https://apple.github.io/swift-nio/docs/current/NIO/Structs/EventLoopPromise.html#/s:3NIO16EventLoopPromiseV12completeWithyyAA0bC6FutureCyxGF

6:51 AM

Promise.completeWith(Future) ともかけるし

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Future.always { result in Promise.completeWith(result) }

6:51 AM

ともかけますが、ここはcascadeが一番オシャレかな。