Usage of _ in scala lambda functions

Question

Can anyone please explain me why I can do:

a.mapValues(_.size)

instead of

a.mapValues(x => x.size)

but I can't do

a.groupBy(_)

instead of a

a.groupBy(x => x)

question from:https://stackoverflow.com/questions/7673545/usage-of-in-scala-lambda-functions

Answer 1

It isn't easy to see it here:

a.groupBy(_)

But it's easier to see it in something like this:

a.mkString("<", _, ">")

I'm partially applying the method/function. I'm applying it to some parameters (the first and last), and leaving the second parameter unapplied, so I'm getting a new function like this:

x => a.mkString("<", x, ">")

The first example is just a special case where the sole parameter is partially applied. When you use underscore on an expression, however, it stands for positional parameters in an anonymous function.

a.mapValues(_.size)
a.mapValues(x => x.size)

It is easy to get confused, because they both result in an anonymous function. In fact, there's a third underscore that is used to convert a method into a method value (which is also an anonymous function), such as:

a.groupBy _