# Our In-House Lua API

Lua comes with a C API.  In general, the functions in this
API use the following calling convention: push the arguments
onto the lua stack, call the function, then pop the return
values from the lua stack.  I find this difficult to use: I
find it hard to remember what's on the stack at any given
time, and which stack position contains which value.  It's
not how I'm used to writing code.

So instead, we created our own in-house lua API called
"LuaStack." It allows you to create C++ local variables that
contain lua values. It has most of the same operations as
the original lua API, but they use different calling
conventions: you don't push arguments, and you don't pop
return values.  Instead, you pass arguments as readable
parameters, and you pass return values as writable
parameters.

There's a caveat: we don't actually store lua values in C++
local variables.  That would make it difficult for the lua
garbage collector to trace values.  Instead, we use a hidden
level of indirection: the C++ local variable contains the
index of a pre-allocated spot on the lua stack.  The
pre-allocated spot on the lua stack actually contains the
lua data.  We'll walk you through the details of exactly how
this works in the following sections.  

In our codebase, you are expected to use LuaStack, not
the original API, whenever possible.

== An Example of our API

This paper will explain LuaStack using the following example
function in Lua, which compares two tables to see if they
are exactly equal:

```lua
function table.equal(table1, table2)
    if (type(table1) ~= "table") then
        error("table1 must be a table")
    end
    if (type(table2) ~= "table") then
        error("table2 must be a table")
    end
    local size1 = table.nkeys(table1)
    local size2 = table.nkeys(table2)
    if (size1 ~= size2) then
        return false
    end
    for key, value1 in pairs(table1) do
        local value2 = table2[key]
        if value1 ~= value2 then
            return false
        end
    end
    return true
end
```

A few notes about that function. You can't just use the
expression "table1==table2." That would tell you if table1
and table2 are the same table (ie, pointer equality). The
function above, by contrast, checks if two different tables
contain the same key-value pairs. The built-in function
*table.nkeys* is something that we added to lua, it returns
the number of key-value pairs in a table, it works on any
table. Once you fully understand the Lua code above, have a
look at the following C++ code, which does the exact same
thing. Don't worry if you don't understand everything yet:

```cpp
LuaDefine(table_equal, "table1, table2",
    "|Return true if two tables are equal."
    "|"
    "|The values in the table are not deep-compared,"
    "|they are compared using pointer comparison."
    ) {
    LuaArg table1, table2;
    LuaVar size1, size2, key, value1, value2;
    LuaRet equalflag;
    LuaDefStack LS(L, table1, table2, size1, size2, key, value1, value2, equalflag);
    LS.cktable(table1);
    LS.cktable(table2);
    LS.set(size1, LS.nkeys(table1));
    LS.set(size2, LS.nkeys(table2));
    if (!LS.rawequal(size1, size2)) {
        LS.set(equalflag, false);
        return LS.result();
    }
    LS.set(key, LuaNil);
    while (LS.next(table1, key, value1)) {
        LS.rawget(value2, table2, key);
        if (!LS.rawequal(value1, value2)) {
            LS.set(equalflag, false);
            return LS.result();
        }
    }
    LS.set(equalflag, true);
    return LS.result();
}
```

There's a lot to unpack there. You might be wondering what
*LuaDefine* is, you might be wondering what *LS* is, you
might be wondering what the *LuaDefStack* constructor does,
and so forth. The rest of this paper is going to pick that
C++ function apart piece by piece. We're going to explain
every class, macro, and template used above. Once you're
done reading this paper, you'll understand at a deep level
how everything works.

## Class LuaArg, Class LuaVar, and Class LuaRet

In C++ example function, the first few lines inside the body
of the LuaDefine are:

```cpp
LuaArg table1, table2;
LuaVar size1, size2, key, value1, value2;
LuaRet equalflag;
```

These are variable declarations. You are meant to think of
these as variables that can hold any lua data type - lua
tables, lua strings, lua numbers, lua closures, lua threads,
any data that the lua interpreter can handle. LuaArg are
function arguments, LuaVar are local variables, and LuaRet
are return values.

