Question

I am wondering if it is possible to use currying on multi parameter-group functions :

scala> def sum(a: Int)(b: Int): Int = { a+b }
sum: (a: Int)(b: Int)Int

scala> sum(3)(4)
res2: Int = 7

scala> val partFunc = sum(3) _
partFunc: Int => Int = <function1>

scala> partFunc(4)
res3: Int = 7

scala> val partFunc2 = sum _ _
<console>:1: error: ';' expected but '_' found.
       val partFunc2 = sum _ _
                             ^

scala> val partFunc2 = (sum _) _
<console>:8: error: _ must follow method; cannot follow Int => (Int => Int)
       val partFunc2 = (sum _) _

Answer 1

Writing simply sum _ does not yet have anything to do with the arguments of sum, but simply distinguishes the function object sum from an application of the function.

Hence, you can write:

scala> val partFunc2 = sum _
partFunc2: Int => (Int => Int) = <function1>

As you can see from the type information, this is already the curried version of sum which takes two Int parameters.

Of course, you can then proceed as before with partFunc2(4) being of type Int => Int and so on.