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Date: Mon, 16 Jun 2025 17:31:20 +0200
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Subject: Re: [PHP-DEV] How hard would it be to add a "superyield" keyword?
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From: rob@bottled.codes ("Rob Landers")

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On Mon, Jun 16, 2025, at 17:18, Olle H=C3=A4rstedt wrote:
> Hello Internals,
>=20
> I was pondering a little about effect handlers today, and how they cou=
ld
> work as a replacement for dependency injection and mocking. Let me sho=
w an
> example:
>=20
> <?php
>=20
> require_once("vendor/autoload.php");
>=20
> use Latitude\QueryBuilder\Engine\MySqlEngine;
> use Latitude\QueryBuilder\QueryFactory;
> use function Latitude\QueryBuilder\field;
>=20
> // Dummy db connection
> class Db
> {
>     public function getQueryBuilder()
>     {
>         return new QueryFactory(new MySqlEngine());
>     }
> }
>=20
> interface Effect {}
>=20
> class QueryEffect implements Effect
> {
>     public $query;
>=20
>     public function __construct($query)
>     {
>         $this->query =3D $query;
>     }
> }
>=20
> class Plugin
> {
>     /* The "normal" way to do testing, by injecting the db object. Not
> needed here.
>     public function __construct(Db $db)
>     {
>         $this->db =3D $db;
>     }
>     */
>=20
>     public function populateCreditCardData(&$receipt)
>     {
>         foreach ($receipt['items'] as &$item) {
>             // 2 =3D credit card
>             if ($item['payment_type'] =3D=3D 2) {
>                 $query =3D $this->db->getQueryBuilder()
>                     ->select('card_product_name ')
>                     ->from('card_transactions')
>                     ->where(field('id')->eq($item['card_transaction_id=
']))
>                     ->compile();
>=20
>                 // Normal way: Call the injected dependency class dire=
ctly.
>                 //$result =3D $this->db->search($query->sql(),
> $query->params());
>=20
>                 // Generator way, push the side-effect up the stacktra=
ce
> using generators.
>                 $result =3D yield new QueryEffect($query);
>                 if ($result) {
>                     $item['card_product_name'] =3D
> $result[0]['card_product_name'];
>                 }
>             }
>         }
>     }
> }
>=20
> // Dummy receipt
> $receipt =3D [
>     'items' =3D> [
>         [
>             'payment_type' =3D> 2
>         ]
>     ]
> ];
> $p =3D new Plugin();  // Database is not injected
> $gen =3D $p->populateCreditCardData($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 st=
ack,
> with "yield from"? :(
>     // Effect handlers solve this by forcing an effect up in the stack
> trace similar to exceptions.
>=20
>     // Dummy db result
>     $rows =3D [
>         [
>             'card_product_name' =3D> 'KLARNA',
>         ]
>     ];
>     $gen->send($rows);
> }
>=20
> // Receipt item now has card_product_name populated properly.
> print_r($receipt);
>=20
> ---
>=20
> OK, so the problem with above code is that, in order for it to work, y=
ou
> 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 JavaSc=
ript.
> Also see the "Which color is your function" article [1].
>=20
> 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].
>=20
> 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 generat=
ors?
> 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)?
>=20
> Thanks for reading :)
>=20
> Olle
>=20
> ---
>=20
> [1] -
> https://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-funct=
ion/
>=20
> [2] - https://ocaml.org/manual/5.3/effects.html

You want to jump out of the current stack frame to one a higher one, and=
 possibly resume execution?

The best way to do that is to use fibers. They basically do exactly this=
 behavior.=20