That sounds straightforward, but there's a catch. The Lua
garbage collector can't allow you to put Lua values into C++
variables. The reason for this is that if you were to put a
pointer to a Lua table into a C++ local variable, the Lua
garbage collector would not know that this pointer exists.
It might think there are no pointers to the table, and it
might free the table. In reality, we can't actually put Lua
values into C++ variables.

So that raises the question, how is it possible to have
LuaArg, LuaRet, and LuaVar variables that seemingly hold lua
values? The answer is that there's a hidden level of
indirection. The values are actually stored on the Lua
stack. In the example function, our variables end up on the
lua stack in the following order:

```
stack[1] holds equalflag
stack[2] holds size1
stack[3] holds size2
stack[4] holds key
stack[5] holds value1
stack[6] holds value2
stack[7] holds table1
stack[8] holds table2
```

In our example function, those eight stack positions are
permanently reserved to hold the values of those eight
variables. We never pop those values off the stack - at
least, not until the end of the lua function.

Here are the C++ declarations of class LuaArg, class LuaRet,
and class LuaVar, with methods omitted:

```cpp
class LuaSlot { int index_; };
class LuaArg : public LuaSlot {};
class LuaVar : public LuaSlot {};
class LuaRet : public LuaSlot {};
```

So as you can see, LuaArg, LuaRet, and LuaVar are all
derived from LuaSlot. It really is true that LuaArg, LuaRet,
and LuaVar don't add anything at all to class LuaSlot: their
class bodies literally are completely empty, as shown above.
A LuaSlot contains nothing but an integer index_.

In our example function, once the LuaSlots are initialized,
the index_ values are as follows:

```
equalflag.index_ == 1
size1.index_ == 2
size2.index_ == 3
key.index_ == 4
value1.index_ == 5
value2.index_ == 6
table1.index_ == 7
table2.index_ == 8
```

Technically, it is not exactly correct to say that the
LuaSlots "contain" lua values. It is more accurate to say
that the LuaSlots contain stack positions, and those stack
positions contain lua values. That is, there's a level of
indirection. However, when you're actually using the
LuaStack API, you don't need to think about that hidden
indirection. Conceptually, you're meant to think of the
LuaSlots as effectively containing lua values. For the
remainder of this paper, I will often say that LuaSlots
"contain" lua values, glossing over the existence of a level
of indirection.

## The LuaDefStack Constructor

In the previous section, I mentioned that the LuaSlots are
all assigned unique stack positions. You might be wondering:
where is the code that assigns unique stack positions to the
LuaSlots?

It's not in the LuaSlot constructor. The LuaSlot constructor
doesn't do anything but set index_ to 0. Since
everything in Lua is numbered from 1, that's an invalid
stack position. It means that the LuaSlot is still not
initialized. Any attempt to use the LuaSlot at this stage
will generate an error.

The machinery to assign unique stack positions lives inside
the LuaDefStack constructor. LuaSlots are useless without a
LuaDefStack to set them up. This section will describe the
LuaDefStack constructor, and all the various functions it
performs.

In our example C++ function, you'll find the LuaDefStack
constructor immediately after the LuaSlot declarations:

```cpp
LuaDefStack LS(L, table1, table2, size1, size2, key, value1, value2, equalflag);
```

As you can see, the first argument to the LuaDefStack
constructor is a capital L. That's boilerplate: the first
argument is always capital L. The declaration of that
variable, *lua_State *L,* is concealed inside the
*LuaDefine* macro. I'll talk about LuaDefine later. That
data type, *lua_State*,* is from the standard Lua API. It's
a C-style pointer to the lua stack. The LuaDefStack
constructor stores this pointer inside the LuaDefStack.

After the capital L, all the LuaSlots have been passed to
the LuaDefStack constructor. They are passed by non-const
reference, which means that the LuaDefStack can mutate them.

The LuaDefStack constructor allocates stack space for the
LuaSlots. In our example function, it counts that there are
eight LuaSlots, so it allocates the first eight positions on
the lua stack to hold them. Then, it chooses a stack
position in the range 1 to 8 for each of the LuaSlot
variables. It mutates each LuaSlot to contain the correct
index_.

