Add ??? Empty Coalesce Operator #2787

khalwat · 2018-12-19T02:54:31Z

Empty Coalesce adds the ??? operator to Twig that will return the first thing that is defined, not null, and not empty. This is particularly useful when you're dealing with a number of fallback/default values that may or may not exist, and may or may not be empty.

The ??? Empty Coalescing operator is similar to the ?? null coalesce operator, but also ignores empty strings (""), 0, 0.0, null, false, and empty arrays ([]) as well.

Because this is an Empty Coalesce Operator, it functions identically to the PHP empty() function in terms of return values.

Example:

{% set bar = null %}
{% set foo = '' %}
{% set baz = [] %}
{% set foobar = woof ??? bar ??? foo ??? baz ??? 'bark' %}
{{ foobar }}

This will output:

bark

...because:

woof is undefined
bar is null
foo is an empty string
baz is an empty array

There is precedence for this operator in languages such as Swift:

https://medium.com/@JanLeMann/providing-meaningful-default-values-for-empty-or-absence-optionals-via-nil-or-empty-coalescing-379abd22ae77

Signed-off-by: Andrew Welch <[email protected]>

stof · 2018-12-19T11:46:56Z

What is the difference with the |default filter we have.

Well, one of them is that it relies on PHP's empty, meaning that '0' is empty while |default does not consider that. But is there any other difference ?

stof · 2018-12-19T11:47:32Z

btw, your implementation also ignores '0', 0 and false, in addition to what you documented as being ignored.

khalwat · 2018-12-19T12:48:23Z

Thanks for looking @stof !

What is the difference with the |default filter we have.

It's the same reason why the ?? null coalescing operator exist in Twig right now, despite default being available. It is far more convenient and much more clear what is going on to have:

{% set foobar = woof ??? bar ??? foo ??? baz ??? 'bark' %}

...using the default filter. It can be done, but it is quite ugly and verbose.

michaelrog · 2018-12-19T18:44:17Z

I'm a big fan of this idea — It comes up frequently enough in my Twig templates (where I'm comparing a chain of values and I want to use/output the first non-empty one) that I've created my own stock extension for it, and it's a default add to all my projects.

I think ??? as an operator could be confusing vis-a-vis ??. And also, the visual difference between ?? and ??? is not super-scannable. Perhaps _? for this.

My only reservation about this in-practice is the case of "0" being empty.... but, one can't really do anything about it; I think ultimately it's reasonable to be consistent with PHP's empty() and expect devs to learn that behavior.

It's definitely a non-standard op, but I think the utility of it is worth adding the sugar to Twig even though no similar operator exists upstream in PHP.

adrienne · 2018-12-19T18:57:20Z

+1 to this PR - i am using @khalwat's null-coalescing operator as a plugin on a site and it is really nice to have in any context where your Twig data contains, for instance, a lot of empty arrays.

nicolas-grekas · 2018-12-19T19:00:06Z

Why not use ?: to provide this behavior?

khalwat · 2018-12-19T19:03:38Z

I think ??? as an operator could be confusing vis-a-vis ??. And also, the visual difference between ?? and ??? is not super-scannable. Perhaps _? for this.

_? is horrible to type; I think ??? is easier to type, and clearer in that it's ?? plus more. The fact that others have independently come up with the same ??? operator for this functionality makes me feel even better about it.

My only reservation about this in-practice is the case of "0" being empty.... but, one can't really do anything about it; I think ultimately it's reasonable to be consistent with PHP's empty() and expect devs to learn that behavior.

It actually uses empty() under the hood, so yes, it'd be consistent with empty()'s behavior, which I think makes sense.

michaelrog · 2018-12-19T19:06:39Z

@nicolas-grekas — The primary benefit is that ?: will throw an error if it encounters an undefined operand. We'd really like this to behave like the null-coalescing operator, which treats undefined things as null.

(Also, it needs to be right-associative... I think ?: is, but I don't remember offhand.)

khalwat · 2018-12-19T19:06:48Z

Why not use ?: to provide this behavior?

