When you write a.groupBy(_)
the compiler understands it as an anonymous function:
x => a.groupBy(x)
According to Scala Specifications ยง6.23, an underscore placeholder in an expression is replaced by a anonymous parameter. So:
_ + 1
is expanded tox => x + 1
f(_)
is expanded tox => 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…