[RFC] Pipe Assignment Operator View as Markdown

Ooooh, fun. Can we informally call |>= the "volcano operator" (like ?: is
the Elvis operator)?

...if only Hershey's Kisses had two paper strips coming out of the top.

mickmackusa

Hi

Thank you. One note regarding “the lowest precedence level,
right-associative”. This is not quite correct. While the assignment
operators have fairly low precedence, they are not the lowest. As an
example, the infamous $foo = bar() or die()`` pattern relies on or
having a lower precedence than assignment. This should be corrected.

The $result = ($x |>= double(...)) |> triple(...); example also
doesn't showcase precedence, because of the explicit parentheses.

Ah, yes. I meant among the symbolic operators but you are correct and I've
updated to make this more clear. I removed the explicit parentheses from the
example.

Thank you, the precedence section LGTM now. And overall I don't have any
further comments regarding the contents of the RFC.

Best regards
Tim Düsterhus

Hi everyone, thanks for the feedback.

Perhaps weirdly, I am not a huge fan of this RFC. I'm open to pipe-compound operators (the
other that's been suggested is a null-safe ?|>, which I'd support), but I'm not sure of the
use case for this one.

That may be because, from a functional programming point of view, piping a back to itself makes
little if any sense. Values should be immutable, so you would be doing

$b = $a |> foo(...) |> bar(...) |> baz(...);

I cannot think of a case where I would want to put $a on the left side, too. How realistic is this
use case?

I don't think this RFC would cause any harm, I suppose, so I probably won't vote against
it. But at the moment I don't see a compelling reason to vote for it. A concrete real-world
use case would help make that case, because I cannot come up with one myself.

--Larry Garfield

I understand the FP perspective, but PHP isn't an immutable language.
We mutate variables constantly; that's why +=, .=, and ??= exist.
The motivation for this RFC came from seeing this pattern and wishing
that such an operator existed.

Here are patterns from a production codebase:

Property self-assignment with filter + reindex:

// before: $this->personnel repeated
$this->personnel = array_values(array_filter(
    $this->personnel,
    fn ($u) => ($u['id'] ?? null) !== $userId,
));

// after: appears once
$this->personnel
    |>= array_filter(?, fn ($u) => ($u['id'] ?? null) !== $userId)
    |> array_values(...);

Here's another common Laravel pattern (real example from codebase),
wrapping a variable in collect() to transform it, then converting back
with ->all():

$filters = collect($filters)
    ->map(function ($value) {
        // logic...
    })
    ->all();

At its core this is just $filters = transform($filters). People
already reach for this self-assignment pattern constantly; collections
just provide the fluent API for the transformation. With |>= and
native array functions you get the same fluency without the round-trip:

$filters |>= some_transform(...);

Even simple $x = func($x) calls benefit; the variable doesn't need
to be duplicated on both sides:

// all real patterns from one codebase
$post_data = json_decode($post_data, true);
$dates = array_reverse($dates);
$model = ltrim($model, '\\/');
$items = array_slice($items, $offset, $per_page);
$formData['search']['name'] = str_replace('\\', '', $formData['search']['name']);

// become
$post_data |>= json_decode(?, true);
$dates |>= array_reverse(...);
$model |>= ltrim(?, '\\/');
$items |>= array_slice(?, $offset, $per_page);
$formData['search']['name'] |>= str_replace('\\', '', ?);

Here's another production example (not the nicest code but you get the
point), it's much cleaner using |>= than repeating the expression:

// before
$params['current']['pallet_quantity'] = intval($params['current']['pallet_quantity']);
$params['current']['pallet_cost']     = floatval($params['current']['pallet_cost']);
$params['current']['box_quantity']    = intval($params['current']['box_quantity']);
$params['current']['cox_cost']        = floatval($params['current']['box_cost']);
$params['current']['cogs']            = floatval($params['current']['cogs']);

// after
$params['current']['pallet_quantity'] |>= intval(...);
$params['current']['pallet_cost']     |>= floatval(...);
$params['current']['box_quantity']    |>= intval(...);
$params['current']['cox_cost']        |>= floatval(...);
$params['current']['cogs']            |>= floatval(...);

This is the same argument as .=; nobody writes $x = $x . ' suffix'
anymore. The benefit scales with variable complexity.

Sequential transforms in business logic also clean up nicely with PFA:

// from real codebase
$model = ltrim($model, '\\/');
$model = str_replace('/', '\\', $model);

// becomes a single expression
$model |>= ltrim(?, '\\/') |> str_replace('/', '\\', ?);

The consistency argument is also worth noting. Every binary operator
where in-place transformation makes sense has a compound assignment
form. We accepted |> as a first-class operator; its compound form
follows the same pattern as +=, .=, ??=, etc.

Hey Larry,

// before
public function makeSlug(string $value): string {
$value = $value |> mb_strtolower(...); // ...
$number = $this->getNextSlugNumber($value); // checks DB
if (0 < $number) { $value .= "-$number"; }
return $value;
}

// after
public function makeSlug(string $value): string {
$value |>= mb_strtolower(...) ; // ...
$number = $this->getNextSlugNumber($value); // checks DB
if (0 < $number) { $value .= "-$number"; }
return $value;
}

Currently we have to make assignment gymnastics even for use cases that
do not require an immutable second value. Would argue many people in PHP
do and will use pipes for readability, not because they suddenly write
PHP from a functional programming point of view.

Nick

Well said, Nick. Exactly this, pipes are a readability tool in PHP,
not a signal that we're writing pure FP like Haskell.

This is a good proposal, IMO.

Thank you! That means a lot.

I would argue that is a very bad practice in php, to alter the actual
function parameter variable. That is because any stack trace obtained will
not reflect the initial value but the modified value.