The LuaDefStack constructor notices that two of the LuaSlots
are of class LuaArg. It knows that LuaArgs are meant to hold
function arguments. So it checks that two function arguments
were passed to the function. If that's not the case, it
throws a lua error. But if all is well, it moves the
function arguments into the stack positions reserved for the
LuaArgs.

This is why we have the declaration of LuaArg which derives
from LuaSlot, but doesn't add any functionality to class
LuaSlot. Even though LuaArg inherits everything from
LuaSlot, the LuaDefStack constructor can differentiate it,
and treat it as special.

The LuaDefStack initializes the contents of all the
LuaSlots. As mentioned above, the LuaArg are initialized
from the function arguments. The LuaRet and LuaVar are
initialized with nil. After the LuaDefStack constructor, all
the LuaSlots have the correct index_ values, and via that
level of indirection, the LuaSlots all contain the correct
lua values.

Meanwhile, the LuaDefStack contains the pointer to the lua
stack. Note that in the example function, the object of
class LuaDefStack is named *LS*. That stands for "Lua
Stack." That is a naming convention that we use throughout
our codebase - the LuaDefStack is called *LS*. But unlike
*lua_State* L*, which is also a pointer to the lua stack,
*LuaDefStack LS* is a C++ style smart pointer, which has
methods.

When the function returns, the only thing
left on the stack is supposed to be the return values.
The LuaDefStack is responsible for making that happen.
In the function LS.return, the stack is rearranged
to put the return values on the stack and remove
everything else.  More on that later.

## Methods of Class LuaDefStack

In our example function, you'll find this line of code:

```cpp
LS.set(size1, LS.nkeys(table1));
```

Let's break that down into pieces. Here's a simpler
statement, using just LS.set:

```cpp
LS.set(size1, 3);
```

Conceptually, you're supposed to think of that in the very
obvious way: it sets *size1* to 3. But you also know that
there's a hidden level of indirection, so you might be
wondering about *how* it sets *size1* to 3.

The function LS.set is a call to the following method:

```cpp
void LuaDefStack::set(const LuaSlot &slot, int value) const;
```

Inside that method, *this* contains the pointer to the lua
stack, *slot* contains the necessary index_, and
*value* contains the value to store. What this method
actually does is set stack[index_] = value. In our
example function we know that *size1.index_* == 2,
and *value* == 3. So this sets stack[2] = 3. But again,
you're meant to ignore the hidden indirection, you're
supposed to think of it as just setting *size1* to 3.

OK, now, let's consider this statement from the example
function:

```cpp
LS.set(size1, LS.nkeys(table1));
```

That's two method calls. The *nkeys* method call fetches the
size of *table1*, returning it as an 'int'. The *set* method
stores the int in the appropriate stack slot for *size1*.

The first argument to LuaDefStack::set must be a LuaSlot.
The second argument is overloaded, it can be any one of
several different data types:

```cpp
LS.set(slota, "hello"); // set to a char*
LS.set(slota, 3.14159); // set to a double
LS.set(slota, true); // set to a bool
LS.set(slota, LuaNil); // set to nil
LS.set(slota, slotb); // copy from another LuaSlot
```

In general, methods of LuaDefStack will automatically
convert the following C++ types to Lua: int, int64_t, float,
double, char *, string, string_view, and bool. If you want
to convert lua values back to C++ values, it's a little
harder. More on that in the next section.

For a complete listing of all the operators in LuaDefStack,
have a look at the header file luastack.hpp, at the class
LuaCoreStack. There, you will find prototypes for all the
functions, along with documentation for each one.

