Rule to mandate/forbid trailing commas

[3flex]

Expected Behavior

ktlint offers a rule that enforces a particular code style regarding trailing commas, which are now permitted by the Kotlin compiler as of 1.3.70, see https://youtrack.jetbrains.com/issue/KT-34743 for more details.

This may not be a good fit for ktlint as it's too opinionated and no conventions exist yet, but if it's suitable it's a better fit here than in detekt where it was originally proposed.

Observed Behavior

No such rule exists.

Steps to Reproduce

N/A, new rule required.

Your Environment

• Version of ktlint used:
• Name and version (or code for custom task) of integration used (Gradle plugin, Maven plugin, command line, custom Gradle task):
• Version of Gradle used (if applicable):
• Operating System and version:
• Link to your project (if it's a public repository):

[alexvanyo]

Ideally this rule could be as flexible as something like ESLint's comma dangling rule, which would allow specifying when trailing commas must be used (never, always, multiline-only, when entry, etc.)

Edit: Perhaps supporting that feature set would better live elsewhere?

[Tapchicoma]

Probably related to #701 to control enable/disable state for cases, though initial rule implementation could just enforce trailing commas and be in experimental set.

[s1monw1]

Any update?

[ivorsmorenburg]

Is breaking so many things this ,
https://stackoverflow.com/questions/65055798/kotlin-linting-rulesetprovider-trailing-comma

[3flex]

Is this on ktlint's roadmap? If not, we could implement in detekt instead. I don't think it makes sense to do that though if ktlint will implement the rule so we can avoid duplication of effort, but I think there's demand for it (I'm sure this is my most 👍 issue ever!)

[ivorsmorenburg]

Hopefully is or it will be 🤞, for now I'm trying to implement my own rule from the link above. I can confirm there is a high demand. Will be nice to autocorrect with ktlint...

[romtsn]

@3flex you are more than welcome to contribute this to ktlint directly and utilize it in detekt afterwards 👀 it is certainly on the roadmap, just the maintainers were sidetracked by the day jobs and we had other priorities in ktlint, but I see this one indeed has a high demand so we'll re-prioritize

[CDehning]

Is this is getting released soon?

[ivorsmorenburg]

Is this is getting released soon?

Maybe try using your own ruleset
https://stackoverflow.com/a/65449838/3438335

[Badneighbour91]

Oui oumême iii Le jeu. 11 mars 2021 à 09:40, Christian Dehning ***@***.***> a écrit :

…

Is this is getting released soon? — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#709 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AR6FQKW2WTD3JDHQ4AAU2DLTDB6V7ANCNFSM4LBDHE4Q> .

[snijsure]

Is this is getting released soon?

Maybe try using your own ruleset
https://stackoverflow.com/a/65449838/3438335

Is there a guideline on integrating this rule into local version of local ktlint?

[christian-draeger]

any news?

[TobiasMende]

We are currently using our own rule for mandating (and automatically fixing) missing trailing commas. Probably it is not ready yet to be added to the ktlint project, but I am happy to have a discussion about it:

class MandateTrailingComma : Rule("mandate-trailing-comma") {

    override fun visit(
        node: ASTNode,
        autoCorrect: Boolean,
        emit: (offset: Int, errorMessage: String, canBeAutoCorrected: Boolean) -> Unit,
    ) {
        if (node.isArgumentOrParameterList() && node.isMultiLine()) {
            val lastArgument = node.lastArgumentOrParameter()
            if (lastArgument != null && lastArgument.treeNext.elementType != COMMA) {
                emit(
                    lastArgument.startOffset + lastArgument.textLength,
                    "Trailing comma is missing",
                    true,
                )
                if (autoCorrect) {
                    node.addChild(LeafPsiElement(COMMA, ","), lastArgument.treeNext)
                }
            }
        }
    }

    private fun ASTNode.isArgumentOrParameterList(): Boolean =
        elementType == VALUE_ARGUMENT_LIST || elementType == VALUE_PARAMETER_LIST

    private fun ASTNode.isMultiLine(): Boolean = children().any { it is PsiWhiteSpace && it.textContains('\n') }
    private fun ASTNode.lastArgumentOrParameter() =
        children().findLast { it.elementType == VALUE_ARGUMENT || it.elementType == VALUE_PARAMETER }
}

[sindrenm]

Fantastic stuff! Just to be clear, “allowing” trailing commas here means requiring trailing commas, right? Will

fun foo(
  param1: Bar,
  param2: Bar
)

be converted into

fun foo(
  param1: Bar,
  param2: Bar, // <-- added trailing comma
)

when either running ktlint -F or the auto-formatter in IJ/AS?

[romtsn]

Yes, this means requiring, that's correct (and that's what IJ does as well, when ticking the "Allow trailing comma" checkbox in settings). I will update README with a better description of the behavior 👍

[snijsure]

I might be doing something wrong, and I don't see ktlint -F is putting missing trailing commas,

Here is my setup:

I have the following in .editorconfig

ij_kotlin_allow_trailing_comma = true
ij_kotlin_allow_trailing_comma_on_call_site = true

And I have code like this

    data class State(
        val tutorialPages: List<TutorialPage> = emptyList(),
        val currentPageNumber: Int = 0
    ) 

When I run command

ktlint -F --editorconfig=.editorconfig --experimental fileName

I see the following error

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.jetbrains.kotlin.com.intellij.util.ReflectionUtil (file:/usr/local/Cellar/ktlint/0.42.1/libexec/ktlint) to field java.lang.Throwable.backtrace
WARNING: Please consider reporting this to the maintainers of org.jetbrains.kotlin.com.intellij.util.ReflectionUtil
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

I don't see that ktlint -F has put the trailing comma.

  data class State(
        val tutorialPages: List<TutorialPage> = emptyList(),
        val currentPageNumber: Int = 0**,**
    ) 

[romtsn]

@snijsure which version are you using? ktlint --version

[snijsure]

@romtsn I am using version 0.42.1

ktlint --version
0.42.1

[romtsn]

yeah, as I mentioned it's only available in the snapshot version for now.

