Playing with concatenative programming.

concat.php

<?hh

// concatenative programming
// based on this paper:
// http://mitarbeiter.hs-heilbronn.de/~herzberg/Publications/ICSOFT.2009.pdf

namespace igorw\concat;

class OpenQuotation {
    var $words;
    function __construct($words = []) {
        $this->words = $words;
    }
    function push($word) {
        $words = $this->words;
        array_push($words, $word);
        return new OpenQuotation($words);
    }
    function close() {
        return new Quotation($this->words);
    }
}

class Quotation {
    var $words;
    function __construct($words = []) {
        $this->words = $words;
    }
    function unquote() {
        return $this->words;
    }
}

function word($word, array $stack) {
    $top = end($stack);
    if ($top instanceof OpenQuotation && !in_array($word, ['[', ']'])) {
        $s = quote($word, $stack);
        return $s;
    }

    if (is_numeric($word)) {
        return number($word, $stack);
    }

    $fns = [
        'dup' => $stack ==> {
            $val = array_pop($stack);
            array_push($stack, $val);
            array_push($stack, $val);
            return $stack;
        },
        '[' => $stack ==> {
            array_push($stack, new OpenQuotation());
            return $stack;
        },
        ']' => $stack ==> {
            $quot = array_pop($stack);
            array_push($stack, $quot->close());
            return $stack;
        },
        '>' => $stack ==> {
            $b = array_pop($stack);
            $a = array_pop($stack);
            array_push($stack, $a > $b);
            return $stack;
        },
        'call' => $stack ==> {
            $quot = array_pop($stack);
            return call($quot, $stack);
        },
        'if' => $stack ==> {
            $quot_false = array_pop($stack);
            $quot_true = array_pop($stack);
            $val = array_pop($stack);
            $quot = $val ? $quot_true : $quot_false;
            return call($quot, $stack);
        },
        '+' => $stack ==> {
            $b = array_pop($stack);
            $a = array_pop($stack);
            array_push($stack, $a + $b);
            return $stack;
        },
    ];

    if (isset($fns[$word])) {
        return $fns[$word]($stack);
    }

    throw new \InvalidArgumentException("Unknown word $word");
}

function quote($word, array $stack) {
    $quot = array_pop($stack);
    array_push($stack, $quot->push($word));
    return $stack;
}

function number($word, array $stack) {
    array_push($stack, (int) $word);
    return $stack;
}

function call(Quotation $quot, array $stack) {
    $words = $quot->unquote();
    return evaluate($words, $stack);
}

function evaluate(array $words, array $stack = []) {
    foreach ($words as $word) {
        var_dump($stack);
        $stack = word($word, $stack);
    }
    return $stack;
}

$code = '6 5 dup [ 3 > ] call [ + ] [ * ] if';
$words = explode(' ', $code);

var_dump(evaluate($words));