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Standard C-style function definitions:

int square(int n) {
    return n * n;
}

auto add(int a, int b) {       // return type inferred
    return a + b;
}

Default arguments like Python:

string greet(string name, string greeting = "Hello") {
    return greeting ~ ", " ~ name ~ "!";
}

~ is D's string concatenation (+ is reserved for arithmetic; trying it gives a compile error — refreshing).

Lambdas with =>:

auto double_it = (int x) => x * 2;
auto add_them = (int a, int b) => a + b;

Functions are first-class — pass them around:

import std.algorithm : map;
auto squared = [1, 2, 3].map!(x => x * x);

Note !() for compile-time template arguments (the lambda is a type parameter to map).