Question

Related: How do you assign a function to a value in Scala?

Given:

class Foo{
  def bar = println("bar")
  def bat = println("bat")
}

How do I create a fnRef such that it points to either Foo.bar or Foo.bat?

def deepFunction(foos : List[Foo], fnRef : ()=>Unit) = {
  foos.map(_.fnRef) //May call either bar or bat
}  

Bonus: Is it possible to constrain the fnRef so that it’s only methods of that signature within the Foo class?

Answer 1

You don’t. 🙂 Instead you write your code in terms of first class functions. What makes this nice in Scala is that it will create a function literal from _.method that’s of type T => R, where T is the type of the parameter, and R is the method’s return type.

So, _.bar and _.bat would both make Foo => Unit:

scala> class Foo{
     |   def bar = println("bar")
     |   def bat = println("bat")
     | }
defined class Foo

scala> def deepFunction(foos: List[Foo], fn: Foo => Unit) {
     |   foos.map(fn)
     | }
deepFunction: (foos: List[Foo], fn: Foo => Unit)Unit

scala> deepFunction(List(new Foo, new Foo), _.bar)
bar
bar

scala> deepFunction(List(new Foo, new Foo), _.bat)
bat
bat

What’s really nice about this approach is that you can use any function you want, not just member functions.

scala> def bam(f: Foo) { println("bam") }
bam: (f: Foo)Unit

scala> deepFunction(List(new Foo, new Foo), bam)
bam
bam