swift - 混淆 map 与平面 map : Cannot invoke 'flatMap' with an argument list of type '(() -> EventLoopFuture)'

Question

我是 Swift 初学者，在 Vapor 框架中工作，开发了一个注册过程，该过程需要几个连续的步骤才能工作。我遇到了以下代码的问题，Xcode 指示标记行上存在错误:

func signup(_ req: Request) throws -> Future<RecordA> {
    return try req.content.decode(RecordACreation.self).flatMap(to: RecordACreation.self) { recordACreation in

        // Prevent a recordA email from being used twice.
        RecordA(on: req).filter(\.email == recordACreation).first().map(to: RecordA.self) { optionalExistingRecordAByEmail in
            if let _ = optionalExistingRecordAByEmail {
                throw Abort(.unprocessableEntity, reason: "RecordA email already exists")
            }

        }.flatMap { // Xcode indicates there's an error here.
            // Prevent an recordB email from being used twice.
            return RecordB.query(on: req).filter(\.email == recordACreation.recordBEmail).first().map(to: RecordA.self) { optionalExistingRecordBByEmail in
                if let _ = optionalExistingRecordBByEmail {
                    throw Abort(.unprocessableEntity, reason: "RecordB email already exists.")
                }
            }

        }.flatMap {
            // If the recordB's password could not be successfully hashed.
            guard let recordBPassword = try? BCrypt.hash(recordACreation.recordBPassword) else {
                throw Abort(.unprocessableEntity, reason: "Password was unhashed")
            }

            // Ensure a verification token is generated successfully.
            guard let optionalVerificationToken = try? VerificationTokenService.generate(), let verificationToken = optionalVerificationToken else {
                throw Abort(.unprocessableEntity, reason: "Verification token could not be generated.")
            }

            let recordA = RecordA(name: recordACreation.recordAName, email: recordACreation.recordAEmail)

            return req.transaction(on: .sqlite) { conn in
                // TODO: Send email on signup success.

                return recordA.save(on: conn).map(to: RecordA.self) { savedRecordA in
                    guard let recordAId = recordA.id else {
                        throw Abort(.unprocessableEntity, reason: "Verification token could not be generated.")
                    }

                    _ = RecordB(name: recordACreation.recordBName, email: recordACreation.recordBEmail, password: recordBPassword, recordAId: recordAId, verificationToken: verificationToken).save(on: conn)
                    return savedRecordA
                }
            }
        }
    }
}

错误如下:

Cannot invoke 'flatMap' with an argument list of type '(() -> EventLoopFuture)'

I don't entirely understand the mechanisms between an appropriate selection of map and flatMap, so that may be the cause of the issue here.想法？

Answer 1

问题是 RecordA 查询之后的 map 调用:

RecordA(on: req).filter(\.email == recordACreation).first().map(to: RecordA.self) { optionalExistingRecordAByEmail in
    if let _ = optionalExistingRecordAByEmail {
        throw Abort(.unprocessableEntity, reason: "RecordA email already exists")
    }
}

您将 RecordA.self 传递给 to 参数，它将闭包的返回类型设置为 RecordA。相反，您不会返回任何东西。您应该删除 to 参数，它应该可以工作:

RecordA(on: req).filter(\.email == recordACreation).first().map { optionalExistingRecordAByEmail in
    if let _ = optionalExistingRecordAByEmail {
        throw Abort(.unprocessableEntity, reason: "RecordA email already exists")
    }
}