Osprey.
Safe, elegant, fast.

effect Files { write: fn(string, string) -> Result<int, Error> }
effect Log { info: fn(string) -> Unit }

// Deep in the app: no disk or logger is passed in.
fn writeLock(lock) = match perform Files.write("osprey.lock", lock) {
    Success _ => "installed"
    Error { message } => "rolled back: ${message}"
}
fn install(name) = {
    perform Log.info("resolving ${name}")
    writeLock("${name} = 2")
}

// The app's edge picks the real disk and logger, once.
let shipped = {
    handle Files { write path text => writeFile(path, text) }
    handle Log { info text => print("log: ${text}") }
    install("serde")
}
print(shipped)

// A test swaps in a full disk. The app code does not change.
test("a full disk rolls back", fn() => {
    handle Files { write _ _ => Error { message: "disk full" } }
    handle Log { info text => expect(text, "resolving serde") }
    expect(install("serde"), "rolled back: disk full")
})