Clean Code - 3. Functions


Chapter 3. Functions - Main Takeaways

Some rules for functions:

Anything that forces you to check the function signature is equivalent to a double-take. It's a cognitive break and should be avoided.



The functions should be small.

Functions should not be 100 lines long. Functions should hardly ever be 20 lines long.

Blocks and Indenting

[Small functions] implies that the blocks within if, else, while statements 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.

Do One Thing

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

the reason we write functions is to decompose a larger concept (in other words, the name of the function) into a set of steps at the next level of abstraction.

[one] way to know that a function is doing more than "one thing" is if you can extract another function from it with a name that is not merely a restatement of its implementation.

One Level of Abstraction per Function

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.

Reading Code from Top to Bottom: 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.

This part from Chapter 5 (The Newspaper Metaphor) is also related:

Think of a well-written newspaper article. You read it vertically. At the top you expect a headline that will tell you what the story is about and allows you to decide whether it is something you want to read. The first paragraph gives you a synopsis of the whole story, hiding all the details while giving you the broad-brush concepts. As you continue downward, the details increase until you have all the dates, names, quotes, claims and other minutia.

(...) The topmost parts of the source file should provide the high-level concepts and algorithms. Detail should increase as we move downward, until at the end we find the lowest level functions and details in the source file.

Use Descriptive Names

You know you are working on clean code when each routine turns out to be pretty much what you expected.

Half of the battle to achieving that principle is choosing good names for small functions that do one thing.

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.

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.

(...) For Example, assertEquals might be better written as assertExpectedEqualsActual(expected, actual). This strongly mitigates the problem of having to remember the ordering of the arguments.

Function Arguments

The ideal number of arguments for a function is zero. Next comes one, followed closely by two. Three arguments should be avoided where possible. More than three requires very special justifications - and then shouldn't be used anyway.

Besides the "easy to read and understand" reason, another one that I liked while reading the book is that "the more arguments a function has, the harder it is to write tests for it".

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!

The solution is to create different functions for each behavior.

Output Arguments

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 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.

(...) Using an output argument instead of a return value for a transformation is confusing. If a function is going to transform its input argument, the transformation should appear as the return value.

Have No Side Effects

Side effects are lies. Your function promises to do one thing, but it also does other hidden things.

In the book (p. 44) it shows an example of a checkPassword() functions that also does Session.initialize(), which is considered a side-effect. A function that promises to check the password should not initialize a session.

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.

Prefer Exceptions to Returning Error Codes

Returning error codes from command functions is a subtle violation of command query separation.

It encourages situations like this:

if (deletePage(page) == E_OK)

This can cause deeply nested structures polluting the logic of the happy path:

if (deletePage(page == E_OK) {
  if (registry.deleteReference( == E_OK) {
    if (configKeys.deleteKey( == E_OK) {
      logger.log("page deleted");
    } else {
      logger.log("configKey not deleted");
  } else {
    logger.log("deleteReference from registry failed");
} else {
  logger.log("delete failed");
  return E_ERROR;

By using exceptions, this 👆 madness could be replaced with:

try {
} catch (Exception e) {

And this 👆 could be improved even more with this:

public void delete(Page page) {
  try {
  } catch (Exception e) {

private void deletePageAndAllReferences(Page page) throws Exception {

private void logError(Exception e) {