You dont need placeholder (_
)
scala> var addto = ({(a :Int, b: Int) => a + b} ).curried
addto: Int => (Int => Int) = <function1>
scala> addto(1)
res0: Int => Int = <function1>
scala> res0(2)
res1: Int = 3
This is because you already have a function object with you here on which you can call curried.
Where as in your previous case
var addto = (adder _ ).curried
You first had to convert the method to a function object (by using placeholder) before doing curried
on it.