Return a LuaObject from a D function

[aubade]

At the moment, when making a D function for use in Lua, there doesn't seem to be a way of accessing the caller's LuaState, and there doesn't seem to be a way to construct a LuaObject to return to Lua without one. Is there any way of doing this at all elegantly? (I know I can just return a D type, but in this case I actually need Lua's polymorphism on the D end of things)

[JakobOvrum]

Provide the LuaState in the function's context, making it a delegate. Methods of doing that include using a nested function, anonymous function or aggregate member function.

[aubade]

I understand; That seems a little less-than-ideal for a program with multiple LuaStates, though (I'm using a couple different ones for sandboxing purposes). Is there any way we could get a function conversion that could check for, e.g. the first parameter of a wrapped function being a LuaState or lua_State* and special case that, a la the raw C api?

[JakobOvrum]

I thought about it, but it would interfere with anyone wanting to push/get LuaState to/from Lua as-is. It could cause hard-to-detect bugs.

[aubade]

That's fair enough; Though this might be another situation where a UDA would be appropriate; if, for example, a function's first parameter was a LuaState, and it was marked with @LuaCallerState, that might be clear enough evidence of intent, along the lines of:

void a (LuaState s) {} //WORKS, requires state passed from Lua code;
@LuaCallerState void b (LuaState s, int a) //WORKS, requires int passed from Lua code, gets internal state
@LuaCallerState void c(int a) //ERROR; no LuaState as first parameter
@LuaCallerState void d (int a, LuaState s) //ERROR, LuaState must be first parameter
@LuaCallerState void e (LuaState s, int a, LuaState t)  //WORKS, requires both an int and a state passed from Lua code

Cases C and D could be detected during compile time, so bugs from that angle would be pretty easy to find. Case A could be ambiguous, but it would at least cause an error the first time the function was called from Lua if the intent was to use the caller state.

[JakobOvrum]

Attributes are attached to symbols/declarations, not types, so using a UDA won't work in the most basic case:

void myFunc() @luaCallerState;
lua["func"] = &myFunc; // RHS expression loses all UDA information

Worse yet, it will compile but won't do what the user expected.

[aubade]

Oh, darn :/ and it doesn't throw errors on attaching it to a type declaration either.

In that case there's probably not a good solution to this. :/ I really need the functionality in my project, so I'm going to patch my own local source tree, but it doesn't sound like there's a good way to have this upstream.

[JakobOvrum]

How come it's not possible for you to use a delegate? It should always be possible.

[aubade]

Because the program has multiple LuaStates, and the code that needs to create a LuaObject can be called from any of them; The only way I can imagine making it possible to communicate the state all the way through D involves global variables on both the D and Lua sides, and that's pretty horrendously messy from my perspective.

[JakobOvrum]

Because the program has multiple LuaStates, and the code that needs to create a LuaObject can be called from any of them;

I don't really see how that's a problem. There's a multitude of ways to achieve it with delegates:

LuaObject delegate() func(LuaState lua)
{
    return () => lua.newTable();
}

foreach(lua; states)
    lua["func"] = func(lua);

Or:

class MyState
{
    private:
    LuaState lua;
    LuaObject func() { return lua.newTable(); }

    public:
    this(LuaState lua)
    {
        this.lua = lua;
        lua["func"] = &func;
    }
}

MyState[] myStates = luaStates.map!(lua => new MyState(lua)).array();

Or using class marshalling etc.

[etcimon]

Or using class marshalling etc.

I'm curious about this method, care to provide an example for it as well?

[JakobOvrum]

I'm curious about this method, care to provide an example for it as well?

Sure:

class MyState
{
    private:
    LuaState lua;

    public:
    this(LuaState lua) { this.lua = lua; }
    LuaObject func() { return lua.newTable(); }
}

lua["myObj"] = new MyState(lua);
lua.doString(`
    local t = myObj:func()
    assert(type(t) == "table" and #t == 0)
`);