Comment by deathanatos | Hacker News Reader

Comment by deathanatos | Hacker News Reader

I'd love to see upvalues diagrammed as they are represented in memory.

It sounds like the stack is perhaps a Stack<Frame pointer¹>, where each Frame contains the locals for that stack frame; then a coroutine just needs to keep a pointer to the stack frame it is closing over. (And then, Frame does too, recursively, incase there is more than one scope being closed over.)

This would be extremely similar to … most any other language … and makes me wonder why Lua gives them such a unique name. It has been hard to really comprehend the Lua spec, when I've tried to understand that facet of it.

(I'd also argue that Lua isn't as simple as it is made out to be: primitives behave wildly different from objects, there's the 1-based indexing, multiple-return is pretty much unique to Lua (vs. returning a tuple, which other languages such as Python, Rust, and sort-of JS, go for; I think that's conceptually simpler).)

¹and note "pointer" here might really be "GC ref", to permit these to be GC'd as necessary, as closures can keep stuff alive far longer than normal.

> It sounds like the stack is perhaps a Stack<Frame pointer¹>,

No, the stack isn't a list of frames where each frame is a separate heap allocated list of slots for values. You're right that that's a very common (but not super fast) implementation technique.

The Lua stack is a single contiguous array of value slots that is used by all call frames. In fact, call frames use overlapping stack windows so that function call arguments don't need to be moved during a call.

That makes implementing closures particularly tricky since when a function returns, its region of the stack is immediately reused by later code.

Upvalues address that by moving just the local variables that are closed over off that contiguous stack array and into the heap. And they do that only when the function where the variable is declared returns so that during that functionn's lifetime, it can be accessed directly from the stack. Also, the compiler is able to manage all this just with a single pass compilation.

It's really clever.

> multiple-return is pretty much unique to Lua (vs. returning a tuple)

Multiple returns are from Common Lisp (I don't know if the history goes back further though), and they aren't tuples. They're the return-position analogue of what are default arguments in argument-position; like default args allow callers to ignore arguments they don't care about, multiple returns allow callers to ignore the return values they don't care about. You can add a secondary return value to a function without breaking existing callers, just like you can add a new default arg.

Barbra Liskov’s CLU language had multiple returns in 1973, ten years before Common Lisp. I was a student of hers back then.

I’m pretty sure I saw multiple returns used in pseudo-code on a class handout the semester before I started working on CLU. The class was “Structure and Interpretation of Computer Programs” that would lead to the writing of SICP.

Looks like POP-2 from 1968 also had multiple return values.

Lisp Machine Lisp is claimed to be the first Lisp to have that feature (end 70s), from where Common Lisp got it.

You're right, CLU is actually where Lua took them from. From "The Evolution of Lua"

> From CLU we took multiple assignment and multiple returns from function calls. We regarded multiple returns as a simpler alternative to reference parameters used in Pascal and Modula and to in-out parameters used in Ada; we also wanted to avoid explicit pointers (used in C).

It's not a Lua VM but the bytecode interpreter in Crafting Interpreters uses upvalues similarly and has some diagrams: https://craftinginterpreters.com/closures.html#upvalues

Edit: And yeah I don't think it's anything unique to Lua, it's an implementation detail of closures and lexical scope generally. It's a non-stack wrapper for closed-over variables, the end of the introduction to chapter 25 explains:

> For locals that aren’t used in closures, we’ll keep them just as they are on the stack. When a local is captured by a closure, we’ll adopt another solution that lifts them onto the heap where they can live as long as needed.

> For locals that aren’t used in closures, we’ll keep them just as they are on the stack. When a local is captured by a closure, we’ll adopt another solution that lifts them onto the heap where they can live as long as needed.

I sort of thought about mentioning something like that; that's in what I categorize as an optimization. (A good one, since the common case is that there probably isn't a closure at all in the scope, and the entire scope can thus be moved to the stack, kept out of heap, and not GC'd, reducing GC pressure.)

Since all upvalues can be determined at “compile” time (cause they all are locals up the lexical scope), I think they don’t pin stack frames^ for their lifetime, instead they create separate upvalue blocks in advance when corresponding activation records get created. One can also create C closures with upvalues from the stack, e.g. push a string and “close” it for few C functions (lua_pushcclosure, lua_upvaluejoin, luaL_setfuncs). But this string gets popped from the stack, and these functions live on their own and have a shared upvalue, which is not in any stack frame. It would be reasonable to assume from the C API that lua_CFunctions and Lua functions have the same upvalue mechanism, as they are completely interchangeable.

^ I mean internal stack structures, not a public stack, which is fully dynamic and may be cleared without affecting anything, e.g. in lua_CFunction. There’s nothing you can refer to on the stack, the same way you don’t want to refer to a temporary technical `push` in Assembly. Consider this:

  function f()
    local arr = []
    for i = 0, 100000000 do
      table.insert(arr, function ()
        return i
      end)
    end
  end

If `i` was created on the stack on every iteration, it would quickly overflow it, because you can’t just erase previous i, since it was closed.

(For those unaware: `for i …` “creates” a new variable/upvalue at each iteration. Every arr[?]() returns a different value in this example.)

For first, I was hoping to omit obvious optimization like splitting unclosed locals into a stack for the simplicity of the conversation, to arrive at a conceptual model for upvalues. That's going to be clearly impossible. That's most of your first paragraph. The Lua C API … almost serves to muddle the issue. (The stack there is … an artifact of the API, really? from a point of view trying to model the language.)

(A corrected version of your counter-example:

  function f()
    local arr = {}
    for i = 0, 5 do
      table.insert(arr, function ()
        return i
      end)
    end
    return arr
  end

I've also changed the constant to make it reasonable to run, without creating an enormous table.)

That's good counter-example, and does run counter to my intuition. (The behavior I describe, e.g., in Python, is different; the closures in Python all close over the same variable.) One might be able to boil this down to instead using "scope" where I was saying "stack frame". (The difference being that for loops in Lua create scopes, vs. Python, where they do not. IMO Lua's is the better of the two options.)

To combine the optimization in the sibling thread (that I was attempting to avoid for simplicity's sake), then the implementation could be a stack of scope-ishes, consisting of the unclosed variables only, and then optionally some structure under GC, containing the closed-over variables (none, if nothing is closed over). The scopes on the stack would need to know of the (potential) GC object holding the closed over variables; a (normal) use of `i` here would have to look up the same `i` as what the closure hold, and as you note, cannot live on the stack.

… this is where, again, I think a diagram would help. I don't know that terms like "upvalue" and "activation record" help build understanding of Lua to new practitioners, and IMO, do more to obscure it. (This isn't your fault, ofc.; these are the terms Lua itself uses for these concepts.)

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