Conal Elliott

Introduction

I’m starting to think about exact numeric computation. As a first step in getting into issues, I’ve been playing with addition on number representations, particularly carry look-ahead adders.

This post plays with adding numbers and explores a few variations, beginning with the standard algorithm I learned as a child, namely working from right to left (least to most significant), propagating carries. For fun & curiosity, I also try out a pseudo-parallel version using circular programming, as well as a state-monad formulation. Each of these variations has its own elegance.

While familiar and simple, right-to-left algorithms have a fundamental limitation. Since they begin with the least significant digit, they cannot be applied numbers that have infinitely many decreasingly significant digits. To add exact real numbers, we’ll need a different algorithm.

Given clear formulations of right-to-left addition, and with exact real addition in mind, I was curious about left-to-right addition. The circular formulation adapts straightforwardly. Delightfully, the monadic version adapts even more easily, by replacing the usual state monad with the backward state monad.

To exploit the right-to-left algorithms in exact real addition, I had to tweak the single-digit addition step to be a bit laxer (less strict). With this change, infinite-digit addition works just fine.