[Lingviston]

When is it going to be live?

[zsmb13]

A new release containing this was just created https://github.com/pinterest/ktlint/releases/tag/0.43.0

[mohsenoid]

I am using 0.43.0 CLI

and enabled the following in my project's .editorconfig

ij_kotlin_allow_trailing_comma = true
ij_kotlin_allow_trailing_comma_on_call_site = true

but I don't get any format fix or lint error from ktlint when I execute this or related Gradle tasks:
ktlint -F "app/src/**/*.kt"

The good news is, the Android studio code styler understands it and does the formatting for me, but I also want it to be a part of the CI check!

[romtsn]

@mohsenoid

Note, that it's available as a --experimental rule for now.

[snijsure]

Is there a way to just enable this rule, out of the experimental ruleset?

If I turn on experimental rules it turns up lots of other warnings that we don't care at this moment

@romtsn

[mohsenoid]

@romtsn Thanks for the hint, I executed this and it works fine:
ktlint -F --experimental "app/src/**/*.kt"

Looking forward to seeing the final rule.

[romtsn]

@snijsure nope, the best you can do is to enable the experimental ruleset and disable the rules you don't care about via disabled_rules option

[mohsenoid]

Since this is an experimental rule, should we report issues here or create a new ticket case by case?

For instance in this case
https://github.com/mohsenoid/Rick-and-Morty/blob/c9fb8dc093c7986055e8892b8e126f60bf0247cc/app/src/main/kotlin/com/mohsenoid/rickandmorty/data/db/DbRickAndMorty.kt#L51

Android Studio works correctly and adds a comma, but Ktlint doesn't and even removes the comma added by Android Studio!

[romtsn]

@mohsenoid please file a new issue

[vojkny]

how can I enable this rule via .editorconfig? I used:

ij_kotlin_allow_trailing_comma = true
ij_kotlin_allow_trailing_comma_on_call_site = true

but seems to have no effect

[romtsn]

#709 (comment)

[vojkny]

#709 (comment)

It does not answer how I can enable it via .editorconfig.

[romtsn]

well, your question was about the trailing comma rule, not about the experimental ruleset. you can't enable experimental via editorconfig

[Egorand]

Can this feature be used with spotless?

[sindrenm]

Can this feature be used with spotless?

Not as far as I can tell, since experimental ktlint flags can't be enabled. (diffplug/spotless#409)

[krisgerhard]

This feature seems to break no-unused-imports rule. I'm getting "unused import" error for all imports that are used on trailing comma lines exclusively

[hongjisung]

I have same unused-imports rule break error with -F --experimental option

I run ktlint -F --experimental three times then the project pass the test and fixed correctly
But, first and second trials are failed with only some modified files

[krisgerhard]

I created a failing test-case and new issue: #1367

[mikaello]

In Ktlint v0.46.0, trailing-comma was promoted from the experimental ruleset to the standard ruleset. Adding this to your .editorconfig should now work without having to specify --experimental flag when running Ktlint:

[*.{kt,kts}]
ij_kotlin_allow_trailing_comma_on_call_site=true
ij_kotlin_allow_trailing_comma=true

If you use the ktlint-maven-plugin, you must use v1.14.0 or later, example:

$ mvn com.github.gantsign.maven:ktlint-maven-plugin:1.14.0:format

[bgolob]

In Ktlint v0.46.0, trailing-comma was promoted from the experimental ruleset to the standard ruleset. Adding this to your .editorconfig should now work without having to specify --experimental flag when running Ktlint:

[*.{kt,kts}]
ij_kotlin_allow_trailing_comma_on_call_site=true
ij_kotlin_allow_trailing_comma=true

If you use the ktlint-maven-plugin, you must use v1.14.0 or later, example:

$ mvn com.github.gantsign.maven:ktlint-maven-plugin:1.14.0:format

I enabled both of the settings for trailing commas. I'm using Ktlint Gradle plugin version 11.0.0. When I run ./gradlew ktlintcheck
I'm not getting any errors for missing trailing commas.

For example here the trailing comma is missing and ktlint didn't throw any errors

data class AdModel(
    val duration: Int,
    val type: String,
    val url: String
)

[mikaello]

I enabled both of the settings for trailing commas. I'm using Ktlint Gradle plugin version 11.0.0. When I run ./gradlew ktlintcheck
I'm not getting any errors for missing trailing commas.

I am not familiar with Ktlint Gradle plugin, but it seems that the newest version of that plugin, v11.0.0, is using Ktlint v0.43.2. Between Ktlint [v0.43.0, v0.46.0), the trailing comma rule was only present in the experimental ruleset.

Skimming trough the issue list of Ktlint Gradle plugin it is at least two issues for upgrading Ktlint, and problems with that, e.g.:

• The plugin does not work when using Ktlint 0.46.0 JLLeitschuh/ktlint-gradle#589
• ktlint 0.47.x breaks plugin JLLeitschuh/ktlint-gradle#607

You should follow those issue to get notified when Ktlint Gradle plugin is compatible with newer versions of Ktlint.