Functional programming

D puts an emphasis on functional programming and provides first-class support for development in a functional style.

In D a function can be declared as pure which implies that given the same input parameters, always the same output is generated. pure functions cannot access or change any mutable global state and are thus just allowed to call other functions which are pure themselves.

int add(int lhs, int rhs) pure {
    // ERROR: impureFunction();
    return lhs + rhs;
}

This variant of add is called strongly pure function because it returns a result dependent only on its input parameters without modifying them. D also allows the definition of weakly pure functions which might have mutable parameters:

void add(ref int result, int lhs, int rhs) pure {
    result = lhs + rhs;
}

These functions are still considered pure and can't access or change any mutable global state. Just passed-in mutable parameters might be altered.

Due to the constraints imposed by pure, pure functions are ideal for multi-threading environments to prevent data races by design. Additionally pure functions can be cached easily and allow a range of compiler optimizations.

The attribute pure is automatically inferred by the compiler for templated functions and auto functions, where applicable (this is also true for @safe, nothrow, and @nogc).

In-depth

rdmd playground.d