Clear Code in Functions - 10 most important things you should know about functions.

1.       Always make it Small

The first rule of functions is that they should be small. The second rule of functions is that they should be smaller than that. This is not an assertion that I can justify. In the eighties we used to say that a function should be no bigger than a screen-full. Of course we said that at a time when VT100 screens were 24 lines by 80 columns, and our editors used 4 lines for administrative purposes. Nowadays with a cranked-down font and a nice big monitor, you can fit 150 characters on a line and a 100 lines or more on a screen. Lines should not be 150 characters long. Functions should not be 100 lines long. Functions should hardly ever be 20 lines long.

• Blocks and Indenting

This implies that the blocks within if statements, else statements, while statements, and so on should be one line long. Probably that line should be a function call. Not only does this keep the enclosing function small, but it also adds documentary value because the function called within the block can have a nicely descriptive name. This also implies that functions should not be large enough to hold nested structures. Therefore, the indent level of a function should not be greater than one or two. This, of course, makes the functions easier to read and understand.

2.       Do one thing

Functions should do one thing. They should do it well. They should do it only.

• ·         Sections within Functions

 This is an obvious symptom of doing more than one thing. Functions that do one thing cannot be reasonably divided into sections.

3.       One Level of Abstraction per Function

In order to make sure our functions are doing “one thing,” we need to make sure that the statements within our function are all at the same level of abstraction. Mixing levels of abstraction within a function is always confusing. Readers may not be able to tell whether a particular expression is an essential concept or a detail. Worse, like broken windows, once details are mixed with essential concepts, more and more details tend to accrete within the function.

• ·         The Stepdown Rule

We want the code to read like a top-down narrative. We want every function to be followed by those at the next level of abstraction so that we can read the program, descending one level of abstraction at a time as we read down the list of functions. I call this The Stepdown Rule.

4.       Switch Statements

It’s hard to make a small switch statement. Even a switch statement with only two cases is larger than I’d like a single block or function to be. It’s also hard to make a switch statement that does one thing. By their nature, switch statements always do N things. Unfortunately we can’t always avoid switch statements, but we can make sure that each switch statement is buried in a low-level class and is never repeated. We do this, of course, with polymorphism.

5.       Use Descriptive Names

Don’t be afraid to make a name long. A long descriptive name is better than a short enigmatic name. A long descriptive name is better than a long descriptive comment. Use a naming convention that allows multiple words to be easily read in the function names, and then make use of those multiple words to give the function a name that says what it does.

Don’t be afraid to spend time choosing a name. Indeed, you should try several different names and read the code with each in place. Modern IDEs like Eclipse or IntelliJ make it trivial to change names. Use one of those IDEs and experiment with different names until you find one that is as descriptive as you can make it.

Choosing descriptive names will clarify the design of the module in your mind and help you to improve it. It is not at all uncommon that hunting for a good name results in a favorable restructuring of the code.

6.       Function Arguments

The ideal number of arguments for a function is zero (niladic). Next comes one (monadic), followed closely by two (dyadic). Three arguments (triadic) should be avoided where possible. More than three (polyadic) requires very special justification—and then shouldn’t be used anyway.

Arguments are hard. They take a lot of conceptual power.

Arguments are even harder from a testing point of view. Imagine the difficulty of writing all the test cases to ensure that all the various combinations of arguments work properly. If there are no arguments, this is trivial. If there’s one argument, it’s not too hard. With two arguments the problem gets a bit more challenging. With more than two arguments, testing every combination of appropriate values can be daunting.

Output arguments are harder to understand than input arguments. When we read a function, we are used to the idea of information going in to the function through arguments and out through the return value. We don’t usually expect information to be going out through the arguments. So output arguments often cause us to do a double-take.

Always try to use Common Monadic Form functions, try to reduce Dyadic form functions, don’t use triads form and flag arguments.

• ·         Common Monadic Forms

Always try to have functions with one argument. That is the best practice and it will help to reduce the complexity of the function.

• ·         Flag Arguments

Flag arguments are ugly. Passing a Boolean into a function is a truly terrible practice. It immediately complicates the signature of the method, loudly proclaiming that this function does more than one thing. It does one thing if the flag is true and another if the flag is false!

• ·         Dyadic Functions

A function with two arguments is harder to understand than a monadic function.

• ·         Triads Functions

Functions that take three arguments are significantly harder to understand than dyads. The issues of ordering, pausing, and ignoring are more than doubled. I suggest you think very carefully before creating a triad.

• ·         Argument Objects

When a function seems to need more than two or three arguments, it is likely that some of those arguments ought to be wrapped into a class of their own. Reducing the number of arguments by creating objects out of them may seem like cheating, but it’s not.

• ·         Argument Lists

Sometimes we want to pass a variable number of arguments into a function. So all the same rules apply. Functions that take variable arguments can be monads, dyads, or even triads. But it would be a mistake to give them more arguments than that.

• ·         Verbs and Keywords

Choosing good names for a function can go a long way toward explaining the intent of the function and the order and intent of the arguments. In the case of a monad, the function and argument should form a very nice verb/noun pair. For example, write(name) is very evocative. Whatever this “name” thing is, it is being “written.” An even better name might be writeField(name), which tells us that the “name” thing is a “field.”

7.       Avoid Side Effects

Side effects are lies. Your function promises to do one thing, but it also does other hidden things. Sometimes it will make unexpected changes to the variables of its own class. Sometimes it will make them to the parameters passed into the function or to system globals. In either case they are devious and damaging mistruths that often result in strange temporal couplings and order dependencies.

8.       Command Query Separation

Functions should either do something or answer something, but not both. Either your function should change the state of an object, or it should return some information about that object. Doing both often leads to confusion.

9.       Prefer Exceptions to Returning Error Codes

Try to convert exceptions into error codes.

• ·         Extract Try/Catch Blocks

Try/catch blocks are ugly in their own right. They confuse the structure of the code and mix error processing with normal processing. So it is better to extract the bodies of the try and catch blocks out into functions of their own.

• ·         Error Handling Is separate from functions

Functions should do one thing. Error handing is one thing. Thus, a function that handles errors should do nothing else. This implies that if the keyword try exists in a function, it should be the very first word in the function and that there should be nothing after the catch/finally blocks.

10.   Don’t Repeat Yourself (DRY)

Duplication may be the root of all evil in software. Many principles and practices have been created for the purpose of controlling or eliminating it. Consider, for example, that all of Codd’s database normal forms serve to eliminate duplication in data. Consider also how object-oriented programming serves to concentrate code into base classes that would otherwise be redundant. Structured programming, Aspect Oriented Programming, Component Oriented Programming, are all, in part, strategies for eliminating duplication. It would appear that since the invention of the subroutine, innovations in software development have been an ongoing attempt to eliminate duplication from our source code.