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Comment by premchai21 | Hacker News Reader

What do you mean by Lua leaving the caller to explicitly specify? Are you referring to earlier versions of Lua that used the explicit ^ upvalue sigil? Lua 5.1 (and 5.2) functions close over all local variables (including the implicit “self” introduced by function definitions with colon syntax), with the innermost ones first and no explicit upvalue sigil, much like Scheme.

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Nah; regular old Lua 5.1. The colon syntax is just syntactic sugar -- self isn't actually bound.

function Foo:Bar(baz) is the same as Foo.Bar = function(self, baz); invoking Foo:Bar("rebar") is sugar for Foo.Bar(Foo, "rebar"). self is never bound - it's just passed in (explicitly, via . syntax, or implicitly, vs : syntax). In all cases, the caller is always specified.

You can pass Foo.Bar around (as it's a function reference), but if you have something like:

    Foo = {}
    function Foo:Bar(baz)
        print(baz)
    end

    local baz = Foo.Bar

Then baz has no binding information to Foo; defining the function with the : syntax is just syntactic sugar. To invoke, you would have to call:

    baz(Foo, "woohoo")

Just calling

    baz("woohoo")

populates self with "woohoo", and the bar parameter would be nil, demonstrating that there is no contextual binding to the function itself.

What do you mean “isn't actually bound”? The function being defined using colon syntax doesn't close over self, since it's a parameter—but functions defined within that function will close over the self parameter, since it's a local from an enclosing scope:

    foo = { x = 3 }
    function foo:bar(baz)
        return function(thud)
            return thud + baz + self.x
        end
    end

    womble = foo:bar(4)
    womble(7) --> 14

Python I believe also closes over self as a variable, and Ruby has similar behavior for local procs, even though « self » is a special form in Ruby.

This is in distinct contrast to JavaScript, where the value of the special form « this » goes nuts inside closures because it's attached to the function:

    foo = { x: 3 };
    foo.bar = function(baz) {
        return function(thud) {
            return thud + baz + this.x;
        };
    };

    womble = foo.bar(4);
    womble(7) # --> NaN

So in what way are you placing Lua and JavaScript together, and Python and Ruby together?

[slightly rearranged for clarity]

Ah, okay, I see what you mean. Yeah, you're absolutely right there. Point conceded.

Mentally, I was separating Javascript and Lua from Ruby because while the caller is explicitly passed in Javascript and Lua (either via call/apply, or as a parameter), Ruby methods are implicitly aware of their scope (and can't really be referentially passed around like Javascript or Lua methods). Lua "methods" aren't aware of their scope (though closures are.

Ruby methods bound to their objects can be passed around; it's just a bit more cumbersome, in the form of « m = obj.method(:foo) » followed by « m.call(…) ». You can even do « m = :foo.to_proc » in recent Ruby and be able to « m.call(obj, …) »; this is quite useful for things like « [1, 2, 3].inject(&:+) ».

So we have that Python's dot always binds, Ruby's dot always calls, and Lua's dot is always a table lookup, with Lua's colon being a separate syntax for (always) the compound operation with injected self-argument.

And then JavaScript is the schizophrenic one: « obj.foo(bar) » is not the same as « var y = obj.foo; y(bar) ». Property lookup and function call insert a hidden step between them when and only when directly composed, and a function call without an immediately adjacent property lookup in a way injects the opposite step of making « this » in the callee be the global object (I think). “Politicians lie in cast iron sinks.”

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