scalaalgebraic-data-typescategory-theoryrecursion-schemescatamorphism

# Efficient implementation of Catamorphisms in Scala

For a datatype representing the natural numbers:

``````sealed trait Nat
case object Z extends Nat
case class S(pred: Nat) extends Nat
``````

In Scala, here is an elementary way of implementing the corresponding catamorphism:

``````  def cata[A](z: A)(l: Nat)(f: A => A): A = l match {
case Z => z
case S(xs) => f( cata(z)(xs)(f) )
}
``````

However, since the recursive call to cata isn't in tail position, this can easily trigger a stack overflow.

What are alternative implementation options that will avoid this? I'd rather not go down the route of F-algebras unless the interface ultimately presented by the code can look pretty much as simple as the above.

EDIT: Looks like this might be directly relevant: Is it possible to use continuations to make foldRight tail recursive?

Solution

• If you were implementing a catamorphism on lists, that would be what in Haskell we call a `foldr`. We know that `foldr` does not have a tail-recursive definition, but `foldl` does. So if you insist on a tail-recusive program, the right thing to do is reverse the list argument (tail-recursively, in linear time), then use a `foldl` in place of the `foldr`.

Your example uses the simpler data type of naturals (and a truly "efficient" implementation would use machine integers, but we'll agree to leave that aside). What is the reverse of one of your natural numbers? Just the number itself, because we can think of it as a list with no data in each node, so we can't tell the difference when it is reversed! And what's the equivalent of the `foldl`? It's the program (forgive the pseudocode)

``````  def cata(z, a, f) = {
var x = a, y = z;
while (x != Z) {
y = f(y);
x = pred(x)
}
return y
}
``````

Or as a Scala tail-recursion,

``````  def cata[A](z: A)(a: Nat)(f: A => A): A = a match {
case Z => z
case S(b) => cata( f(z) )(b)(f)
}
``````

Will that do?