Kotlin Quality

Kotlin Quality — detekt + ktlint

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: detekt or ktlint.

Tool Comparison

Use both ktlint and detekt — complementary. ktlint for formatting, detekt for everything else.

Setup — ktlint (Gradle Plugin)

// build.gradle.kts (root)
plugins {
    id("org.jlleitschuh.gradle.ktlint") version "12.1.1" apply false
}

subprojects {
    apply(plugin = "org.jlleitschuh.gradle.ktlint")

    configure<org.jlleitschuh.gradle.ktlint.KtlintExtension> {
        version.set("1.4.1")
        verbose.set(true)
        outputToConsole.set(true)
        coloredOutput.set(true)
        ignoreFailures.set(false)
        enableExperimentalRules.set(false)
        filter {
            exclude("**/generated/**")
            exclude("**/build/**")
            include("**/kotlin/**")
        }
    }
}

./gradlew ktlintCheck                              # check all modules
./gradlew ktlintFormat                              # auto-fix
./gradlew :shared:ktlintCheck                       # one module

ktlint Configuration via .editorconfig

# .editorconfig
root = true

[*.{kt,kts}]
indent_size = 4
max_line_length = 120
ij_kotlin_imports_layout = *,java.**,javax.**,kotlin.**,^

# ktlint-specific
ktlint_standard = enabled
ktlint_experimental = disabled
ktlint_function_naming_ignore_when_annotated_with = Composable,Test
ktlint_compose = enabled

Setup — detekt

// build.gradle.kts (root)
plugins {
    id("io.gitlab.arturbosch.detekt") version "1.23.7" apply false
}

subprojects {
    apply(plugin = "io.gitlab.arturbosch.detekt")

    configure<io.gitlab.arturbosch.detekt.extensions.DetektExtension> {
        config.setFrom(files("$rootDir/config/detekt/detekt.yml"))
        baseline = file("$projectDir/detekt-baseline.xml")
        buildUponDefaultConfig = true
        autoCorrect = false                          // CI-safe
        parallel = true
    }

    dependencies {
        "detektPlugins"("io.gitlab.arturbosch.detekt:detekt-formatting:1.23.7")
        "detektPlugins"("io.nlopez.compose.rules:detekt:0.4.16")    // Compose rules
    }
}

tasks.withType<io.gitlab.arturbosch.detekt.Detekt>().configureEach {
    reports {
        html.required.set(true)
        xml.required.set(true)
        sarif.required.set(true)
    }
}

./gradlew detekt
./gradlew detektBaseline                              # generate baseline
./gradlew :shared:detekt

detekt.yml — Production Config

# config/detekt/detekt.yml
build:
  maxIssues: 0
  weights:

complexity:
  CyclomaticComplexMethod:
    threshold: 15
  LongMethod:
    threshold: 60
  LongParameterList:
    functionThreshold: 8
  TooManyFunctions:
    thresholdInClasses: 30
    ignoreAnnotatedFunctions:
      - 'Composable'

empty-blocks:
  active: true

exceptions:
  TooGenericExceptionCaught:
    exceptionNames:
      - 'Exception'
      - 'RuntimeException'
      - 'Throwable'
    allowedExceptionNameRegex: '_|(ignore|expected).*'

naming:
  FunctionNaming:
    functionPattern: '[a-z][a-zA-Z0-9]*'
    excludes: ['**/test/**', '**/androidTest/**']
    ignoreAnnotated:
      - 'Composable'                                  # PascalCase composables OK

performance:
  active: true

potential-bugs:
  HasPlatformType:
    active: true                                       # KMP-friendly: catch missing nullability

style:
  MagicNumber:
    ignoreNumbers: ['-1', '0', '1', '2', '100', '1000']
    ignoreEnums: true
  MaxLineLength:
    maxLineLength: 120
  ReturnCount:
    max: 4
  WildcardImport:
    active: true

formatting:                                            # ktlint-formatting plugin
  active: true
  android: false
  autoCorrect: false

Baseline (For Legacy Code)

./gradlew detektBaseline    # generates detekt-baseline.xml — commit it

New issues fail CI; existing ones silently allowed. Reduce baseline over time.

Compose Rules

Twitter/Slack/Mrtn maintain Compose-specific lint rules:

dependencies {
    "detektPlugins"("io.nlopez.compose.rules:detekt:0.4.16")
    // OR for ktlint
    "ktlintRuleset"("io.nlopez.compose.rules:ktlint:0.4.16")
}

Catches:

• @Composable not in PascalCase
• Modifier parameter not first
• remember { mutableStateOf() } instead of by remember
• Side effects outside LaunchedEffect
• Missing key in LazyColumn.items
• Composable returning unit named like accessor

CI Integration — GitHub Actions

# .github/workflows/quality.yml
name: Kotlin Quality

on: [push, pull_request]

jobs:
  ktlint-detekt:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-java@v4
        with: { java-version: '17', distribution: 'temurin' }
      - uses: gradle/actions/setup-gradle@v4

      - name: ktlint
        run: ./gradlew ktlintCheck

      - name: detekt
        run: ./gradlew detekt

      - name: Upload SARIF (GitHub Code Scanning)
        if: always()
        uses: github/codeql-action/upload-sarif@v3
        with:
          sarif_file: build/reports/detekt/detekt.sarif

      - name: Upload reports on failure
        if: failure()
        uses: actions/upload-artifact@v4
        with:
          name: quality-reports
          path: |
            **/build/reports/ktlint/
            **/build/reports/detekt/

Pre-Commit Hook

# lefthook.yml
pre-commit:
  parallel: true
  commands:
    ktlint:
      glob: "*.{kt,kts}"
      run: ./gradlew ktlintFormat -PfilesToFormat={staged_files}
      stage_fixed: true
    detekt:
      glob: "*.{kt,kts}"
      run: ./gradlew detekt

Custom detekt Rules

class NoPrintStatement : Rule() {
    override val issue = Issue(
        id = "NoPrintStatement",
        severity = Severity.Style,
        description = "Use Logger.d() instead of println()",
        debt = Debt.FIVE_MINS,
    )

    override fun visitCallExpression(expression: KtCallExpression) {
        super.visitCallExpression(expression)
        val name = expression.calleeExpression?.text
        if (name in setOf("println", "print")) {
            report(CodeSmell(issue, Entity.from(expression), "Use a logger"))
        }
    }
}

class CustomRuleSet : RuleSetProvider {
    override val ruleSetId: String = "custom"
    override fun instance(config: Config) = RuleSet(ruleSetId, listOf(NoPrintStatement()))
}

Register via META-INF/services/io.gitlab.arturbosch.detekt.api.RuleSetProvider.

Wallet App Quality Checklist

For BHODL-style production:

• ktlintCheck and detekt in CI
• No baseline regressions (new issues block PR)
• Compose rules enabled (consistent composable patterns)
• No println / Log.d (use Kermit / Timber)
• No TODO/FIXME in main code (use issue tracker)
• Magic numbers explained or named constants
• Cyclomatic complexity ≤ 15 per function
• No @Suppress without comment explaining why
• All public types documented (KDoc)
• No platform types from Java interop without explicit nullability
• No !! force-unwrap in main code (review usage)

detekt Rule Sets Cheat Sheet

Anti-Patterns

Troubleshooting

When NOT to Use This Skill