data and logic related to Foo is. Public getters is a leaky interface, but are necessary when using global functions. Otherwise how can the logic be performed without access to the internals of an object? Therefore, a simple class method effectively encapsulates both data and logic for that particular class.

class PupilService
{
    public function findOldestIn(array $aListOfPupils)
    {
        $compareAge = function(Pupil $carry = null, Pupil $item) {
            return ($carry == null || $item->isOlderThan($carry)) ? $item : $carry;
        };
        return array_reduce($aListOfPupils, $compareAge);
    }
}

max_by(__, method(@getAge, [], __)) is not elegant - it's incoherent. Attempts to remove syntactical noise, verbosity, and ceremony should not sacrifice clarity.

edit: convert static method to service method

Ok, in order to have a truly simple and reusable function, you want find_by, not max_by. See my fork.

class PupilService
{
    public function findOldestIn(array $aListOfPupils)
    {
        $pupilIsOlder = function(Pupil $pupilA, Pupil $pupilB) {
            return $pupilA->isOlderThan($pupilB);
        };
        return find_by($aListOfPupils, $pupilIsOlder);
    }
}

http://3v4l.org/6O94O