地图与flatMap的混淆：无法使用类型为（（（）-> EventLoopF​​uture）'的参数列表调用'flatMap'

Question

我是一名Swift初学者，在Vapor框架中工作，开发了一个注册过程，该过程需要依次执行几个步骤。我遇到以下代码的问题，该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
                }
            }
        }
    }
}

该错误显示为：

  

无法使用类型为'（（）-> EventLoopF​​uture）'的参数列表调用'flatMap'

我不完全了解正确选择map和flatMap之间的机制，因此这可能是造成此问题的原因。有想法吗？

Answer 1

问题是在map查询之后的RecordA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，该参数将闭包的返回类型设置为RecordA。相反，您什么也不返回。您应该删除to参数，它应该可以正常工作：

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