Unix : Compactness and Orthogonality

Code is not the only sort of thing with an optimal chunk size. Languages and APIs (such as sets of library or system calls) run up against the same sorts of human cognitive constraints that produce Hatton's U-curve.

Accordingly, Unix programmers have learned to think very hard about two other properties when designing APIs, command sets, protocols, and other ways to make computers do tricks: compactness and orthogonality.

4.2.1 Compactness

Compactness is the property that a design can fit inside a human being's head. A good practical test for compactness is this: Does an experienced user normally need a manual? If not, then the design (or at least the subset of it that covers normal use) is compact.

Compact software tools have all the virtues of physical tools that fit well in the hand. They feel pleasant to use, they don't obtrude themselves between your mind and your work, they make you more productive—and they are much less likely than unwieldy tools to turn in your hand and injure you.

Compact is not equivalent to 'weak'. A design can have a great deal of power and flexibility and still be compact if it is built on abstractions that are easy to think about and fit together well. Nor is compact equivalent to 'easily learned'; some compact designs are quite difficult to understand until you have mastered an underlying conceptual model that is tricky, at which point your view of the world changes and compact becomes simple. For a lot of people, the Lisp language is a classic example of this.

Very few software designs are compact in an absolute sense, but many are compact in a slightly looser sense of the term. They have a compact working set, a subset of capabilities that suffices for 80% or more of what expert users normally do with them. Practically speaking, such designs normally need a reference card or cheat sheet but not a manual. We'll call such designs semi-compact, as opposed to strictly compact.

The concept is perhaps best illustrated by examples. The Unix system call API is semi-compact, but the standard C library is not compact in any sense. While Unix programmers easily keep a subset of the system calls sufficient for most applications programming (file system operations, signals, and process control) in their heads, the C library on modern Unixes includes many hundreds of entry points, e.g., mathematical functions, that won't all fit inside a single programmer's cranium.