beautiful.php

<?php

$addOne = partial(operator('+'), 1);
$double = partial(operator('*'), 2);
$greaterThan3 = partial(operator('>'), …(), 3);

assert(
    map([1, 2, 3], $double)->§
    == [2, 4, 6]
);


assert(
    map([1, 2, 3], $double)->filter($greaterThan3)->§
    == [4, 6]
);


assert(
    map([1, 2, 3], compose($addOne, $double))->§
    == [4, 6, 8]
);


assert(
    map([1, 2, 3], compose($double, $addOne))->fold(operator('+'))
    == 15
);

ugly.php

<?php

// threw together some code stolen from various FP libs all over da githubz

/**
 * @property $§ Returns the Collection as an array
 */
final class Collection extends ArrayObject
{

    public function __construct(array $input)
    {
        parent::__construct($input, $flags = 0, $iterator_class = 'ArrayIterator');
    }

    public function __get($name)
    {
        switch($name) {
            case '§':
                return array_values($this->getArrayCopy());
                break;
            default:
                throw new BadMethodCallException;
        }
    }

    public function map(callable $f) {
        return new Collection(array_map($f, $this));
    }

    public function filter(callable $f) {
        return new Collection(array_filter($this->§, $f));
    }

    public function fold(callable $f, $initial = 0) {
        return array_reduce($this->§, $f, $initial);
    }


}


function compose($f, $g) {
    return function() use($f,$g) {
        $x = func_get_args();
        return $g(call_user_func_array($f, $x));
    };
}
 
function map(array $array, callable $f) {
    return new Collection(array_map($f, $array));
}
 
function filter(array $array, callable $f) {
    return new Collection(array_filter($array, $f));
}
 
function fold(array $array, callable $f, $initial = null) {
    return  new Collection(array_reduce($array, $f, $initial));
}


function operator($operator, $arg = null) {
    $functions = [
        'instanceof' => function($a, $b) { return $a instanceof $b; },
        '*'   => function($a, $b) { return $a *   $b; },
        '/'   => function($a, $b) { return $a /   $b; },
        '%'   => function($a, $b) { return $a %   $b; },
        '+'   => function($a, $b) { return $a +   $b; },
        '-'   => function($a, $b) { return $a -   $b; },
        '.'   => function($a, $b) { return $a .   $b; },
        '<<'  => function($a, $b) { return $a <<  $b; },
        '>>'  => function($a, $b) { return $a >>  $b; },
        '<'   => function($a, $b) { return $a <   $b; },
        '<='  => function($a, $b) { return $a <=  $b; },
        '>'   => function($a, $b) { return $a >   $b; },
        '>='  => function($a, $b) { return $a >=  $b; },
        '=='  => function($a, $b) { return $a ==  $b; },
        '!='  => function($a, $b) { return $a !=  $b; },
        '===' => function($a, $b) { return $a === $b; },
        '!==' => function($a, $b) { return $a !== $b; },
        '&'   => function($a, $b) { return $a &   $b; },
        '^'   => function($a, $b) { return $a ^   $b; },
        '|'   => function($a, $b) { return $a |   $b; },
        '&&'  => function($a, $b) { return $a &&  $b; },
        '||'  => function($a, $b) { return $a ||  $b; },
    ];

    if (!isset($functions[$operator])) {
        throw new \InvalidArgumentException("Unknown operator \"$operator\"");
    }

    $fn = $functions[$operator];
    if (func_num_args() === 1) {
        return $fn;
    } else {
        return function($a) use ($fn, $arg) {
            return $fn($a, $arg);
        };
    }
}

function not($function) {
    return function() use ($function) {
        return !call_user_func_array($function, func_get_args());
    };
}



function partial(callable $f /* , $args...*/)
{
    $args = func_get_args();
    array_shift($args);

    return function () use ($f, $args) {
        return call_user_func_array($f, mergeParameters($args, func_get_args()));
    };
}

/**
 * @return Placeholder
 */
function …()
{
    return Placeholder::create();
}

/**
 * @return Placeholder
 */
function placeholder()
{
    return …();
}

/** @internal */
function mergeParameters(array $left, array $right)
{
    foreach ($left as $position => &$param) {
        if ($param instanceof Placeholder) {
            $param = $param->resolve($right, $position);
        }
    }

    return array_merge($left, $right);
}



final class Placeholder
{
    private static $instance;

    private function __construct()
    {
    }

    public static function create()
    {
        if (self::$instance === null) {
            self::$instance = new self();
        }

        return self::$instance;
    }

    public function resolve(array &$args, $position)
    {
        if (count($args) === 0) {
            throw new \InvalidArgumentException(
                sprintf('Cannot resolve parameter placeholder at position %d. Parameter stack is empty', $position)
            );
        }

        return array_shift($args);
    }
}