Behavioral Attributes: pure

pure functions

As you have seen in the functions chapter, functions can produce return values and side effects. When possible, return values should be preferred over side effects because functions that do not have side effects are easier to make sense of, which in turn helps with program correctness and maintainability.

A similar concept is the purity of a function. Purity is defined differently in D than most other programming languages. In D, a function that does not access a mutable global or static state is pure.

Since input and output streams are considered as mutable global state, pure functions cannot perform input or output operations either.

In other words, a function is pure if it produces ...