Maybe monad in D

gistfile1.d

import std.stdio;
import std.traits;
import std.conv;

@safe:

struct Maybe(T) {
    T payload;
    private bool _just;
}

auto just(T)(T t) pure nothrow {
    return Maybe!T(t, true);
}

auto nothing(T)() pure nothrow {
    return Maybe!T(T.init, false);
}

auto bind(alias F, T)(auto ref Maybe!T maybe) {
    return maybe._just ? F(maybe.payload) : nothing!T;
}

class MyClass {
    private int i;
    this(int i) pure nothrow const { this.i = i;}
    override string toString() pure const { return "MyClass(" ~ i.to!string ~ ")";}
}

void main() {
    writeln("value of Just(5): ", just(5).bind!(a => just(a * 2)).bind!(a => just(a * 3)));
    writeln("value of Nothing: ", nothing!int.bind!(a => just(a * 2)).bind!(a => just(a * 3)));

    writeln("just class: ", just(new MyClass(5)).bind!(a => just(new MyClass(a.i * 2))).bind!(a => just(new MyClass(a.i * 3))));
    MyClass oops; //null
    writeln("noth class: ", nothing!MyClass.bind!(a => just(new MyClass(a.i * 2))).bind!(a => just(new MyClass(a.i * 3))));
}