forms - 如何为 play2 的表单定义 StopOnFirstFail dsl？

Question

在这个问题中:If a form field has multi validators, how to let play verify them one by one, not all? , Julien 给了我一个名为 stopOnFirstFail 的方法解决我的问题:

def stopOnFirstFail[T](constraints: Constraint[T]*) = Constraint { field: T =>
  constraints.toList dropWhile (_(field) == Valid) match {
    case Nil => Valid
    case constraint :: _ => constraint(field)
  }
}

它的用法是:

val loginForm = Form(
  "name" -> (text verifying stopOnFirstFail( nonEmpty, minLength(4) ))
)

但我希望定义一个可以用作的 dsl:

val loginForm = Form(
  "name" -> (text verifying ( nonEmpty or minLength(4) ))
)

我试图为 play.api.data.validation.Constraint 定义一个隐式方法:

import play.api.data.validation._

implicit def _Constraint[T](cons: Constraint[T]) = new {

  def or[T](other: Constraint[T]) = Constraint { field: T =>
    cons(field) match {              // (!)
      case Valid => other(field)
      case invalid => invlaid
    }
  }
}

但无法编译，错误在 (!) 行，消息为:

type mismatch; 
found: field.type (with underlying type T) required: T 
Note: implicit method _Constraint is not applicable here
      because it comes after the application point and it lacks an explicit result type

如何解决？

Answer 1

or方法不接受类型参数:

implicit def toLazyOr[T](cons: Constraint[T]) = new {
  def or(other: Constraint[T]) = Constraint { field: T =>
    cons(field) match {
      case Valid => other(field)
      case Invalid => Invalid
    }
  }
}