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Date: Mon, 16 Jun 2025 13:10:49 -0500
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Subject: Re: [PHP-DEV] How hard would it be to add a "superyield" keyword?
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From: larry@garfieldtech.com ("Larry Garfield")

On Mon, Jun 16, 2025, at 10:18 AM, Olle H=C3=A4rstedt wrote:
> Hello Internals,
>
> 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:
>
> <?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 =3D $query;
>     }
> }
>
> class Plugin
> {
>     /* The "normal" way to do testing, by injecting the db object. Not
> needed here.
>     public function __construct(Db $db)
>     {
>         $this->db =3D $db;
>     }
>     */
>
>     public function populateCreditCardData(&$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();
>
>                 // Normal way: Call the injected dependency class dire=
ctly.
>                 //$result =3D $this->db->search($query->sql(),
> $query->params());
>
>                 // 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'];
>                 }
>             }
>         }
>     }
> }
>
> // 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.
>
>     // Dummy db result
>     $rows =3D [
>         [
>             'card_product_name' =3D> '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, 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].
>
> 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 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)?
>
> Thanks for reading :)
>
> Olle

Algebraic effects is a... big and interesting topic. :-)  If we were to =
go that route, though, I would want to see something more formal than ju=
st a "yield far."  That's basically another kind of unchecked exception,=
 whereas I want us to move more toward checked exceptions.

--Larry Garfield