Usage of _ in scala lambda functions -

can please explain me why can do:

a.mapvalues(_.size) 

instead of

a.mapvalues(x => x.size) 

but can't do

a.groupby(_) 

instead of

a.groupby(x => x) 

it isn't easy see here:

a.groupby(_) 

but it's easier see in this:

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

i'm partially applying method/function. i'm applying parameters (the first , last), , leaving second parameter unapplied, i'm getting new function this:

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

the first example special case sole parameter partially applied. when use underscore on expression, however, stands positional parameters in anonymous function.

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

it easy confused, because both result in anonymous function. in fact, there's third underscore used convert method method value (which anonymous function), such as:

a.groupby _