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Date: Mon, 16 Jun 2025 17:18:43 +0200
Message-ID: <CAJpLVh3Kxk4iCWs8b3jyT7OfvxpBUbZbBo4F81Lgc7qegYT2wg@mail.gmail.com>
Subject: [PHP-DEV] How hard would it be to add a "superyield" keyword?
To: PHP internals <internals@lists.php.net>
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From: olleharstedt@gmail.com (=?UTF-8?Q?Olle_H=C3=A4rstedt?=)

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Hello Internals,

I was pondering a little about effect handlers today, and how they could
work as a replacement for dependency injection and mocking. Let me show an
example:

<?php

require_once("vendor/autoload.php");

use Latitude\QueryBuilder\Engine\MySqlEngine;
use Latitude\QueryBuilder\QueryFactory;
use function Latitude\QueryBuilder\field;

// Dummy db connection
class Db
{
    public function getQueryBuilder()
    {
        return new QueryFactory(new MySqlEngine());
    }
}

interface Effect {}

class QueryEffect implements Effect
{
    public $query;

    public function __construct($query)
    {
        $this->query = $query;
    }
}

class Plugin
{
    /* The "normal" way to do testing, by injecting the db object. Not
needed here.
    public function __construct(Db $db)
    {
        $this->db = $db;
    }
    */

    public function populateCreditCardData(&$receipt)
    {
        foreach ($receipt['items'] as &$item) {
            // 2 = credit card
            if ($item['payment_type'] == 2) {
                $query = $this->db->getQueryBuilder()
                    ->select('card_product_name ')
                    ->from('card_transactions')
                    ->where(field('id')->eq($item['card_transaction_id']))
                    ->compile();

                // Normal way: Call the injected dependency class directly.
                //$result = $this->db->search($query->sql(),
$query->params());

                // Generator way, push the side-effect up the stacktrace
using generators.
                $result = yield new QueryEffect($query);
                if ($result) {
                    $item['card_product_name'] =
$result[0]['card_product_name'];
                }
            }
        }
    }
}

// Dummy receipt
$receipt = [
    'items' => [
        [
            'payment_type' => 2
        ]
    ]
];
$p = new Plugin();  // Database is not injected
$gen = $p->populateCreditCardData($receipt);
foreach ($gen as $effect) {
    // Call $db here instead of injecting it.
    // But now I have to propagate the $gen logic all over the call stack,
with "yield from"? :(
    // Effect handlers solve this by forcing an effect up in the stack
trace similar to exceptions.

    // Dummy db result
    $rows = [
        [
            'card_product_name' => 'KLARNA',
        ]
    ];
    $gen->send($rows);
}

// Receipt item now has card_product_name populated properly.
print_r($receipt);

---

OK, so the problem with above code is that, in order for it to work, you
have to add "yield from" from the top to the bottom of the call stack,
polluting the code-base similar to what happens with "async" in JavaScript.
Also see the "Which color is your function" article [1].

For this design pattern to work seamlessly, there need to be a way to yield
"all the way", so to speak, similar to what an exception does, and how
effect handlers work in OCaml [2].

The question is, would this be easy, hard, or very hard to add to the
current PHP source code? Is it conceptually too different from generators?
Would it be easier to add a way to "jump back" from a catched exception
(kinda abusing the exception use-case, but that's how effect handlers work,
more or less)?

Thanks for reading :)

