Polylinguistic Code Improvement September 24th, 2012
Patrick Stein

I have started the Coursera course: Functional Programming Principles in Scala. The first assignment was a few simple recursion problems: calculate a given entry in Pascal’s triangle, verify that the parens are balanced in a list of characters, and count the number of ways one can give change for a given amount with given coin denominations. I did those assignments six days ago.

Last night, for grins, I thought I would implement them in Common Lisp. I started out doing them without referencing the Scala code that I’d written. Then, I got lazy and pulled up the Scala code for the last problem and a half.

For all three problems, I came up with better solutions that translated back into the Scala. Some of it was just the benefit of looking at the previous solution again. Some of it was thinking about it with a new language in mind. Some of it was being more comfortable with Lisp. It was such a dramatic improvement in the code that even though I had scored 10/10 six days ago, I still reworked the Scala code and resubmitted it last night.

As it relates to Common Lisp development specifically…. One of the first Lisp books that I read was Paul Graham’s On Lisp. In that book, he does a great deal of turning recursive functions into tail-recursive functions. I had followed his idiom of calling the internal function rec:

(defun factorial (n)
  (labels ((rec (n accum)
             (if (zerop n)
               (rec (1- n) (* n accum)))))
    (rec n 1)))

For the Scala, I started out naming the internal function with a Rec suffix on the function name:

def factorial (n: Int): Int =
  def factorialRec (n: Int, accum: Int): Int =
    if (n == 0) accum
    else factorialRec(n-1, n*accum)

Now, I’m partial to just re-using the function name:

(defun factorial (n)
  (labels ((factorial (n accum)
             (if (zerop n)
               (factorial (1- n) (* n accum)))))
    (factorial n 1)))