More efficient Solution with tailrecursion? - scala

Question

I have the following ADT for Formulas. (shortened to the important ones)
sealed trait Formula
case class Variable(id: String) extends Formula
case class Negation(f: Formula) extends Formula
abstract class BinaryConnective(val f0: Formula, val f1: Formula) extends Formula
Note that the following methods are defined in an implicit class for formulas.
Let's say i want to get all variables from a formula.
My first approach was:
Solution 1
def variables: Set[Variable] = formula match {
case v: Variable => HashSet(v)
case Negation(f) => f.variables
case BinaryConnective(f0, f1) => f0.variables ++ f1.variables
case _ => HashSet.empty
}
This approach is very simple to understand, but not tailrecursive. So I wanted to try something different. I implemented a foreach on my tree-like formulas.
Solution 2
def foreach(func: Formula => Unit) = {
#tailrec
def foreach(list: List[Formula]): Unit = list match {
case Nil =>
case _ => foreach(list.foldLeft(List.empty[Formula])((next, formula) => {
func(formula)
formula match {
case Negation(f) => f :: next
case BinaryConnective(f0, f1) => f0 :: f1 :: next
case _ => next
}
}))
}
foreach(List(formula))
}
Now I can implement many methods with the help of the foreach.
def variables2 = {
val builder = Set.newBuilder[Variable]
formula.foreach {
case v: Variable => builder += v
case _ =>
}
builder.result
}
Now finally to the question. Which solution is preferable in terms of efficieny? At least I find my simple first solution more aesthetic.

Answer 1

I would expect Solution 2 to be more efficient, because you aren't create many different HashSet instances and combining them together. It is also more general.
You can simplify your Solution 2, removing the foldLeft:
def foreach(func: Formula => Unit) = {
#tailrec
def foreach(list: List[Formula]): Unit = list match {
case Nil =>
case formula :: next => {
func(formula)
foreach {
formula match {
case Negation(f) => f :: next
case BinaryConnective(f0, f1) => f0 :: f1 :: next
case _ => next
}
}
}
}
foreach(List(formula))
}