@nicolas-grekas what would the Twig code for this be using the ?: operator for this construct:

{% set foobar = woof ??? bar ??? foo ??? baz ??? 'bark' %}

In PHP ?: just determines truthiness, and would throw an error if a variable was undefined, unlike ???

nicolas-grekas · 2018-12-19T19:13:20Z

and would throw an error if a variable was undefined

twig is a different language, we can remove this behavior if we want to.

khalwat · 2018-12-19T19:15:58Z

twig is a different language, we can remove this behavior if we want to.

That's definitely true, but I'd rather define a new operator for new functionality than hoist new functionality on an operator for which there's already a defined behavior.

Many people who write in Twig also write in PHP, and it might be very confusing if ?: operated differently in Twig than it does in PHP... at least to me.

stof · 2018-12-20T08:54:24Z

twig is a different language, we can remove this behavior if we want to.

that would require changing the existing operator, and that makes mistake detection harder in case you make typos in your variable names. I would vote for keeping the existing behavior of the operator.

brandonkelly · 2019-01-16T23:38:54Z

Another vote to not modify existing operator behaviors! (And a vote for ???)

khalwat · 2019-01-22T13:20:38Z

Welp, looks like the ??= operator is being added in PHP 7.4: https://twitter.com/nikita_ppv/status/1087662379037528064

It's not the same thing, but it does show some precedence for a three character operator... so I hope ??? can make it into Twig!

lib/Twig/Node/Expression/EmptyCoalesce.php

brandonkelly · 2019-04-01T19:25:41Z

btw, your implementation also ignores '0', 0 and false, in addition to what you documented as being ignored.

If we’re calling this the “Empty Coalesce Operator”, I would expect that any value PHP considers “empty” would also be considered empty here, so '0' and 0 should in fact invoke the following operator argument. Safe to assume that if PHP ever adds the same operator, they would go with the same behavior as empty() as well.

khalwat · 2019-04-01T19:28:29Z

Yeah that's the idea @brandonkelly -- it's called the Empty Coalesce Operator because the behavior is what we'd expect from chained/nested empty() calls.

jfcherng · 2019-04-01T19:53:47Z

https://github.com/twigphp/Twig/pull/2787/files#diff-b445caeb1cc1391b4cc1966bd1b1f76cR28

After compilation, wouldn't the current implementation eventually evaluate both the left-hand side and the right-hand side twice at runtime? I mean it's possible that func1 or func2 in func1(arg1) ??? func2(arg2) be executed twice? In PHP, both L and R in L ?? R would be only evaluated once.

If there is a static variable (such as a static counter to log how many time the func has been executed) in func1 or func2, a counterintuitive behavior would happen?

Signed-off-by: Andrew Welch <[email protected]>

khalwat · 2019-04-30T03:36:40Z

@jfcherng suggestions on how to mitigate the behavior you've mentioned?

jfcherng · 2019-04-30T06:57:39Z

@khalwat Sorry I cannot really answer that. But that is what I saw when I was following the ??= RFC. Initially, L ??= R was implemented as a simple syntax sugar for L = L ?? R, but soon problems came out.

For example, consider the expression $a[print 'X'] ??= $b. A simple desugaring into $a[print 'X'] = $a[print 'X'] ?? $b will result in 'X' being printed twice. However, this is not how all other existing compound assignment operators behave: They will print X only once, as the LHS is only evaluated once. I assume that ??= would behave the same way.

        $compiler
            ->raw('(('.self::class.'::empty(')
            ->subcompile($this->getNode('left'))
            ->raw(') ? null : ')
            ->subcompile($this->getNode('left'))
            ->raw(') ?? ('.self::class.'::empty(')
            ->subcompile($this->getNode('right'))
            ->raw(') ? null : ')
            ->subcompile($this->getNode('right'))
            ->raw('))')
        ;

With this, I am assuming L ??? R will be de-sugared into something like

(empty(L) ? null : L) ?? (empty(R) ? null : R)

The worst case is evaluating both L and R twice when executing the compiled template.

