Extending a Function1 in Scala

Question

In few examples, I have seen that an object or a class extends Function1 .

Eg object Cash extends (CashProduct => String) in Hidden features of Scala

(I suppose A => B means Function1 )

What it the benefit of extending a Function1 ?

Answer 1

The full example of what you provided:

object Cash extends (CashProduct => String) {
  def apply(p: CashProduct) = p.currency.name + "="

  def unapply(s: String)(implicit ps: ProductService): Option[CashProduct] = {
    if (s.endsWith("=") 
      Some(ps.findCash(s.substring(0,3))) 
    else None
  }
}

Shows that OP wanted to gain the syntactical benefit of the apply method, which allows your to create an instance calling Cash(...) .

But why would you really want to extend a function? Lets look at a better case perhaps, List[T] .

If we look up the long inheritance hierarchy, we'll see that:

trait Seq[+A] extends PartialFunction[Int, A]  

Hmm, why does Seq extend PartialFunction[Int, A] (which in turns inherits Function1[A, B] ? Because if we think about it, if I pass a List[A] an Int , representing the index of the element I'm seeking , it will (not efficiently) return me the element at that given index (if present).

Answer 2

The benefit of extending Function1 as compared to just defining apply is just that you can pass this object where a Function1 is expected. Eg

val products: List[CashProduct] = ...
products.map(Cash)

Without the extends it would have to be written as

val products: List[CashProduct] = ...
products.map(Cash(_)) 
// or products.map(Cash.apply)