=E2=80=94 Rob
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<!DOCTYPE html><html><head><title></title></head><body><div><br></div><d=
iv><br></div><div>On Mon, Jun 16, 2025, at 17:18, Olle H=C3=A4rstedt wro=
te:</div><blockquote type=3D"cite" id=3D"qt" style=3D""><div dir=3D"ltr"=
><div>Hello Internals,</div><div><br></div><div>I was pondering a little=
 about effect handlers today, and how they could</div><div>work as a rep=
lacement for dependency injection and mocking. Let me show an</div><div>=
example:</div><div><br></div><div>&lt;?php</div><div><br></div><div>requ=
ire_once("vendor/autoload.php");</div><div><br></div><div>use Latitude\Q=
ueryBuilder\Engine\MySqlEngine;</div><div>use Latitude\QueryBuilder\Quer=
yFactory;</div><div>use function Latitude\QueryBuilder\field;</div><div>=
<br></div><div>// Dummy db connection</div><div>class Db</div><div>{</di=
v><div>&nbsp; &nbsp; public function getQueryBuilder()</div><div>&nbsp; =
&nbsp; {</div><div>&nbsp; &nbsp; &nbsp; &nbsp; return new QueryFactory(n=
ew MySqlEngine());</div><div>&nbsp; &nbsp; }</div><div>}</div><div><br><=
/div><div>interface Effect {}</div><div><br></div><div>class QueryEffect=
 implements Effect</div><div>{</div><div>&nbsp; &nbsp; public $query;</d=
iv><div><br></div><div>&nbsp; &nbsp; public function __construct($query)=
</div><div>&nbsp; &nbsp; {</div><div>&nbsp; &nbsp; &nbsp; &nbsp; $this-&=
gt;query =3D $query;</div><div>&nbsp; &nbsp; }</div><div>}</div><div><br=
></div><div>class Plugin</div><div>{</div><div>&nbsp; &nbsp; /* The "nor=
mal" way to do testing, by injecting the db object. Not</div><div>needed=
 here.</div><div>&nbsp; &nbsp; public function __construct(Db $db)</div>=
<div>&nbsp; &nbsp; {</div><div>&nbsp; &nbsp; &nbsp; &nbsp; $this-&gt;db =
=3D $db;</div><div>&nbsp; &nbsp; }</div><div>&nbsp; &nbsp; */</div><div>=
<br></div><div>&nbsp; &nbsp; public function populateCreditCardData(&amp=
;$receipt)</div><div>&nbsp; &nbsp; {</div><div>&nbsp; &nbsp; &nbsp; &nbs=
p; foreach ($receipt['items'] as &amp;$item) {</div><div>&nbsp; &nbsp; &=
nbsp; &nbsp; &nbsp; &nbsp; // 2 =3D credit card</div><div>&nbsp; &nbsp; =
&nbsp; &nbsp; &nbsp; &nbsp; if ($item['payment_type'] =3D=3D 2) {</div><=
div>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; $query =3D $=
this-&gt;db-&gt;getQueryBuilder()</div><div>&nbsp; &nbsp; &nbsp; &nbsp; =
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; -&gt;select('card_product_name=
 ')</div><div>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &n=
bsp; &nbsp; -&gt;from('card_transactions')</div><div>&nbsp; &nbsp; &nbsp=
; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; -&gt;where(field('id'=
)-&gt;eq($item['card_transaction_id']))</div><div>&nbsp; &nbsp; &nbsp; &=
nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; -&gt;compile();</div><di=
v><br></div><div>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;=
 // Normal way: Call the injected dependency class directly.</div><div>&=
nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; //$result =3D $th=
is-&gt;db-&gt;search($query-&gt;sql(),</div><div>$query-&gt;params());</=
div><div><br></div><div>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;=
 &nbsp; // Generator way, push the side-effect up the stacktrace</div><d=
iv>using generators.</div><div>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;=
 &nbsp; &nbsp; $result =3D yield new QueryEffect($query);</div><div>&nbs=
p; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if ($result) {</div>=
<div>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbs=
p; $item['card_product_name'] =3D</div><div>$result[0]['card_product_nam=
e'];</div><div>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }=
</div><div>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }</div><div>&nbsp; =
&nbsp; &nbsp; &nbsp; }</div><div>&nbsp; &nbsp; }</div><div>}</div><div><=
br></div><div>// Dummy receipt</div><div>$receipt =3D [</div><div>&nbsp;=
 &nbsp; 'items' =3D&gt; [</div><div>&nbsp; &nbsp; &nbsp; &nbsp; [</div><=
div>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 'payment_type' =3D&gt; 2</=
div><div>&nbsp; &nbsp; &nbsp; &nbsp; ]</div><div>&nbsp; &nbsp; ]</div><d=
iv>];</div><div>$p =3D new Plugin(); &nbsp;// Database is not injected</=
div><div>$gen =3D $p-&gt;populateCreditCardData($receipt);</div><div>for=
each ($gen as $effect) {</div><div>&nbsp; &nbsp; // Call $db here instea=
d of injecting it.</div><div>&nbsp; &nbsp; // But now I have to propagat=
e the $gen logic all over the call stack,</div><div>with "yield from"? :=
(</div><div>&nbsp; &nbsp; // Effect handlers solve this by forcing an ef=
fect up in the stack</div><div>trace similar to exceptions.</div><div><b=
r></div><div>&nbsp;&nbsp;&nbsp; // Dummy db result</div><div>&nbsp; &nbs=
p; $rows =3D [</div><div>&nbsp; &nbsp; &nbsp; &nbsp; [</div><div>&nbsp; =
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 'card_product_name' =3D&gt; 'KLARNA',=
</div><div>&nbsp; &nbsp; &nbsp; &nbsp; ]</div><div>&nbsp; &nbsp; ];</div=
><div>&nbsp; &nbsp; $gen-&gt;send($rows);</div><div>}</div><div><br></di=
v><div>// Receipt item now has card_product_name populated properly.</di=
v><div>print_r($receipt);</div><div><br></div><div>---</div><div><br></d=
iv><div>OK, so the problem with above code is that, in order for it to w=
ork, you</div><div>have to add "yield from" from the top to the bottom o=
f the call stack,</div><div>polluting the code-base similar to what happ=
ens with "async" in JavaScript.</div><div>Also see the "Which color is y=
our function" article [1].</div><div><br></div><div>For this design patt=
ern to work seamlessly, there need to be a way to yield</div><div>"all t=
he way", so to speak, similar to what an exception does, and how</div><d=
iv>effect handlers work in OCaml [2].</div><div><br></div><div>The quest=
ion is, would this be easy, hard, or very hard to add to the</div><div>c=
urrent PHP source code? Is it conceptually too different from generators=
?</div><div>Would it be easier to add a way to "jump back" from a catche=
d exception</div><div>(kinda abusing the exception use-case, but that's =
how effect handlers work,</div><div>more or less)?</div><div><br></div><=
div>Thanks for reading :)</div><div><br></div><div>Olle</div><div><br></=
div><div>---</div><div><br></div><div>[1] -</div><div><a href=3D"https:/=
/journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function/">htt=
ps://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function/<=
/a></div><div><br></div><div>[2] - <a href=3D"https://ocaml.org/manual/5=
.3/effects.html">https://ocaml.org/manual/5.3/effects.html</a></div></di=
v></blockquote><div><br></div><div>You want to jump out of the current s=
tack frame to one a higher one, and possibly resume execution?</div><div=
><br></div><div>The best way to do that is to use fibers. They basically=
 do exactly this behavior.&nbsp;</div><div><br></div><div id=3D"sig12122=
9152">=E2=80=94 Rob</div></body></html>
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