Maybe worth a note in the docs if this cannot be resolved?

dharkness · 2019-05-22T20:18:03Z

How about using ??: instead? From what I can tell, this new operator combines the effects of the existing ?? and ?: operators in PHP.

brandonkelly · 2019-05-22T20:35:41Z

@dharkness ?: can’t be used in succession like ?? can though, e.g. foo ?? bar ?? baz

dharkness · 2019-05-22T21:15:25Z

@brandonkelly I can only speak for PHP, but both ?? and ?: can be used in a series.

> $y = null;
> echo $x ?? $y ?? 'foo';
foo
> echo 0 ?: '' ?: 'foo';
foo

lib/Twig/Node/Expression/EmptyCoalesce.php

stof · 2019-05-23T08:11:46Z

lib/Twig/Extension/Core.php


-@trigger_error(sprintf('Using the "Twig_Extension_Core" class is deprecated since Twig version 2.7, use "Twig\Extension\CoreExtension" instead.'), E_USER_DEPRECATED);
+final class CoreExtension extends AbstractExtension


this change is wrong

Correct, the error message is wrong (a copy and paste error) but if we want this removed as per your earlier comment, then it probably doesn't matter. It'd be fixed by just removing the whole deprecation nonsense.

I pushed a change to remove the pointless Twig_Node_Expression_EmptyCoalesce class entirely, which should resolve this.

this is not fixed. You are still making a huge mess in the CoreExtension file, reverting the legacy file to a full class.

I'm using this to merge into: https://github.com/twigphp/Twig/blob/2.x/src/Extension/CoreExtension.php

Should I be using it from a different branch?

@stof I'd love to get this addressed, but I could use some guidance.

The only reason there is a change to lib/Twig/Extension/Core.php is to include the ??? operator. To fix this so that it's not creating a "huge mess" I just need to know what branch/source I should be diff'ing with to add this line?

you should not be making any change in lib/Twig/Extension/Core.php, which is a legacy BC file (except you are reverting this to not be a BC file anymore). The change should be done only in src/Extension/CoreExtension.php

your PR is reverting #2863 on this file, which is what the huge mess is.

I see. That change to make lib/Twig/Extension/Core.php wasn't made until after the PR I made, thus my confusion about what's going on here.

I will revert the now legacy lib/Twig/Extension/Core.php and make my changes in src/Extension/CoreExtension.php

Signed-off-by: Andrew Welch <[email protected]>

GromNaN · 2020-02-21T15:02:38Z

Your example is easy to do with operators ?? and ?:.

{% set bar = null %}
{% set foo = '' %}
{% set baz = [] %}
{% set foobar = woof ?? null ?: bar ?: foo ?: baz ?: 'bark' %}
{{ foobar }}

khalwat · 2020-02-21T15:49:22Z

@GromNaN that presupposes that you know the state of the variables. The point is that any of them could be undefined, null, or empty, and you don't know ahead of time.

GromNaN · 2020-02-28T09:27:38Z

I did't imply this feature was not useful, just trying to help using the existing features.
So <variable> ??? <default> would be a shortcut for (<variable> ?? null) ?: <default>.

If the existence of any variable is unknown, the example can be:

{% set foobar = (woof ?? null) ?: (bar ?? null) ?: (foo ?? null) ?: (baz ?? null) ?: 'bark' %}

brandonkelly · 2020-02-28T13:16:57Z

@GromNaN I don’t think anyone here is unaware of existing syntax options. You could have made the same point about ?? … that <variable> ?? <default> it is the same thing as doing (<variable> is defined and <variable> is not same as(null) ? <variable> : <default>. The point is to simplify, to make templates more readable & maintainable.

GromNaN · 2020-02-28T14:12:04Z

Excepted that with ?? the <variable> have to be repeated several times. Which is very verbose when <variable> contains a complex expression.

acalvino4 · 2022-08-26T01:27:22Z

What's the status of this? Thought I'd bump it up since it seems it got abandoned without any particular reason.

