Boolean Parameters (Flag Arguments Smell)

Also Known As

TL;DR

Explanation

Martin Fowler calls boolean parameters (flag arguments) a smell because they advertise that the function does two different things. A call like sendEmail($user, true) tells the reader nothing; sendEmail($user, $sendCopy=true) is marginally better but still bifurcates the function. The remedy is to split into two clearly-named functions: sendEmail($user) and sendEmailWithCopy($user), or use an enum/value object to express the mode explicitly. Named arguments (PHP 8.0) mitigate the readability problem but don't fix the underlying design issue.

Common Misconception

Why It Matters

Common Mistakes

• Multiple boolean parameters: render($template, true, false, true) — impossible to read at call site.
• Using a boolean to switch between two fundamentally different behaviours — create two methods instead.
• Not using named arguments (PHP 8) to document boolean flags at call sites.
• Booleans that change over time to tri-state (null/false/true) — use an enum instead.

Code Examples

function createUser(string $name, bool $isAdmin): User { ... }

function createUser(string $name): User { ... }
function createAdminUser(string $name): User { ... }

References

• ↗ https://martinfowler.com/bliki/FlagArgument.html

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