V4Validations.swift

func createUser(request: Request) -> EventLoopFuture<HTTPResponseStatus> {

    let password: String? = request.content["password"]
    let email: String? = request.content["email"]

    User.query(on: request.db).filter(\.email == email).count().map {
        var validations = NewUser.validations()
        validations.add("passwordAgain", as: String, is: .in(["password"])) // we need a .equals validator
        if email != nil {
            validations.add("email", result: UniqueEmailValidatorResult())
        }
        return validations
    }.flatMapThrowing {
        try $0.validate(request).assert()
    }.map { .ok } // obviously we need to create the user here as well
}

struct UniqueEmailValidatorResult: ValidatorResult {
    public var isFailure: Bool {
        true
    }

    public var successDescription: String? {
        nil
    }

    public var failureDescription: String? {
        "email already exists"
    }
}

struct NewUser {
    let password: String
    let passwordAgain: String
    let email: String
}