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Date: Thu, 14 Aug 2025 14:30:08 -0500
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Subject: [PHP-DEV] Pipe precedence challenges
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From: larry@garfieldtech.com ("Larry Garfield")

Hi folks.  We have discovered a subtle issue with the pipes implementation that needs to be addressed, although we're not entirely sure how.  Specifically, Derick found it while trying to ensure Xdebug plays nice with pipes.

The problem has to do with the operator precedence of short closures vs pipes.  For example:

$result = $arr
  |> fn($x) => array_map(strtoupper(...), $x)
  |> fn($x) => array_filter($x, fn($v) => $v != 'O');
  
It's logical to assume that would be parsed as 

$result = $arr
  |> (fn($x) => array_map(strtoupper(...), $x))
  |> (fn($x) => array_filter($x, fn($v) => $v != 'O'));

Which then compiles into, essentially:

$temp = (fn($x) => array_map(strtoupper(...), $x))($arr);
$temp = (fn($x) => array_filter($x, fn($v) => $v != 'O'))($temp);
$result = $temp;

Or 

$result = (fn($x) => array_filter($x, fn($v) => $v != 'O'))((fn($x) => array_map(strtoupper(...), $x))($arr));

(depending on how you want to visualize it.)

That was the intent of the RFC.

However, because short closures are "greedy" and assume anything before the next semi-colon is part of the closure body, it's actually getting parsed like this:

$result = $arr
    |> fn($x) => (
        array_map(strtoupper(...), $x)
            |> fn($x) => (
                array_filter($x, fn($v) => $v != 'O')
            )
    )
;

Which would compile into something like:

$result = (fn($x) => (fn($x) => array_filter($x, fn($v) => $v != 'O'))(array_map(strtoupper(...), $x)))($arr);

Which is not the intent.

Humorously, if all the functions and closures involved are pure, this parsing difference *usually* doesn't matter.  The result is still computed as intended.  That's why it wasn't caught during earlier reviews or by automated tests.  However, there are cases where it matters.  For example:

42
    |> fn($x) => $x < 42
    |> fn($x) => var_dump($x);
    
One would expect that to evaluate to false in the first segment, then var_dump() false.  But it actually var_dump()s 42, because it gets interpreted as (42 |> fn($x) => var_dump($x)) first.

The incorrect wrapping also makes debugging vastly harder, even if the computed result is the same, as the expression is broken up "wrong" into multiple nested closures, stack traces are different than one would expect, etc.

The challenge is conflicting requirements.  Closures have extremely low precedence right now, specifically so that they will grab everything that comes after them as a single expression.  However, we also want pipes to allow a step to be a closure; that would typically mean making pipe bind even lower than closures, but that's not viable because it would result in 

$result = 'x' |> fn ($x) => strtoupper($x) 

being interpreted as

($result = 'x') |> (fn ($x) => strtoupper($x))

Which would be rather pointless.

So far, the best suggestion that's been put forward (though we've not tried implementing it yet) is to disallow a pipe inside a short-closure body, unless the body is surrounded by ().  So this:

fn($x) => $x 
  |> fn($x) => array_map(strtoupper(...), $x)
  |> fn($x) => array_filter($x, fn($v) => $v != 'O');
 
Today, that would run somewhat by accident, as the outer-most closure would claim everything after it.  With the new approach, it would be interpreted as passing `fn($x) => $x` as the argument to the first pipe segment, which would then be mapped over, which would fail.  You'd instead need to do this:

fn($x) => ($x 
  |> (fn($x) => array_map(strtoupper(...), $x))
  |> (fn($x) => array_filter($x, fn($v) => $v != 'O'))
);

Which is not wonderful, but it's not too bad, either.  That's probably the only case where pipes inside a short-closure body would be useful anyway.  And if PFA (https://wiki.php.net/rfc/partial_function_application_v2) and similar closure improvements pass, it will greatly reduce the need for mixing short closures and pipes together in either precedence, so it won't come up very often.

There are a few other operators that bind lower than pipe (see https://github.com/php/php-src/blob/fd8dfe1bfda62a3bd9dd1ff7c0577da75db02fcf/Zend/zend_language_parser.y#L56-L73), which would therefore need wrapping parentheses.  For many of them we do want them to be lower than pipe, so just moving pipe's priority down isn't viable. However, most of those are unlikely to be used inside a pipe segment, so are less likely to come up.  The most likely would be a bail-out exception:

$value = null;
$value
    |> fn ($x) => $x ?? throw new Exception('Value may not be null')
    |> fn ($x) => var_dump($x);

Which would currently be interpreted something like:

$c = function ($x) {
  $c = function ($x) {
    return var_dump($x);
  };
  return $x ?? throw $c(new Exception('Value may not be null'));
};
$c(null);

This would not throw the exception as expected, unless parentheses are added.  It would var_dump() an exception and then try to throw the return of varl_dump(), which would fatal.

RM approval to address this during the 8.5 beta phase has been given, but we still want to have some discussion to make sure we have a good solution.

So, the question:

1. Does this seem like a good solution, or is there a problem we've not spotted yet?
2. Does anyone have a better solution to suggest?

Thanks to Derick, Ilija, and Tim for tracking down this annoying edge case.

-- 
  Larry Garfield
  larry@garfieldtech.com