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All that might work to a certain extent as long as everything can be traced through pure lua code. An interesting thought. But i imagine it gets nigh impossible as soon as a C function is called anywhere (including standard library calls). C functions can potentially change anything anywhere in the entire lua environment, with effects that are completely unpredictable to the type checker (including types/values that aren't even related to the current context of the call like arguments, returned values, etc).

Strictly speaking, this means that any C function call in a control flow as traced by the type checker, would invalidate everything in the type checker's state that it had gathered up to that point. Also I suspect metatables can introduce some complexity in this issue, if not unpredictability. There are ways to formalize this of course and to provide hints to the checker, but this would probably involve fundamental extensions to the language, and I don't suspect it's possible to actually enforce it (which essentially means it would still be unpredictable, at best extremely limited).

From: Rici Lake <>
Reply-To: Lua list <>
To: Lua list <>
Subject: Re: From dynamic to static
Date: Mon, 31 Oct 2005 17:15:03 -0500

On 31-Oct-05, at 4:51 PM, David Given wrote:

You'd also need a limitation on variable assignments to one particular type.

local foo = 7
foo = "bar"
...would fail a type constraint.

I don't see why, except for trying to do "static" type checking. You could equally well check the assertion that foo has the correct type at every use-point, which would mean keeping track of the (set of) types foo might contain at the entry to each basic block. If you know the (possibly generic) function prototypes of every function, you could pull that off, and if you didn't know that, you wouldn't be able to accomplish this:

However, this also means that:

local foo = 7
foo = bar()

...would mean that, for it to be valid, bar() must return a number.

This sort of thing is done by some Scheme compilers, for example. Lua goes out of its way to make it difficult, of course, because it is very hard to track assignments to closed variables. But you don't have to get it perfect: you can still insert run-time checks if the compiler can't prove that static checks are sufficient. (Dylan does this, and the original Apple IDE also colour-coded the program to show you where the run-time checks were inserted, in case you felt like adding declarations, etc.)

An interesting example of this sort of thing:

--[[ line 1 ]]  local a = i
--[[ line 2 ]]  assert(type(a) == "number")
--[[ line 3 ]]  local b = math.sin(a)
--[[ line 4 ]]  return b

Say that at line 1, the compiler knew that i was a number (perhaps because there was a "local i = 3" somewhere leading up to that point.) In that case, it can deduce that a is a number, and the assert check at line 2 is a compile-time constant, so it doesn't need to actually be generated. Furthermore, it can use a non-checking (i.e. fast) version of math.sin (if it knows that the math table hasn't been altered), and it can demonstrate to itself that the function returns a number, assuming that there are no other return statements.

OK, suppose it couldn't prove anything about i. In that case, it would have to compile the test at line 2, but it would still know that a was a number at line 3, and it would still be able to demonstrate that the function returns a number.

Now, suppose the programmer hadn't put in the assert statement. In that case, the compiler would pretty well have to do it itself, or it would have to use a version of math.sin which did that. However, it could still conclude that the function returns a number. In that case, hypothesizing a sufficiently integrated IDE, it could even suggest to the programmer that the assert() be made explicit, which might be a clue to investigate why it wasn't possible to demonstrate that i was a number.

Although this seems like more work for the compiler, I think it makes for a more interesting programming environment than "static type-checking".

You know, this actually sounds *possible*. Good grief.

Sure. But it also sounds like a different language :)