Teaches AI coding agents to write idiomatic Kotlin instead of Java-in-a-.kt-file. Six rules covering the highest-leverage Kotlin idioms, three skills that perform the common conversions, and a CI script that gates the JUnit-to-Kotest migration deterministically.

Why This Plugin Exists

A coding agent without Kotlin context writes Kotlin like a Java developer who learned the syntax over a long weekend: regular class instead of data class, manual equals / hashCode, var properties everywhere, Optional<String> for nullables, JUnit assertions in test files. It compiles. It runs. It's Java in a .kt file. This plugin closes the gap — one tessl install and your agent knows the difference.

Installation

tessl install jbaruch/kotlin-tutor

What's Included

Rules (6)

ID	Rule	Summary
K-1	prefer-val	Properties default to `val`; `var` is opt-in for mutation
K-2	nullable-question-mark	`String?` not `Optional<String>`; lean on `?.` and `?:`
K-3	use-data-class	Value types are `data class`; never hand-roll equals/hashCode/toString
K-4	kotest-over-junit	Tests use Kotest matchers and specs, not `assertEquals` and `@Test`
K-5	prefer-stdlib-scope	Use `.let` / `.also` / `.apply` over imperative Java patterns
K-6	extension-over-util	Extension functions, not `*Utils` classes with static methods

Skills (3)

Skill	Description
kotlinify-tests	Convert JUnit-style test classes to idiomatic Kotest specs; delegates to `verify-no-junit-assertions.sh` for the deterministic gate
pojoify-to-dataclass	Refactor a Java-style POJO (manual equals/hashCode/toString, bean accessors) into an idiomatic Kotlin `data class`
nullable-cleanup	Replace `Optional<T>` with `T?` and the `?.` / `?:` / `?.let { }` operators

Scripts (1)

Script	Purpose
verify-no-junit-assertions.sh	Fails CI if any test file under `src/test/` still uses JUnit-style assertions. Delegated from the `kotlinify-tests` skill; runs standalone as a CI gate. Enforces rule K-4.

Eval Scenarios

Scenario	Targets skill	Purpose
scenario-0, scenario-1, scenario-2	kotlinify-tests	Real evals (auto-generated, curated to remove bleeding)
scenario-pojoify-subscription	pojoify-to-dataclass	Java-style POJO → idiomatic data class refactor
scenario-nullable-user-service	nullable-cleanup	`Optional<T>` API → Kotlin nullable types, caller updated alongside
trial-1-task-leaks-language	—	Pedagogical — slide-59 demo, task leaks language, baseline 100%
trial-2-rubric-grades-features	—	Pedagogical — slide-59 demo, rubric still grades features only, baseline still 100%
trial-3-rubric-weights-language	—	Pedagogical — slide-59 demo, rubric weights idiomatic Kotlin 80%, lift becomes measurable

The pedagogical scenarios are intentionally bad eval design — they ship publicly so audiences of the talk "You're Absolutely Right (and Other Lies My AI Told Me)" can browse them in the registry and see why the failures land at 100% / 100% / ~20%-then-~95%.

The Skill / Script Delegation Pattern

The kotlinify-tests skill demonstrates the script-delegation pattern in miniature: the skill does the judgment work (which Kotest matcher fits this JUnit call? what spec style preserves the original structure?), and verify-no-junit-assertions.sh does the deterministic gate (did any JUnit assertion slip through?). Judgment lives in the skill; mechanism lives in the shell. The script also runs when the agent isn't present — human commits go through the same gate in CI.

Conventions

All six rules apply per-turn (frontmatter alwaysApply: true); agents see them on every prompt
Skills are intent-discovered via the trigger phrases in their description field
Rule IDs (K-1 through K-6) are talk-shorthand for the descriptive filenames; the registry uses the filenames

See CHANGELOG.md for version history.

Workspace: jbaruch
Visibility: Public
Created: about 21 hours ago
Last updated: about 20 hours ago
Publish Source: CLI
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