How to restrict a anonymous function's boundary?

Question

I saw a below code:

Map(1 -> "one", 2 -> "two") map _._1

this return a Iterable[Int], but if I want to do nothing with map, how to do it?

I want to do something like below, but the below code can't compile, I know because it a object instance not a function, but how to create a function to do x => x and use placeholder:

Map(1 -> "one") map (_)  // map (Int, String) to (Int, String) by nothing change
// I test some other way, but all can't compile

how to do this?

UPDATED

Sorry for confuse passionate person. I want to know why map (_) != map (x => x), compiler transform this code to (x$1) => Map(1.$minus$greater("one")).map(x$1) why not Map('a'.$minus$greater(1)).map((x$1) => x$1), and does there has a way can use _ make this code?

I used below code to help compiler inferred the _ type:

Map(1 -> "one") map (_:((Int, String))=>(Int, String))
// but it return (((Int, String)) => (Int, String)) => scala.collection.immutable.Map[Int,String] = <function1>

It seem the parser was not sure where to put the beginning of the anonymous function. So my new question is "How to help the parser to restrict a anonymous function's boundary?"

Answer 1

Hi!

  Map(1 -> "one") map((x)=>x)

Answer 2

Not sure I understand the question, but identity is maybe what you're looking for:

scala> Map(1 -> "one") map (identity)
res66: scala.collection.mutable.Map[Int,java.lang.String] = Map((1,one))

or, do some tricks:

scala> def __[A](x: A): A = x
__: [A](x: A)A

scala> Map(1 -> "one") map (__)
res1: scala.collection.immutable.Map[Int,java.lang.String] = Map((1,one))

Answer 3

I can't see any value in what you're trying to do here, the correct way to map a collection to itself is not to call map!

Wrong:

Map(1 -> "one") map (_)

Right:

Map(1 -> "one")

It isn't even useful as a shallow copy operation, the default Scala Map is immutable and there's no point in copying it.

Answer 4

I find a answer by Daniel, http://stackoverflow.com/questions/3505125/anonymous-functions-and-maps-in-scala/3507163#3507163 , this answer let me clear how the parser process placeholder in this case. thanks for all.