在学习Scalaz 6时,我正在尝试编写返回验证的类型安全读取器。以下是我的新型:
type ValidReader[S,X] = (S) => Validation[NonEmptyList[String],X]
type MapReader[X] = ValidReader[Map[String,String],X]
我有两个为int和string(*)创建映射读取器的函数:
def readInt( k: String ): MapReader[Int] = ...
def readString( k: String ): MapReader[String] = ...
给定以下地图:
val data = Map( "name" -> "Paul", "age" -> "8" )
我可以写两个读者来检索姓名和年龄:
val name = readString( "name" )
val age = readInt( "age" )
println( name(data) ) //=> Success("Paul")
println( age(data) ) //=> Success(8)
一切都很好,但现在我想组合两个阅读器来构建一个Boy
实例:
case class Boy( name: String, age: Int )
我最好的选择是:
val boy = ( name |@| age ) {
(n,a) => ( n |@| a ) { Boy(_,_) }
}
println( boy(data) ) //=> Success(Boy(Paul,8))
它按预期工作,但对于两个层次的应用构建器来说,表达方式很尴尬。有没有办法让以下语法发挥作用?
val boy = ( name |@| age ) { Boy(_,_) }
(*)完整且可运行的实施方式:https://gist.github.com/1891147
更新:这是我在尝试上面的行或Daniel建议时收到的编译器错误消息:
[error] ***/MapReader.scala:114: type mismatch;
[error] found : scalaz.Validation[scalaz.NonEmptyList[String],String]
[error] required: String
[error] val boy = ( name |@| age ) { Boy(_,_) }
[error] ^
这个怎么样?
val boy = (name |@| age) {
(Boy.apply _).lift[({type V[X]=ValidationNEL[String,X]})#V]
}
或使用类型别名:
type VNELStr[X] = ValidationNEL[String,X]
val boy = (name |@| age) apply (Boy(_, _)).lift[VNELStr]
这是基于控制台上的以下错误消息:
scala> name |@| age apply Boy.apply
<console>:22: error: type mismatch;
found : (String, Int) => MapReader.Boy
required: (scalaz.Validation[scalaz.NonEmptyList[String],String],
scalaz.Validation[scalaz.NonEmptyList[String],Int]) => ?
所以我只是举起Boy.apply
来取所需的类型。
Reader
和Validation
(具有半群E)都是可应用的,所以它们的组合也是可应用的。使用scalaz 7,这可以表示为:
import scalaz.Reader
import scalaz.Reader.{apply => toReader}
import scalaz.{Validation, ValidationNEL, Applicative, Kleisli, NonEmptyList}
//type IntReader[A] = Reader[Int, A] // has some ambigous implicit resolution problem
type IntReader[A] = Kleisli[scalaz.IdInstances#Id, Int, A]
type ValNEL[A] = ValidationNEL[Throwable, A]
val app = Applicative[IntReader].compose[ValNEL]
现在我们可以在组合的应用程序:上使用单个|@|
操作
val f1 = toReader((x: Int) => Validation.success[NonEmptyList[Throwable], String](x.toString))
val f2 = toReader((x: Int) => Validation.success[NonEmptyList[Throwable], String]((x+1).toString))
val f3 = app.map2(f1, f2)(_ + ":" + _)
f3.run(5) should be_==(Validation.success("5:6"))