Did the team decide this isn't desirable? the comment voting from the community seems to suggest otherwise.
Is it that the PR needs more work? as far as I can tell there isn't any specific request of what it needs.

So I guess the question is

Are the maintainers open to this?
If not, why, given that the community wants it and every objection so far has been answered?
If so, is there something that needs to be done on the PR to get it merged?

khalwat · 2022-08-26T05:10:47Z

Currently, the PR is borked. I'd be happy to attempt to redo it if there is interest from the maintainers.

maxstrebel · 2022-09-13T13:32:29Z

I want to add my +1 on that. And thanks for the work on that.

{% set theme = entry is defined and entry.theme != "" ? entry.theme : defaults.theme != "" ? defaults.theme : "dark" %}
vs.
{% set theme = entry.theme ??? defaults.theme ??? "dark" %}

The readability is just a whole other level.

jdreesen · 2022-09-13T13:38:35Z

Wouldn't ??: be the better choice, since it's a combination of the operators ?? and ?:?

maxstrebel · 2022-09-13T13:47:52Z

Wouldn't ??: be the better choice, since it's a combination of the operators ?? and ?:?

I guess you're right.

I stumbled upon this via a plugin that allows to use ??? and it felt natural to me. But I actually do not care too much as long as I can use it 😄

VincentLanglet · 2022-10-20T20:33:49Z

Just my two cents, but the first thing I thought about is ??? was "?:` does the same thing until I read

In PHP ?: just determines truthiness, and would throw an error if a variable was undefined

??: seems to be more understandable then.
$a ??: $b = ($a ?? null) ?: $b

It could even be a RFC proposed to PHP first, in order to have the same symbol for both twig and php.

khalwat · 2022-10-20T20:39:37Z

@VincentLanglet would love to see an RFC for it in PHP, too.

The reason I prefer ??? is simply that it's easier to type, and it's been used in other languages before, to do the same thing.

VincentLanglet · 2022-10-20T20:41:42Z

The reason I prefer ??? is simply that it's easier to type, and it's been used in other languages before, to do the same thing.

Do you have examples of other languages with ??? ?

khalwat · 2022-10-20T21:12:48Z

@VincentLanglet link is in the first post in the thread; someone added it to Swift using ???

khalwat added 3 commits December 18, 2018 21:39

Add ??? Empty Coalesce Operator

0f1d747

Signed-off-by: Andrew Welch <[email protected]>

Add ??? Empty Coalesce Operator

d6cf583

Signed-off-by: Andrew Welch <[email protected]>

Fix test

00fbf87

Signed-off-by: Andrew Welch <[email protected]>

fabpot requested changes Apr 1, 2019

View reviewed changes

lib/Twig/Node/Expression/EmptyCoalesce.php Outdated Show resolved Hide resolved

khalwat and others added 2 commits April 29, 2019 23:18

Use twig_test_empty() instead of PHP’s empty(), updated namespacing

2d74692

Signed-off-by: Andrew Welch <[email protected]>

Merge branch '2.x' into 2.x

b42d8a3

fabpot force-pushed the 2.x branch 3 times, most recently from c2a7ac3 to d997512 Compare May 1, 2019 13:44

stof reviewed May 23, 2019

View reviewed changes

Remove pointless Twig_Node_Expression_EmptyCoalesce class

2e08267

Signed-off-by: Andrew Welch <[email protected]>

kylecotter mentioned this pull request May 2, 2020

Support for ??? "Empty Coalesce" operator trivago/prettier-plugin-twig-melody#47

Open

khalwat mentioned this pull request Feb 2, 2024

What would you like to see as major changes that would lead to a BC break for the next major release of Twig? #3951

Open


		@trigger_error(sprintf('Using the "Twig_Extension_Core" class is deprecated since Twig version 2.7, use "Twig\Extension\CoreExtension" instead.'), E_USER_DEPRECATED);
		final class CoreExtension extends AbstractExtension

Add ??? Empty Coalesce Operator #2787

Are you sure you want to change the base?

Add ??? Empty Coalesce Operator #2787

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