Coding Techniques; Software Development; Computer Science
A guard clause is a computer construct that allows or disallows sections of code to run based on preconditions. This is achieved with conditional expressions, Boolean operators, check functions, and other elements of computer programming language. Guard clauses can be used for a wide variety of purposes, including preventing errors from occurring, responding to errors that do occur, and refactoring code to replace nested conditional statements.
A guard clause is a computer construct that uses conditional expressions to determine whether to allow blocks of code to execute. They allow simple, local processing of exceptions and exit from subroutines. Some common forms of guard clauses are combined Boolean expressions, Boolean operators, and check functions. All essentially work by determining whether preconditions are true or false. A clause “guards” against invalid preconditions at the beginning of a block of code. It allows subsequent code to run if preconditions are met.
Guard clauses serve many purposes. For example, they are often used to check that the data passed to procedures such as functions and methods is of the correct data type, or to validate the data in some other way. This prevents the execution of code for no reason or the execution of code that will generate errors or exceptions. Since guard clauses are often used to report on errors or throw exceptions, they can also serve as a type of error handling code. Many programmers also favor using guard clauses in place of nested conditional statements. This can be done when writing code or when refactoring, or redesigning, existing code. It often makes code easier to read, understand, and maintain.
The same functionality can often be provided using a series of nested conditional statements or using a guard clause. However, multiple nested conditional statements grow increasingly difficult to read as more are added. In most programming languages and development environments, each if-statement is usually indented from the surrounding statement in the following way:
In complex code, this leads to developers having to scroll back and forth while reading, which can be inefficient. This also makes it difficult to immediately see where any needed modifications to the code must be inserted. Using guard statements to replace nested if-statements can make the code flatter, shorter, and simpler.
However, this approach is not universally accepted. Many traditional approaches to programming operate under the principle that a procedure should do whatever it was designed to accomplish and then provide a single point of return to the calling code. For this reason, nested conditional statements are favored by those who follow a strict structured programming paradigm. Advocates of pure structured programming do not believe that the readability benefits of guard clauses justify abandoning the single point of return approach.
Supporters of the guard clause approach counter that use of the single point of return has diminished due to the increased use of shorter procedures. They also challenge the idea that the single point of return approach is superior to the guard clause approach on a fundamental level. To many programmers, the guard clause approach makes the functionality of a procedure easier to understand.
A common source of errors in a computer program occurs when procedures are passed null values when called. Guard clauses can be used to prevent these types of errors from occurring. For example, take a software developer for a movie theater chain creating a function to determine the total daily attendance based on a given number of days. This takes the following form:
where x is the total attendance and y is the given number of days.
A guard clause could be used to check that neither of the input values is null before processing the calculation:
The IsNull function returns true if the value of the parameter passed to the function is null. The guard clause uses the IsNull function to check if either x or y is null. If so, it immediately exits the function, preventing an error from occurring. If neither is null, the average is calculated and returned.
Guard clauses are a useful part of a developer's toolkit. As they can help programs avoid and respond to errors, they are used for a wide variety of purposes. They can validate data to prevent errors from occurring when updating database tables or when storing values in data structures such as trees structures and arrays. They can be used to ensure that a long section of code in a large program does not waste system resources by executing unnecessarily. They can improve the development process by refactoring code to make it more efficient. With all of these benefits to offer, guard clauses are sure to remain in widespread use in many development environments.
—Maura Valentino, MSLIS
Friedman, Daniel P., and Mitchell Wand. Essentials of Programming Languages. 3rd ed., MIT P, 2008.
MacLennan, Bruce J. Principles of Programming Languages: Design, Evaluation, and Implementation. 3rd ed., Oxford UP, 1999.
Schneider, David I. Introduction to Programming Using Visual Basic. 10th ed., Pearson, 2016.
Scott, Michael L. Programming Language Pragmatics. 4th ed., Morgan Kaufmann Publishers, 2016.
Van Roy, Peter, and Seif Haridi. Concepts, Techniques, and Models of Computer Programming. Massachusetts Institute of Technology, 2004.