examples of continuation-passing style in PHP

01_factorial_plain.php

<?php

function factorial($n) {
    if ($n === 0) {
        return 1;
    } else {
        return $n * factorial($n - 1);
    }
}

print(factorial(3));

print("\n");

02_factorialHelper_plain.php

<?php
function factorial($n) {
    return factorialHelper($n, 1);
}
function factorialHelper($n, $a) {
    if ($n === 0) {
        return $a;
    } else {
        return factorialHelper($n - 1, $n * $a);
    }
}
print(factorial(3));
print("\n");

03_factorial_cps.php

<?php
function factorial($n, $c) {
    if ($n === 0) {
        $c(1);
    } else {
        factorial($n - 1, function ($v) use ($c, $n) { $c( $n * $v); });
    }
}
factorial(3, function ($v) { print($v); });
print("\n");

04_factorialHelper_cps.php

<?php
function factorial ($n, $c) {
    factorialHelper($n, 1, $c);
}
function factorialHelper($n, $a, $c) {
    if ($n === 0) {
        $c($a);
    } else {
        factorialHelper($n - 1, $n * $a, $c);
    }
}
factorial(3, function ($v) { print($v); });
print("\n");

05_amnxy_plain.php

<?php
function p($v) { print($v . "\n"); }
function a() { p("a"); m(); p("a"); x(); p("a"); }
function m() { p("m"); n(); p("m"); }
function n() { p("n"); }
function x() { p("x"); y(); p("x"); }
function y() { p("y"); }
a();

06_amnxy_cps.php

<?php
function p($v) { print($v . "\n"); }
function a($c) { p("a"); m(function () use($c) { p("a"); x(function () use($c) { p("a"); $c(); }); }); }
function m($c) { p("m"); n(function () use($c) { p("m"); $c(); }); }
function n($c) { p("n"); $c(); }
function x($c) { p("x"); y(function () use($c) { p("x"); $c(); }); }
function y($c) { p("y"); $c(); }
a(function () {});

07_amnxy_cps_loop.php

<?php
$s = 0;
function p($v) { print($v . "\n"); }
function a($c) { p("a"); m(function () use($c) { p("a"); x(function () use($c) { p("a"); $c(); }); }); }
function m($c) { p("m"); n(function () use($c) { p("m"); $c(); }); }
function n($c) { global $s; $s = $c; p("n"); $c(); }
function x($c) { p("x"); y(function () use($c) { p("x"); $c(); }); }
function y($c) { p("y"); global $s; $s(); }
a(function () {});

08_factorial_trampoline.php

<?php
function factorial($n, $c) {
    if ($n === 0) {
        return function () use ($c) { return $c(1); };
    } else {
        return function () use ($c, $n) {
            return factorial(
                $n - 1,
                function ($v) use ($c, $n) {
                    return function () use ($c, $n, $v) { return $c($n * $v); };
                }
            );
        };
    }
}
function trampoline($t) {
    while (isset($t)) {
        $t = $t();
    }
}
trampoline( function () { return factorial(3, function ($v) { print($v); return null; }); });
print("\n");

09_factorialHelper_trampoline.php

<?php
function factorial($n, $c) {
    return function () use($n, $c) { return factorialHelper($n, 1, $c); };
}
function factorialHelper($n, $a, $c) {
    if ($n === 0) {
        return function () use ($c, $a) { return $c($a); };
    } else {
        return function () use ($c, $n, $a) {
            return factorialHelper($n - 1, $n * $a, $c);
        };
    }
}
function trampoline($t) {
    while (isset($t)) {
        $t = $t();
    }
}
trampoline(function () { return factorial(10, function ($v) { print($v); return null; }); });
print("\n");

10_factorial_trampoline_threaded.php

<?php
function p($_) {
    $args = func_get_args();
    print(implode(" ", $args) . "\n");
}
function factorial($id, $n, $c) {
    p($id, $n);
    if ($n === 0) {
        return function () use ($c) { return $c(1); };
    } else {
        return function () use ($id, $c, $n) {
            return factorial(
                $id,
                $n - 1,
                function ($v) use ($c, $n) {
                    return function () use ($c, $n, $v) { return $c($n * $v); };
                }
            );
        };
    }
}
function trampoline($q) {
    while (count($q)) {
        $t = array_shift($q);
        $t = $t();
        if (isset($t)) {
            array_push($q, $t);
        }
    }
}
trampoline(
    [
        function () { return factorial("a", 1, function ($v) { p("==> $v"); return null; }); },
        function () { return factorial("b", 6, function ($v) { p("==> $v"); return null; }); }
    ]
);

11_factorialHelper_trampoline_threaded.php

<?php
function factorial($label, $n, $continuation) {
    return factorialHelper($label, $n, 1, $continuation);
}
function factorialHelper($label, $n, $accumulator, $continuation) {
    print("$label: $n ($accumulator)\n");
    if ($n === 0) {
        return function () use ($continuation, $accumulator) {
            return $continuation($accumulator);
        };
    } else {
        return function () use ($label, $continuation, $n, $accumulator) {
            return factorialHelper(
                $label,
                $n - 1,
                $n * $accumulator,
                $continuation
            );
        };
    }
}
function trampoline($queue) {
    while (count($queue)) {
        $thread = array_shift($queue);
        $thread = $thread();
        if (isset($thread)) {
            array_push($queue, $thread);
        }
    }
}
function makeThread($label, $n) {
    return function () use ($label, $n) {
        return factorial(
            $label,
            $n,
            function ($v) use($label, $n) {
                print("$label: factorial of $n is $v\n");
                return null;
            }
        );
    };
}
trampoline(
    [
        makeThread("thread A", 2),
        makeThread("thread B", 4),
        makeThread("thread C", 6)
    ]
);