If simpler is better, is fewer better? Is one better than two? When designing programming constructs, there is a tendency towards extremist programming. There is power and beauty in a grand unified approach that has just one class of elements. Alluring as singularity may be, these one-element approaches tend to fail in ways that more complex designs do not. Hybrid approaches tend to win in the end. Why? And how should this reality inform our designs? Can we predict and enable hybrid solutions?

During the 17th century, opposing theories of light were being considered: particles vs waves. A great deal of science was conducted on behalf of either camp, but no clear victor emerged. It wasn’t until the 20th century that the dual nature of light was widely accepted. I’m far from the first to notice the parallels to logic and computing. Similarities are plentiful here. We even use the same word, “duality,” to informally describe the frequent occurrence of this two-element, yin-and-yang pattern in computing.

Where do dualities come from? Why are there so many of them? Why aren’t there more trinities? Quaternities?

Binary classifications arrises when we define a domain of discourse, then segregate that domain by a predicate. That is, some elements of the domain possess some property and the other elements do not. When this property is defined in terms of some bounded context, the binary classifications becomes a metaphysical, particle/wave-style duality. An element of such a domain may or may not satisfy such a predicate depending on your frame of reference!

Consider the famous Rubin’s Vase illusion:

In each image, either a vase or a pair of face profiles can be scene. Consider which subset of pixels satisfies the foreground predicate, and which satisfies its negation or dual, the background predicate. While the “meta-image” has a white background, the left and rightmost sub-images have a black background. Inverting the colors can switch which object appears in the foreground. However, adding a border can achieve the same illusion! Despite the center image being a pixel-perfect, color-matching, sub-rectangle of the rightmost image, the vase and faces appear to have swapped between the foreground and background. The border provides the bounded context that governs the foreground/background duality.

Returning to the duality of light: What property separates particles from waves? Unlike waves, particles possess a definite position in space. Waves have indefinite position, existing only with respect to some medium. A physical medium serves as a bounded context. There’s no splash without a pool of water; just as there is no background without a border.

Faced with duality, there is natural tendency to seek a preference between the two sides. For many binary classifications, there is no clear winner. Heads or tails? White or black? There’s no right answer. However, the equation is not as balanced when it comes to context-bounded dualities. Our preference is necessarily situational. On a white canvas, we paint a black vase in the foreground, but just the opposite on a black canvas. We must consider the context.

Frequently, the “particle” side of a duality is easier to recognize and reason about. The particle theory of light was proposed in the 11th century, nearly 600 years prior to the wave theory. What makes waves more difficult to recognize or reason about? Typically, context is implicit. The choice of context is often already made for you. When I set out to write this blog post, the background of my blog was already white. On a small beach, it’s easy to see the particles of sand, but it’s just as easy to see the waves of the ocean. Our relative scale influences our perspective, and that grants dominance to one of the two perspectives on the duality. Luckily, perspective can be shifted. Placing a handful of sand in to a carefully selected context, such as the surface of a speaker, will instantly reveal a wave. Thinking in terms of bounded contexts can put reasoning about waves on a level playing field with particles.

One last bit of metaphysics is worthy of mention here: spacetime. It is especially difficult to reason about time as a contextual bound on space. Notably, many waves only appear when observing a medium over time, or by projecting the time dimension on to a spatial dimension. Despite being unbounded at a universal scale, time is scarce for us mortals. We can’t capture time, holding on to it for study. At best, we can capture information about the past, or make predictions about the future, effectively transforming time in to the space such information takes up.

A good design takes things apart with an eye towards putting them back together again. However, focusing on the pieces to the exclusion of their contexts will yield a poor design. Even if the parts snap together nicely, a gestalt may not arise, or the composition may clash with its surroundings. The particles and waves or spacetime dualities can be found hiding under every rock in computer science: Data vs interfaces, logic with negation, values and state, imperative vs declarative, structural vs relational, and many more. Learning to recognize these dualities will enable you to design better systems and to produce better compositions within existing systems.

In future posts, I intend to discuss a wide range of programming topics with an eye for context and gestalt. We’ll use the lense of duality to deepen our understanding of the design tradeoffs in systems ranging from calculators to databases and beyond. I’ll then abuse the perspective to argue against the fetishization of modern programming ideals, such as functional purity, or the overuse of constructs such as promises and observables.