The Robert C. Martin Clean Code Collection (Collection) (Robert C. Martin Series)

by Robert C. Martin

There are two very common reasons to pass a single argument into a function. You may be asking a question about that argument, as in boolean fileExists(“MyFile”). Or you may be operating on that argument, transforming it into something else and returning it. For example, InputStream fileOpen(“MyFile”) transforms a file name String into an InputStream return value. These two uses are what readers expect when they see a function. You should choose names that make the distinction clear, and always use the two forms in a consistent context. (See Command Query Separation below.) A somewhat less common, but still very useful form for a single argument function, is an event. In this form there is an input argument but no output argument. The overall program is meant to interpret the function call as an event and use the argument to alter the state of the system, for example, void passwordAttemptFailedNtimes(int attempts). Use this form with care. It should be very clear to the reader that this is an event. Choose names and contexts carefully. Try to avoid any monadic functions that don’t follow these forms, for example, void includeSetupPageInto(StringBuffer pageText). Using an output argument instead of a return value for a transformation is confusing.