A Philosophy of Software Design

by John Ousterhout

When students had to implement this class, many of them chose a line-oriented interface, with methods to read, insert, and delete whole lines of text. This resulted in a simple implementation for the class, but it created complexity for higher level software. At the level of the user interface, operations rarely involve whole lines. For example, keystrokes cause individual characters to be inserted within an existing line; copying or deleting the selection can modify parts of several different lines. With the line-oriented text interface, higher-level software had to split and join lines in order to implement the user interface. A character-oriented interface such as the one described in Section 6.3 pulls complexity downward. The user interface software can now insert and delete arbitrary ranges of text without splitting and merging lines, so it becomes simpler. The implementation of the text class probably becomes more complex: if it represents the text internally as a collection of lines, it will have to split and merge lines to implement the character-oriented operations. This approach is better because it encapsulates the complexity of splitting and merging within the text class, which reduces the overall complexity of the system.

Reducing complexity