Cats Effect Resource (Scala)

Quick start

• Model every owned disposable value with Resource.make, Resource.makeCase, or Resource.fromAutoCloseable.
• Never return an acquired handle from .use; it is released when .use completes. Return only values computed inside the scope.
• Classes must not allocate owned resources in constructors, fields, or top-level vals. Use companion/factory methods returning Resource[F, Service], then pass acquired private dependencies into constructors.
• Wrap Java AutoCloseable values with Resource.fromAutoCloseable(Sync[F].blocking(...)) or IO.blocking(...).
• Compose resources with flatMap, mapN, parMapN, or helper constructors; expose Resource[F, A] from lifecycle-owning APIs.
• Read references/resource.md for patterns, best practices, and type-checked samples.

Workflow

1. Identify every value that needs disposal or lifecycle shutdown.
2. Move acquisition and release into a Resource[F, A]; suspend side effects in F (Sync[F].delay/blocking, IO.blocking, etc.).
3. Build classes from already-acquired constructor parameters; put construction in resource/create factories that return Resource.
4. Compose lower-level resources into higher-level resources before .use.
5. Run with .use at the lifecycle boundary (IOApp, server startup, test fixture), and keep raw handles inside the use scope.
6. Use allocated only for interop that truly needs separate acquire/release, and guarantee the finalizer is called exactly once.

Usage guidance

• Prefer Resource over try/finally or bracket when composition and cancelation safety matter.
• Use IO.blocking (or Sync[F].blocking) for acquisition and release when calling blocking JVM APIs.
• Prefer IO.interruptible/Sync[F].interruptible for blocking use-phase operations inside .use.
• Use Resource.eval only for effects that do not own a finalizer; it is not a substitute for resource acquisition.
• use acquires a fresh resource each time. If the same handle must be shared for multiple operations, do those operations inside one .use.
• For background fibers, use Resource, .background, or Supervisor to ensure cleanup on cancelation.
• When adding or changing Scala examples, update and compile the bundled scripts/verify-examples.scala script.

References

• Load references/resource.md for API details, patterns, and examples; the representative check is scripts/verify-examples.scala.
• For Kotlin/Arrow parallels, see the arrow-resource skill.
• Install this skill with npx skills add https://github.com/alexandru/skills --skill cats-effect-resource.