Pipe precedence challenges

So, the question:

1. Does this seem like a good solution, or is there a problem we've not spotted yet?

Yes, I think this will be a good (temporary) solution until PFAs make it into the language, when it becomes less of an issue.

cheers
Derick

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

Hi Larry,

What would happen to this:

$x = fn($y) => $y |> strtoupper(…) |> var_dump(…);

with this change? I would expect $x to be a chain — which is what it currently is. But if I understand correctly, it will become:

$x = (fn($y) => $y) |> strtoupper(...) |> var_dump(...);

which would be interesting... but this that you shared as the current "correct" way:

fn($x) => ($x
|> (fn($x) => array_map(strtoupper(...), $x))
|> (fn($x) => array_filter($x, fn($v) => $v != 'O'))
);

This looks correct to me… but I’ve been using pipes since they were merged. So, I might be biased. I would have written the above like this though:

fn($x) => $x
|> __(array_map(...))(strtoupper(...), __)
|> (array_filter(...))(, fn($v) => $v != 'O');

or if I was feeling wordy:

$array_map = __(array_map(...);
$array_filter = __(array_filter(...);
fn($x) => $x => $array_map(strtoupper(...), ) |> $array_filter(, fn($v) => $v != 'O');

This suggested change would completely break that if I'm understanding correctly. Then PFA would reallow this syntax? I dunno ... changing it makes it sound inconsistent to me.

FWIW, I like it how it is for writing reusable pipes chains.

PS. The code above is implemented like so:

function __(...$steps): Closure {
$first = array_shift($steps) ?? throw new LogicException();

return function(...$steps) use ($first) {
    return function($x) use ($first, $steps) {
        foreach($steps as &$val) {
            if ($val === __) {
                $val = $x;
            }
        }
        return $first(...$steps);
    };
};

}
define('__', __(...));

ish.

— Rob

Does anyone have a better solution to suggest?

I've been following the discussion on the pipe/short-closure precedence
issue, and while the parentheses-based solution works, it does lead to
syntactic noise that undermines the readability benefits of pipes.

I'd like to suggest an alternative that turns this problem into an
opportunity for a more elegant syntax.

Inspired by prior art like Haskell's do notation, I propose what I
call "short closure canned pipes." The idea is to leverage the greedy
nature of short closures to define a clean, linear pipe chain within a
closure context. Instead of fighting the precedence, we embrace it to
create a new form:

Current problematic example (even with parentheses):

fn($x) => ($x
|> (fn($x) => array_map(strtoupper(...), $x))
|> (fn($x) => array_filter($x, fn($v) => $v != 'O'))
);

Proposed "canned pipes" syntax:

fn($x) =>
|> array_map(strtoupper(...), $x)
|> array_filter($x, fn($v) => $v != 'O')
;

This syntax:

• Resolves the ambiguity: The fn($x) => explicitly "cans" the value
  for the entire pipe chain, making the precedence clear without
  parentheses.

• Reduces boilerplate: It eliminates the repetition of fn($x) => and
  nested parentheses, making the code more readable and maintainable.

• Aligns with intent: It reads as a simple sequence of transformations,
  which is the core goal of the pipe operator.

This approach effectively provides syntactic sugar for the common case
of processing a value through a pipe chain within a closure. It turns
the late-discovered defect into a win for developer ergonomics and
future-proofing.

I understand that implementation feasibility needs to be assessed, but
if possible, this could be a more satisfying solution than mandatory
parentheses. Thanks to the team for highlighting this issue -- it's
sparked a creative way to enhance the feature.

Would love to hear thoughts on this.

Best,
hakre

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.

Ilija has implemented the above solution, and it has now been merged. It's not ideal, but it seems like the best option at present. Hopefully, we can land both composition and PFA in 8.6, which should make this situation a fairly rare occurrence anyway.

Thanks, Ilija!

--Larry Garfield