Combining partial functions - scala

Question

I came from Java and would like to combine two partial functions like this:
def sum(a: Int, b: Int, c: Int) : Int = a + b + c
I want write something like this:
val l = List(1, 2, 3)
l.foreach(println _ sum (1, _ : Int, 3) ) // It's supposed to apply
// the partial sum(1, _: Int, 3)
// and println computed value.
But it refuses to compile. Is there a way to fix it concisely?

Answer 1

There are few problems with what you were trying to do. First of all, println is an overloaded method, so when you are trying to eta-expand it to a function like this println _, Scala is taking the shortest form, which is the one with 0 arguments:
scala> println _
res0: () => Unit = $$Lambda$1048/48841776#58ec44ec
So you need to be more specific:
scala> println(_: Int)
res1: Int => Unit = $$Lambda$1064/1621799037#1e8bddc4
Now that we have correct function there is one more thing to do: to add compose and proper parentheses (additional parentheses wrap anonymous functions):
scala> l.foreach((println(_: Int)) compose (sum (1, _: Int, 3)))
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or in the other direction:
scala> l.foreach((sum (1, _: Int, 3)) andThen (println(_: Int)))
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Answer 2

Assuming I reading what you want correctly (and even the code it's a huge assumption), here a snippet that might achieve it:
scala> def sum(a: Int, b: Int, c: Int) : Int = a + b + c
sum: (a: Int, b: Int, c: Int)Int
scala> val sum13 = sum(1, _ : Int, 3)
sum13: Int => Int = <function1>
scala> val printInt = println( _ : Int )
printInt: Int => Unit = <function1>
scala> List(1,2,4) foreach { printInt compose sum13 }
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Notice the compose. Another way would be explicitly compose x => printInt(sum13(x)).