Functional purity is a valuable concept for writing maintainable code, though outside of functional programming languages like Haskell, it’s often treated like a nice-but-expensive luxury. But it turns out that pure functions that aren’t quite so pure can be cheap while still having concrete benefits for code in non-functional languages like C++, Java and Python. For D code, this is supported by the language itself, but there’s nothing D-specific about the overall idea.