[英]Scala map with partial function as value
In Twitter's Scala school collections section , they show a Map with a partial function as a value: 在Twitter的Scala学校 集合部分 ,他们显示了一个具有部分功能的Map作为值:
// timesTwo() was defined earlier.
def timesTwo(i: Int): Int = i * 2
Map("timesTwo" -> timesTwo(_))
If I try to compile this with Scala 2.9.1 and sbt I get the following: 如果我尝试用Scala 2.9.1和sbt编译它,我得到以下内容:
[error] ... missing parameter type for expanded function ((x$1) => "timesTwo".$minus$greater(timesTwo(x$1)))
[error] Map("timesTwo" -> timesTwo(_))
[error] ^
[error] one error found
If I add the parameter type: 如果我添加参数类型:
Map("timesTwo" -> timesTwo(_: Int))
I then get the following compiler error: 然后我得到以下编译器错误:
[error] ... type mismatch;
[error] found : Int => (java.lang.String, Int)
[error] required: (?, ?)
[error] Map("timesTwo" -> timesTwo(_: Int))
[error] ^
[error] one error found
I'm stumped. 我很难过。 What am I missing?
我错过了什么?
It thinks you want to do this: 它认为你想这样做:
Map((x: Int) => "timesTwo".->timesTwo(x))
When you want this: 如果你想要这个:
Map("timesTwo" -> { (x: Int) => timesTwo(x) })
So this works: 这样可行:
Map( ("timesTwo", timesTwo(_)) )
Map("timesTwo" -> { timesTwo(_) })
Note this is not an usual error, see 请注意,这不是通常的错误,请参阅
(and probably more) (可能更多)
You are missing telling scalac
that you want to lift the method timesTwo
into a function . 你不想告诉
scalac
你想把方法 timesTwo
提升到一个函数中 。 This can be done with an underscore as follows 这可以通过下划线完成,如下所示
scala> Map("timesTwo" -> timesTwo _)
res0: scala.collection.immutable.Map[java.lang.String,Int => Int] = Map(timesTwo -> <function1>)
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