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)

Answer 1

When you write a.groupBy(_) the compiler understands it as an anonymous function:

x => a.groupBy(x)

According to Scala Specifications §6.23, an underscore placeholder in an expression is replaced by a anonymous parameter. So:

• _ + 1 is expanded to x => x + 1
• f(_) is expanded to x => f(x)
• _ is not expanded by itself (the placeholder is not part of any expression).

The expression x => a.groupBy(x) will confuse the compiler because it cannot infer the type of x. If a is some collection of type E elements, then the compiler expects x to be a function of type (E) => K, but type K cannot be inferred...