Scala: type mismatch in function composition, found (Int, Int) => Seq[Int] require ? => Seq[Int]

Question

I'm trying to compose 2 functions in a script and getting a type mismatch I can not solve. Here is the sample code:

def generate(start: Int, end: Int): Seq[Int] = (start until end).toSeq
def restrain(seq: Seq[Int]) = seq.dropWhile(_ < 20).takeWhile(_ < 60)

val com: (Int, Int) => Seq[Int] = (restrain _ compose generate)

By loading this in the REPL with:

:load test.sc

I get the following error:

       val com: (Int, Int) => Seq[Int] = (restrain _ compose generate)
                                                             ^
test.sc:1: error: type mismatch;
        found   : (Int, Int) => Seq[Int]
        required: ? => Seq[Int]

What am I doing wrong?

Answer 1

The types Function2[Int, Int, Seq[Int]] and Function1[(Int, Int), Seq[Int]] are not the same. The (generate _) produces the former, whereas for this composition, you need the latter. Try:

restrain _ compose (generate _).tupled