A recipe for the Perfect Use Case

Question

Today we’re cooking the perfect Use Case — and CQRS will be our recipe.

Ingredients

• Action — fire and forget (logout, clear cache)
• Query — read data (list products)
• Command — write data (update profile)
• Exchange — transform data (login)

All four types extend a sealed interface:

sealed interface UseCase<Input, Output> {
    interface Action : UseCase<Unit, Unit>
    interface Query<Output> : UseCase<Unit, Output>
    interface Command<Input> : UseCase<Input, Unit>
    interface Exchange<Input, Output> : UseCase<Input, Output>
}

Three Ways to Season It

Arrow (Typed Errors) — the chef’s choice:

class GenerateSeed(
    private val seedService: SeedService
) : UseCase.Action {
    override suspend fun Raise<Throwable>.action() =
        seedService.generateSeed().bind()
}

Result (Standard Wrapper) — zero dependencies:

class GenerateSeed(private val service: SeedService) : UseCase.Action {
    override suspend fun action() = service.generateSeed().getOrThrow()
}

Raw (Direct Execution) — zero overhead:

class GenerateSeed(private val service: SeedService) : UseCase.Action {
    override suspend fun action() = service.generateSeed()
}

Why This Recipe Works

• No more UseCase<Unit, Unit> noise
• Every use case follows the same structure
• Query or Command makes intent obvious

Full implementation in the GitHub repository.