The Cardinal Rule of Functions

Functions should be:

• short
• easy to understand
• well-named
• reusable
• maintainable

All these desirable properties follow from obeying the Cardinal Rule of Functions.

One Function to a Function

That is, the code implementing a function should have one and only one task to do. Example tasks are:

• Calculate: using primitive operators, calculate a value; this includes arithmetic, string operations, list building, and so on
• Read or write: get data from or send data to files or users
• Parse or format: convert text into structured data or construct text representations of structured data
• Combine: apply several other functions to a set of arguments
• Dispatch: decide which of several other functions to apply to a set of arguments
• Iterate: repeatedly apply another function to a list of argument values

Notice that the latter tasks correspond to classic control structures (composition of function calls, conditional branching, and iteration). A symptom that the rule has been broken is when several of these control structures appear in one function. Another symptom is when you have trouble keeping a function to less than a dozen lines of code, and/or its lines to less than 72 characters.
Another good sign that the rule has been broken is difficulty naming the function clearly, or using names like scan-and-compress-and-print.