How to compose partial functions with multiple parameters in Scala-CodePudding

    val f1 = (x: Int) => x match {
      case x1 => x * 2
    }
    
    val f2 = (x: Int, y: Int) => (x, y) match {
      case (x1, y1) => x1   y1
    }
    
    val f3 = f1.compose(f2)

Expecting f3 to be a partial function from (Int, Int) => Int but I get the following error

Found:    (f2 : (Int, Int) => Int)
Required: Any => Int

CodePudding user response：

As the documentation suggests, compose specifically composes two instances of Function1, that is two instances of a unary function.

If you want to keep your code as is, the way to do this is

f1 andThen f2.curried

If you look at the documentation closely

compose returns a new function f s.t. f(x) == apply(g(x))

andThen returns a new function f s.t. f(x) == g(apply(x))

Note the difference in order of application. Yes, I know the naming is rather is confusing.

Edit: It is confusing which way you want these to apply. The other way around is provided in Jorg's answer.

CodePudding user response：

Both scala.Function1.compose and scala.PartialFunction.compose only take functions as parameters which have a single parameter. But f2 has two parameters, therefore, it is not type-compatible with the parameter type of compose.

You can convert f2 to a single-parameter function which takes its parameters as a scala.Tuple using the scala.Function2.tupled method:

val f3 = f1.compose(f2.tupled)

Note: f3 is now a Function1[Tuple2[Int, Int], Int] aka ((Int, Int)) => Int, i.e. it needs to be applied like this:

f3((2, 3))

To turn it into a Function2[Int, Int, Int] aka (Int, Int) => Int, you can use the scala.Function.untupled method:

val f4 = Function.untupled(f3)

Scastie link

Also note that neither f1 nor f2 are scala.PartialFunctions. f1 is a scala.Function1[Int, Int] aka (Int) => Int and f2 is a scala.Function2[Int, Int, Int] aka (Int, Int) => Int.