functor

auto getFive = functor!(() => 5)();
assert(getFive() == 5);

auto getValue = functor!(n => n)(5);
assert(getValue() == 5);

// Functor construction is a bit like currying, though mutation of
// curried arguments (here, state) is explicitly allowed.

auto addValue = functor!((n, i) => n + i)(2);
assert(addValue(5) == 7);

auto accumulator = functor!((ref n, i) => n += i)(0);
accumulator(2); accumulator(5);
assert(accumulator.state[0] == 7);

// Regular D closures are still supported. Not @nogc!

auto n = 2;
auto addValue = functor!(i => i + n)();
assert(addValue(5) == 7);

auto m = 3;
auto addMul = functor!(i => addValue(i * m))();
assert(addMul(5) == 17);