Functional Programming in C#, Second Edition

3 Why function purity matters

This chapter covers

• What makes a function pure or impure
• Why purity matters in concurrent scenarios
• How purity relates to testability
• Reducing the impure footprint of your code

The initial name for this chapter was “The irresistible appeal of purity.” But if it was so irresistible, we’d have more functional programmers, right? Functional programmers, you see, are suckers for pure functions: functions with no side effects. In this chapter, you’ll see what that means exactly and why pure functions have some very desirable properties.

Unfortunately, this fascination with pure functions is partly why FP as a discipline has become disconnected from the industry. As you’ll soon realize, there’s little purity in most ...