main.swift

import FDB
import NIO

enum E: Error {
    case recordExists
}

let fdb = FDB()

let key = "somekey".bytes

// using blocking API

try fdb.withTransaction { tr in
    guard try tr.get(key: key, snapshot: false, commit: false) == nil else {
        throw E.recordExists
    }

    try tr.set(key: key, value: "somevalue".bytes, commit: true) as Void
}

// or if you're using NIO (which is, again, recommended)

let eventLoopGroup: EventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: System.coreCount)
let resultFuture: EventLoopFuture<Void> = fdb.withTransaction(on: eventLoopGroup.next()) { tr in
    tr
        .get(key: key, snapshot: false, commit: false)
        .flatMapThrowing { (maybeValue: Bytes?) in
            guard maybeValue == nil else {
                throw E.recordExists
            }
        }
        .flatMap {
            tr.set(key: key, value: "somevalue".bytes, commit: true)
        }
        .map { _ in Void() }
}

public extension String {
    var bytes: Bytes {
        Bytes(self.utf8)
    }
}