Compile-time Ducks

[Andrej Mitrovic]

I couldn't believe my eyes when I tried out this code, but this thing
actually works:

import std.string;
import std.stdio;

// remove clashes
string safeName(T)() { return T.stringof.toLower() ~ "Res"; }

struct Result(T)
{
    bool ok = true;
    auto opCast(T = bool)() { return ok; }
    mixin("T " ~ safeName!T() ~ ";");
    mixin("alias " ~ safeName!T() ~ " this;");
}

Result!Node getNode()
{
    typeof(return) node;

    node.values["foo"] = "bar";
    //node.ok = false;  // try commenting it out!

    return node;
}

struct Node
{
    string[string] values;
}

void main()
{
    Node node;

    if (auto result = getNode())
    {
        node = result;
        writeln("assigned");
    }
    else
    {
        writeln("no assignment!");
    }

    writeln(node);
}

No need for any tuple unpacking at all! 'auto return' actually calls
into opCast(bool), but it returns a node. How crazy is that?!