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Date: Tue, 14 Dec 2021 09:21:28 +0000
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Subject: Re: [PHP-DEV] Allow default parameters before non-default ones?
From: rowan.collins@gmail.com (Rowan Tommins)

On 13/12/2021 23:12, Kirill Nesmeyanov wrote:
> I'll add that leading optional parameters are needed to implement currying and partial application.
>
> ```
> function foo(int $opt = 42, int $req) {}
>
> $foo = curry(foo(...), 23);
>
> // $foo = fn($opt = 42, $req = 23);
> ```


I'm confused what the optional parameter is doing here - is it just to 
avoid telling the curry function which argument you're fixing? And 
wouldn't both currying and partial application result in a 
single-argument closure there, not just making both parameters optional?

I would expect it to look something like this:


```
function foo(int $a, int $b) {}

$foo = partial(foo(...), 23, 0);

// $foo = fn($b) => foo(23, $b);

$foo = partial(foo(...), 23, 1);

// $foo = fn($a) => foo($a, 23);

$bar = curry(foo(...));

// $bar = fn($a) => fn($b) => foo($a, $b);
```


> While this is not a popular practice in PHP, this deprecation notification «breaks» all code that uses functional pradigm/concepts.

That's clearly an exaggeration; it's clearly possible to write 
functional-style code without making use of this particular trick. If it 
is a *common* trick, then that's worth considering, but it would be good 
to see some evidence of that.

Regards,

-- 
Rowan Tommins
[IMSoP]