## Converting Lua Values to C++ Values

Suppose that you have a function, callable from lua, that is
supposed to accept a string as a parameter:

```cpp
LuaDefine(frob, "str", "Frobnicate a string.") {
    LuaArg str;
    LuaDefStack LS(L, str);
    …etc…
```

The variable 'str' is a LuaSlot. Suppose you want to convert
that to a C++ string. LuaDefStack provides a method that can
do that:

```cpp
eng::string s = LS.ckstring(str);
```

The method *ckstring* verifies that *str* really is a lua
string. If it is a string, it converts it to an eng::string
and returns it. If it's not a string, ckstring throws a lua
error: "value must be a string". That's not an especially
good error message, it lacks all context. You can make it
slightly better, "str must be a string", by passing the
variable's name to ckstring:

```cpp
std::string s = LS.ckstring(str, "str");
```

Class LuaDefStack provides a full catalog of conversion
functions that return different C++ data types:

```cpp
bool             ckboolean(LuaSlot s, const char *argname = "value") const;
lua_Integer      ckinteger(LuaSlot s, const char *argname = "value") const;
int              ckint(LuaSlot s, const char *argname = "value") const;
lua_Number       cknumber(LuaSlot s, const char *argname = "value") const;
eng::string      ckstring(LuaSlot s, const char *argname = "value") const;
std::string_view ckstringview(LuaSlot s, const char *argname = "value") const;
lua_State *      ckthread(LuaSlot s, const char *argname = "value") const;
LuaToken         cktoken(LuaSlot s, const char *argname = "value") const;
util::DXYZ       ckxyz(LuaSlot s, const char *argname = "value") const;
void             cktable(LuaSlot s, const char *argname = "value") const;
void             cknil(LuaSlot s, const char *argname = "value") const;
void             ckfunction(LuaSlot s, const char *argname = "value") const;
void             ckcfunction(LuaSlot s, const char *argname = "value") const;
void             cktangible(LuaSlot s, const char *argname = "value") const;
```

Notice that cktable returns void. That's because you can't
convert a lua table to a C++ anything. So cktable just
checks the type and if it's not a table, it throws a lua
error, "value must be a table". If it is a table, cktable
doesn't do anything at all. Note that we used *LS.cktable*
in the example table_equal function, to verify that the
caller passed in two tables.

OK, so what if you want to convert the LuaSlot str to a C++
string, but you want to handle the non-string exception case
yourself? Use this code:

```cpp
std::optional<eng::string> s = LS.trystring(str);
```

You may not be familiar with C++ std::optional, so I'll
summarize here. An optional<string> can either contain a
string, or it can contain nothing. There are methods that
can check if the optional is empty, and methods to extract
the string value, if present.

What LS.trystring(str) does is: if str is a string,
trystring returns an optional<string> containing the string.
If str is not a string, trystring returns an
optional<string> that is empty.

Unlike the ck-functions functions which throw errors on type
mismatch, the try-functions never throw an error. Using the
try-functions is ideal in code where throwing errors would
be bad.

What if you just want to know if it's a string, but you
don't need the actual string? In that case, use *isstring:*

```cpp
bool b = LS.isstring(str);
```

You can also use LS.type to fetch the type of a lua value as
an enum.

## Dealing with Return Values

Our sample function contains this code:

```cpp
LuaRet equalflag;
…
if (!LS.rawequal(size1, size2)) {
    LS.set(equalflag, false);
    return LS.result();
}
```

The variable declaration, "LuaRet equalflag", declares that
this function has a return value whose name is equalflag. In
our API, return values are just variables. You have to
assign a value to the variable before you return from the
function. In the code above, when size1 != size2, it stores
*false* into equalflag. Then, there is a return-statement.

Lua calling conventions dictate that at the end of a
function, there shouldn't be anything on the lua stack
except for the return values. That is accomplished by the
method LS.result. This takes everything off the stack except
for the LuaRet values. Another lua calling convention is
that the function must return the number of return values
that are on the lua stack. Therefore, LS.result returns the
number of LuaRet values that were declared.

So, in every LuaDefine, the process of returning from a
function always consists of two steps:

1. You must set the LuaRet slots to whatever values you want
   to return.
2. There must be an explicit return-statement that must
   always be *return LS.result()*

## The LuaDefine Macro

Our sample function starts with this:

```cpp
LuaDefine(table_equal, "table1, table2",
    "|Return true if two tables are equal."
    "|"
    "|The values in the table are not deep-compared,"
    "|they are compared using pointer comparison."
    )
{ ... function body here ...}
```

LuaDefine is a C macro that takes three parameters: the
function name, the parameter list docstring, and the big
docstring.  Note that in C++, when you put two strings right
next to each other, C++ automatically concatenates them into
one string, and we've used that in the big docstring above.

