There are several ways to categorize programming languages. One is to distinguish between applicative and concatenative evaluation. Most languages are applicative – functions are applied to data. In contrast, a concatenative language moves a single store of data or knowledge along a ‘pipeline’ with a sequence of functions each operating on the store in turn. The output of one function is the input of the next function, in a ‘threaded’ fashion. This sequence of functions is the program. Forth is an example of a concatenative language, with the stack serving as the data store that is passed along the thread of functions (‘words’). “Forth is a simple, natural computer language” – Charles Moore, inventor of Forth.

One of the great advantages of concatenative languages is the opportunity for extreme simplicity. Since each function really only needs to know about its own input, machinery, and output, there is a greatly reduced need for overall architecture. The big picture, or state, of the entire program is neither guiding nor informing each step. As long as a function can read, compute, and write, it can be an entity unto itself, with little compliance or doctrine to worry about. In fact, in Forth, beyond the stack and the threaded execution model, there’s precious little doctrine anyway! Program structure is a simple sequence, with new words appended to the list (concatenated). The task of the programmer is just to get each word right, then move on to the next.

In nature, the concatenative approach is the only game in town. Small genetic changes occur due to several causes, random mutation being one of them. Each change is then put through the survivability sieve of natural selection, with large changes accumulating over large time scales (evolution). (Evolution is active across the entire spectrum of abstraction levels. Hierarchies emerge naturally, not through doctrine or design.) Concatenation is the way by which these small changes are accumulated. Much of the epic and winding road of human evolution is recorded in our DNA, which is billions of letters long.

This process can be seen happening right now in molecular biology. Consider the ribosome. This is the little machine inside a cell that reads a piece of RNA (a chain of nucleotides) and translates it into a protein (a chain of amino acids). There is no Master Control Program assigning names, delegating work, and applying functions. There is only a concatenative program, built up over the ages by evolution. So, basic life uses a fairly powerful and successful form of computation: DNA for storage, RNA for code, ribosome for computing, protein for application.

(and natural selection for testing) 🙂

We flatter ourselves when we talk of our ‘invention’ of levers, gears, factories, and computers. Nature had all that stuff and much more long before we ever came down from the trees. Math, engineering, and science are great not because of their products, but rather because they enable 3-pound hominid brains to explore nature and ponder the possibilities.