Resource management with on_exit and Scope

on_exit attaches a finalizer that runs when the effect settles, on success and failure alike. A Scope collects finalizers from a whole sub-tree: acquire_and_release registers a release for an acquired resource, and .scoped() provides the Scope and runs the collected finalizers in reverse order when the wrapped effect settles. It composes with provide chains — program.provide(Db)(db).scoped() — and is a no-op on effects that never acquired a Scope, so it can uniformly terminate a chain.

E.success(21).on_exit(E.log_info("done"))  # finalizer runs on success and failure alike
 
conn = E.acquire_and_release(
    E.sync(lambda: pool.connect()),  # acquire
    lambda c: E.sync(c.close),  # release, guaranteed by the enclosing scope
)  # Effect[Connection, Never, Scope]
 
program = conn.flat_map(
    run_queries
).scoped()  # Scope discharged; close() runs when program settles

on_exit also takes a function that receives the Exit and returns the finalizer, so cleanup can depend on how the effect settled:

E.success(21).on_exit(
    lambda exit: E.log_info("settled", exit)
)  # Succeeded(21), or Failure(cause) carrying Fail, Die or Interrupt

A finalizer that dies doesn't skip the remaining finalizers; its defect surfaces in the final Exit.

More examples: test_scope.py.