Scala underscore explanation

问题

Have a look at these scala snippets: if we have something like this:

List(List(1, 2), List(3, 4), List(5)) map (x => (x.size))

we can shorten it to:

List(List(1, 2), List(3, 4), List(5)) map ((_.size))

but, if we have something like this:

List(List(1, 2), List(3, 4), List(5)) map (x => (x.size, x.size))

why can't we shorten it to:

List(List(1, 2), List(3, 4), List(5)) map ((_.size, _.size))

?

回答1:

An amount of placeholders should be equals amount of function parameters. In your case map has 1 parameter that's why you can't use two placeholders.

回答2:

Because List(List(1, 2), List(3, 4), List(5)) map ((_.size, _.size)) has a different meaning, namely

List(List(1, 2), List(3, 4), List(5)) map ((x => x.size, y => y.size))

(you can see this from the error message). This obviously doesn't compile, because map doesn't accept a tuple of two functions.

来源：https://stackoverflow.com/questions/37589210/scala-underscore-explanation