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Date: Sat, 9 Sep 2017 16:38:18 +0200
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To: Andrea Faulds <ajf@ajf.me>
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Subject: Re: [PHP-DEV] [RFC] [Discussion] Operator functions
From: nikita.ppv@gmail.com (Nikita Popov)

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On Fri, Sep 8, 2017 at 11:41 PM, Andrea Faulds <ajf@ajf.me> wrote:

> Hi everyone!
>
> Here's an RFC for a small, simple, self-contained feature with no
> backwards-compatibility breaks and which in fact doesn't even touch the
> language's syntax (it's 50%+1 eligible!) but which could make PHP a bit
> more expressive and consistent, especially with potential later features.
> It even has a test designed to impose minimal maintenance burden while
> testing a fairly large possibility space!
>
> Anyway, the RFC in question is this: https://wiki.php.net/rfc/opera
> tor_functions
>
> Please tell me what you think and suggest any potential improvements or
> anything you think might have been an omission.
>
> Thanks!


I like the general idea here, but have some comments.

My main observation is that this proposal is only really useful in
combination with a form of partial application. Passing operators to
array_reduce() is cute, but it's not a major application, especially as we
already have built-in functions for the two common operations (array_sum
and array_product).

Where operators-as-functions really shine is in cases where only one of the
operands is bound. You acknowledge this in the RFC, and provide a few
examples using a (not yet existing) partialApply() function:

    // Select only the positive numbers
    $positiveSubset = array_filters($numbers, partialApply('>', 0));

However, this code is subtly broken. Partial application (at least without
specifying a more specific behavior) operates from left to right, so this
code would be equivalent to:

    // Select only the positive numbers
    $positiveSubset = array_filters($numbers, function($n) { return 0 > $n;
});

As such, it would return all negative numbers, not all positive numbers.

This is a general issue of partial application in combination with
operators: For the operations that do not commute, you nearly always want
to bind the right operand, not the left.

For my own purposes, I define an operator() function as follows:
https://github.com/nikic/iter/blob/master/src/iter.fn.php#L60

This function either accepts a single argument such as operator('+'), in
which case it is essentially equivalent to this proposal. Or it accepts two
arguments, in which case the right operand will be bound, such as
operator('>', 0).

I wonder if providing such a function might not be a better solution to
this problem. It also has the additional advantage that it can be easily
polyfilled in older PHP versions.

Nikita

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