我正在努力学习 scalaz和Learn You A Haskell For Greater Good,我想知道如何将 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)
^
和
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)
^