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Date: Tue, 18 May 2021 21:51:05 +0100
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Subject: Re: [PHP-DEV] [RFC] Partial function application
From: rowan.collins@gmail.com (Rowan Tommins)

Hi David,

On 18/05/2021 20:56, David Gebler wrote:
> function foo(int $a, int $b, int ...$p) { ... }
> $partial = foo(?, 10);
>
> $partial(5, 15, 25);
>
> Intuitively, because the existing convention is extra unused parameters in
> user defined functions are silently ignored, I think I would expect the
> above to be equivalent to something like:
>
> $partial = fn(int $a) => foo($a, 10);
> $partial(5, 15, 25); // 15 and 25 are lopped off to no effect
>
> and not
>
> $partial = fn(int $a, int ...$params) => foo($a, 10, ....$params);

Did you see my message yesterday about the two mental models of the 
feature? https://externals.io/message/114157#114492

Your expectation there is in line with the "start with an empty closure 
and add things" model; the behaviour proposed in the RFC is in line with 
the "copy the full signature and then splice in fixed values" model.

I do worry that users of the language will assume the "wrong" mental 
model, though, unless we pick a syntax that more clearly matches the 
"copy and splice" model. I think that would mean having a way to say 
"make this is a partial", and a way to indicate which arguments to 
"splice", without any "placeholders".

Yet Another Bad Unsolicited Syntax Suggestion:

$partial = partial foo(); // no bindings; current RFC $partial = foo(?);
$partial = partial foo(b: 10); // bind named parameter $b; current RFC 
$partial = foo(?, b: 10);
$partial = partial foo(1: 10); // bind positional parameter #1 
(zero-indexed); current RFC $partial = foo(?, 10);


Regards,

-- 
Rowan Tommins
[IMSoP]