首页 文章

Scalaz作家Monad和filterM

提问于
浏览
0

我正在通过learning scalazLearn You A Haskell For Greater Good工作,并想知道如何将filterM示例从LYAHFGG转换为Scala .

fst $ runWriter $ filterM keepSmall [9,1,5,2,10,3]

keepSmall 定义为

keepSmall :: Int -> Writer [String] Bool  
keepSmall x  
    | x < 4 = do  
        tell ["Keeping " ++ show x]  
        return True  
    | otherwise = do  
        tell [show x ++ " is too large, throwing it away"]  
        return False

我天真的方法以编译错误结束,我不知道如何解决这个问题!

val keepSmall: (Int => WriterT[Id, Vector[String], Boolean]) = (x: Int) => 
      if (x < 4) for {
        _ <- Vector("Keeping " + x.shows).tell
      } yield true
      else for {
        _ <- Vector(x.shows + " is too large, throwing it away").tell
      } yield false

println(List(9,1,5,2,10,3) filterM keepSmall)

编译错误:

Error:(182, 32) no type parameters for method filterM: (p: Int => M[Boolean])(implicit evidence$4: scalaz.Applicative[M])M[List[Int]] exist so that it can be applied to arguments (Int => scalaz.WriterT[scalaz.Scalaz.Id,Vector[String],Boolean])
 --- because ---
argument expression's type is not compatible with formal parameter type;
 found   : Int => scalaz.WriterT[scalaz.Scalaz.Id,Vector[String],Boolean]
 required: Int => ?M[Boolean]
    println(List(9,1,5,2,10,3) filterM keepSmall)
                               ^

Error:(182, 40) type mismatch;
 found   : Int => scalaz.WriterT[scalaz.Scalaz.Id,Vector[String],Boolean]
 required: Int => M[Boolean]
    println(List(9,1,5,2,10,3) filterM keepSmall)
                                       ^

1 回答

  • 2

    问题是由于Scala无法真正知道如何将具有三个孔的类型拟合到 filterM 所期望的参数中,该参数只有一个孔填充 Boolean .

    你可以使用像这样的奇怪的类型lambda语法来解决你的问题(未经测试,可能无效):

    val keepSmall: (Int => ({type L[T] = WriterT[Id, Vector[String], T]})#L) = ...
    

    或者(通过引入类型别名更容易),如下所示:

    type MyWriter[T] = WriterT[Id, Vector[String], T]
    val keepSmall: (Int => MyWriter[Boolean]) = ...
    

    这将确保 filterM 所期望的参数类型与您提供的参数类型相匹配 .

相关问题