And generally, it is a bad practice to override a variable with some other
value that represents something different. You could have had
incrementally: $lowercaseValue, and $slug/$valueWithSlugNumber.

Alex

I wouldn't call it "very bad practice", it may not be the "best" practice
but it is still extremely common in PHP codebases and the language has
never discouraged it. We're not the parameter reassignment police 🙃.
That said, |>= isn't even limited to parameters; it works on
properties, array dimensions, and locals.

$this->message |>= trim(...);

That's the same pattern as $this->count += 1. The type and semantic
meaning don't change; it's the same data, transformed.

PHP already has $x .= ' suffix' instead of requiring
$suffixedX = $x . ' suffix'. Nobody argues .= encourages bad
practice. |>= is the same pattern; a transformation, not a
replacement with something semantically different.

Using different variable names ($lowercaseValue, $slug) is a valid
stylistic/preference choice. But in practice, people reuse variables for
sequential transforms, and |>= just makes that existing pattern cleaner.

While I agree, we need to accept that often people do not use the best
practices when coding. PHP makes the barrier to entry low, which invites
people who are still learning what best practices are.

I wanted to argue against this for exactly this reason, but it's really
not that bad. When combined with FCC, PFA, and pipe chaining, it can
really condense code.

Holly

Agreed! The more I've played around with this the more I've come to love
how expressive and concise it is.

My only hesitation is that what we're going to end up with is a lot of:

$foo |>= a(…)
$foo |>= b(…)
$foo |>= c(…)

This completely defeats the spirit of this operation (and probably has
terrible performance).

Even if folks do choose to write it that way then I would argue
that it is still cleaner than:

$foo = a($foo);
$foo = b($foo);
$foo = c($foo);

But I think the operator naturally encourages piping, so I don't think it
will be too much of an issue:

$foo |>= a(...) |> b(...) |> c(...);

Now I imagine that you will (potentially) see a lot of regular function
/ method calls using this to avoid having to repeat the variable
expression:

// before
$someVeryLongVariableName = array_values($someVeryLongVariableName);

// after
$someVeryLongVariableName |>= array_values(...);

On the performance note, there should be zero overhead. Each |>= compiles
to the exact same opcodes as $x = $x |> ...; there is no difference.
For complex LHS expressions like the following, |>= is actually
more performant (because it evaluates sub-expressions once instead of
twice, same memoization mechanism as ??=) and more concise (how do you
beat that?):

//before
$array[$this->computeKey($object)] = strtolower($array[$this->computeKey($object)]);

// or you introduce a temp var
$key = $this->computeKey($object);
$array[$key] = strtolower($array[$key]);

// after: performant and concise without having to use temp var
$array[$this->computeKey($object)] |>= strtolower(...);

Best,
Caleb

I just have one more question to the RFC author, which I see in the
implementation, but the RFC is not explicitly noting:
Is it intentional that fetching is repeated?
It will literally desugar $a->b->c |>= strtolower(...); to
$a->b->c = strtolower($a->b->c); resulting in double execution
of e.g. property get hooks.

I would consider that a bug in the implementation, given that the
"Single-Evaluation Guarantee" section mentions:

When the LHS contains sub-expressions, they are evaluated
exactly once:

Thus if it behaves differently to ??=, it's a bug in the
implementation :-)

Hi Bob, Tim,

Thanks for flagging this. I tested the scenario Bob described and
|>= behaves identically to ??= here:

class Inner {
    public string $c = "hello" {
        get { echo "Inner::c GET\n"; return $this->c; }
        set(string $v) { echo "Inner::c SET\n"; $this->c = $v; }
    }
}
class Outer {
    public Inner $b {
        get { echo "Outer::b GET\n"; return $this->b; }
    }
    public function __construct() { $this->b = new Inner(); }
}

$a = new Outer();
$a->b->c |>= strtoupper(...);
// Outer::b GET  (read)
// Inner::c GET  (read)
// Outer::b GET  (write-back)
// Inner::c SET  (write-back)

$a->b->c ??= "default";
// Outer::b GET  (read)
// Inner::c GET  (read)
// Outer::b GET  (write-back)
// Inner::c SET  (write-back)

Both |>= and ??= fetch the intermediate chain twice (once for the
read, once for the write-back). The single-evaluation guarantee in
the RFC refers to sub-expressions like $arr[expensive_call()],
where the call itself is evaluated only once. That works correctly:

$arr[track()] |>= strtoupper(...);
// track() is called exactly once

So the behavior is consistent with ??=. Bob, your observation about
optimizing intermediate chain fetches with ad-hoc references is
interesting, but that would be an improvement to all compound
assignment operators, not something specific to |>=.

I've updated the RFC to clarify this in the Single-Evaluation
Guarantee section.

1. my point is: why |>= and not =|>?

Hi Nick,

Tim already covered this well, but I agree with him: |>= follows
the established op= convention for compound assignments (+=,
.=, ??=). =|> looks like a special form of => rather than
an assignment, and consistency with existing patterns is worth more
than theoretical future extensibility for operators that may never
materialize.

2. Holly's example without semicolons

Yeah, that was just a missing semicolon in the email example;
standard parse error, same as with |>. Nothing to address there.

3. I recently discovered a bug [1] when pipes are combined with
   property hooks.

Thanks for flagging this. I've added a test (assign_pipe_018.phpt)
that covers |>= with get/set hooks, virtual properties, and
readonly properties. The |>= side works correctly; the bug you
found (#22587) is specific to the base |> operator and is not
introduced or affected by this RFC.

Thanks for testing the implementation and for the detailed feedback!

Best,
Caleb