Olle

---

[1] -
https://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function/

[2] - https://ocaml.org/manual/5.3/effects.html

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<div dir=3D"ltr">Hello Internals,<br><br>I was pondering a little about eff=
ect handlers today, and how they could<br>work as a replacement for depende=
ncy injection and mocking. Let me show an<br>example:<br><br>&lt;?php<br><b=
r>require_once(&quot;vendor/autoload.php&quot;);<br><br>use Latitude\QueryB=
uilder\Engine\MySqlEngine;<br>use Latitude\QueryBuilder\QueryFactory;<br>us=
e function Latitude\QueryBuilder\field;<br><div><br></div><div>// Dummy db =
connection</div>class Db<br>{<br>=C2=A0 =C2=A0 public function getQueryBuil=
der()<br>=C2=A0 =C2=A0 {<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 return new QueryFac=
tory(new MySqlEngine());<br>=C2=A0 =C2=A0 }<br>}<br><br>interface Effect {}=
<br><br>class QueryEffect implements Effect<br>{<br>=C2=A0 =C2=A0 public $q=
uery;<br><br>=C2=A0 =C2=A0 public function __construct($query)<br>=C2=A0 =
=C2=A0 {<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 $this-&gt;query =3D $query;<br>=C2=
=A0 =C2=A0 }<br>}<br><br>class Plugin<br>{<br>=C2=A0 =C2=A0 /* The &quot;no=
rmal&quot; way to do testing, by injecting the db object. Not<br>needed her=
e.<br>=C2=A0 =C2=A0 public function __construct(Db $db)<br>=C2=A0 =C2=A0 {<=
br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 $this-&gt;db =3D $db;<br>=C2=A0 =C2=A0 }<br>=
=C2=A0 =C2=A0 */<br><br>=C2=A0 =C2=A0 public function populateCreditCardDat=
a(&amp;$receipt)<br>=C2=A0 =C2=A0 {<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 foreach =
($receipt[&#39;items&#39;] as &amp;$item) {<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 // 2 =3D credit card<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 if ($item[&#39;payment_type&#39;] =3D=3D 2) {<br>=C2=A0 =C2=A0 =C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 $query =3D $this-&gt;db-&gt;getQuery=
Builder()<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 =C2=A0 -&gt;select(&#39;card_product_name &#39;)<br>=C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 -&gt;from(&#39;card_transa=
ctions&#39;)<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 -&gt;where(field(&#39;id&#39;)-&gt;eq($item[&#39;card_transactio=
n_id&#39;]))<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 -&gt;compile();<br><br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 =C2=A0 =C2=A0 // Normal way: Call the injected dependency class directly.<=
br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 //$result =3D $t=
his-&gt;db-&gt;search($query-&gt;sql(),<br>$query-&gt;params());<br><br>=C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 // Generator way, push=
 the side-effect up the stacktrace<br>using generators.<br>=C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 $result =3D yield new QueryEffect=
($query);<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 if ($r=
esult) {<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 $item[&#39;card_product_name&#39;] =3D<br>$result[0][&#39;card_produ=
ct_name&#39;];<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 }=
<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 }<br>=C2=A0 =C2=A0 =C2=A0 =C2=
=A0 }<br>=C2=A0 =C2=A0 }<br>}<br><br>// Dummy receipt<br>$receipt =3D [<br>=
=C2=A0 =C2=A0 &#39;items&#39; =3D&gt; [<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 [<br=
>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 &#39;payment_type&#39; =3D&gt; 2=
<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 ]<br>=C2=A0 =C2=A0 ]<br>];<br>$p =3D new Pl=
ugin(); =C2=A0// Database is not injected<br>$gen =3D $p-&gt;populateCredit=
CardData($receipt);<br>foreach ($gen as $effect) {<br>=C2=A0 =C2=A0 // Call=
 $db here instead of injecting it.<br>=C2=A0 =C2=A0 // But now I have to pr=
opagate the $gen logic all over the call stack,<br>with &quot;yield from&qu=
ot;? :(<br>=C2=A0 =C2=A0 // Effect handlers solve this by forcing an effect=
 up in the stack<br><div>trace similar to exceptions.</div><div><br></div><=
div>=C2=A0=C2=A0=C2=A0 // Dummy db result</div>=C2=A0 =C2=A0 $rows =3D [<br=
>=C2=A0 =C2=A0 =C2=A0 =C2=A0 [<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 &#39;card_product_name&#39; =3D&gt; &#39;KLARNA&#39;,<br>=C2=A0 =C2=A0 =C2=
=A0 =C2=A0 ]<br>=C2=A0 =C2=A0 ];<br>=C2=A0 =C2=A0 $gen-&gt;send($rows);<br>=
}<br><br>// Receipt item now has card_product_name populated properly.<br>p=
rint_r($receipt);<br><br>---<br><br>OK, so the problem with above code is t=
hat, in order for it to work, you<br>have to add &quot;yield from&quot; fro=
m the top to the bottom of the call stack,<br>polluting the code-base simil=
ar to what happens with &quot;async&quot; in JavaScript.<br>Also see the &q=
uot;Which color is your function&quot; article [1].<br><br>For this design =
pattern to work seamlessly, there need to be a way to yield<br>&quot;all th=
e way&quot;, so to speak, similar to what an exception does, and how<br>eff=
ect handlers work in OCaml [2].<br><br>The question is, would this be easy,=
 hard, or very hard to add to the<br>current PHP source code? Is it concept=
ually too different from generators?<br>Would it be easier to add a way to =
&quot;jump back&quot; from a catched exception<br>(kinda abusing the except=
ion use-case, but that&#39;s how effect handlers work,<br>more or less)?<br=
><br>Thanks for reading :)<br><br><div>Olle</div><div><br></div>---<br><br>=
[1] -<br><a href=3D"https://journal.stuffwithstuff.com/2015/02/01/what-colo=
r-is-your-function/">https://journal.stuffwithstuff.com/2015/02/01/what-col=
or-is-your-function/</a><br><br>[2] - <a href=3D"https://ocaml.org/manual/5=
.3/effects.html">https://ocaml.org/manual/5.3/effects.html</a></div>

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