The two strings passed to the LuaDefine are just
documentation. Neither has any effect on how the function
works. They are collected automatically by our documentation
system, which can print them out on demand.

The big docstring doesn't contain newlines. Instead, we use
the pipe character to designate where new lines should
start.  I find it harder to screw up.

Before I tell you what LuaDefine macro expands into, I'd
like to tell you how you write C functions that are intended
to be inserted into Lua:

```cpp
int table_equal(lua_State *L) {
    …
}
```

The C prototype is always the same: the return value is
always *int,* and the only argument is *lua_State *L*, a
pointer to the lua stack. A C function must have this
prototype to be compatible with the lua runtime.

Next, I will show you the macro expansion of a LuaDefine to
help you understand what it does. Here is the macroexpansion
of the LuaDefine above:


```cpp
int lfn_table_equal(lua_State *L);
const char *lfnarg_table_equal = "table1, table2";
const char *lfndoc_table_equal =
    "|Return true if two tables are equal."
    "|"
    "|The values in the table are not deep-compared,"
    "|they are compared using pointer comparison.";
LuaFunctionReg reg_table_equal(
    "table_equal", lfnarg_table_equal, lfndoc_table_equal, false, lfn_table_equal);
int lfn_table_equal(lua_State *L)
```

So first, you can see that there's a prototype for the
function lfn_table_equal. As you can see, it returns *int*
and accepts *lua_State *L,* which makes it compatible with
the lua runtime. This is a forward declaration.

Next, there are the two documentation strings.

Next, class LuaFunctionReg implements a registry of
LuaDefines.  It is the means by which we can loop over all
LuaDefines and insert them into a lua environment. The
constructor of LuaFunctionReg runs at program initialization
time. The constructor stores its arguments: the function
name, the documentation strings, the flags, and the C
function pointer. These are all stored inside
reg_table_equal. Then, reg_table_equal is linked
into a global linked list of LuaFunctionReg objects. This
linked list makes up the registry of all LuaDefines.

Finally, we have a second copy of the prototype for
lfn_table_equal. This time, it's not a forward declaration:
there's no semicolon. This prototype is meant to be followed
by the function body.  A LuaDefine must always be followed
by a function body.

The upshot of all this is that LuaDefine doesn't just
declare a function. It also adds that function to the
registry of all LuaDefines.  The registry contains not only
the function name and function pointer, but also the
function documentation. Remember also that in C++, static
objects like reg_table_equal are constructed before *main*.
So by the time *main* starts, the registry is ready-to-use.

The registry of all LuaDefines has two primary uses:

1. The routine source_load_cfunctions traverses the registry
   and inserts every LuaDefine into the lua environment.
   It's fully automatic: simply writing a LuaDefine causes
   the function to appear in Lua.

2. The documentation system traverses the registry to find
   the documentation strings. It uses these strings to
   auto-generate the manual.

## Class LuaExtStack

What if you want to write C++ code that manipulates Lua
data, but which *isn't* called from inside Lua? In that
case, LuaDefStack won't work, because LuaDefStack is
hardwired to assume that a lua caller *exists*,
and that it wants to pass arguments and return values:

- It checks that the caller passed the right number of
  arguments.

- It transfers the arguments from the caller to the LuaArg
  slots.

- It transfers the LuaRet slots back to the caller at
  function end.

By contrast, class LuaExtStack is for a situation where
you're in the middle of doing some C++ processing, and
you suddenly decide you need to store some more lua
variables.  Class LuaExtStack will allocate space for
the variables.

- LuaExtStack doesn't accept LuaArg or LuaRet parameters -
  it only allows LuaVar parameters.

- There is no code in LuaExtStack to check that the caller
  passed the right number of arguments, since again, it
  is not meant to be used in that context.
  
- Class LuaExtStack doesn't have a method LS.result, since
  there's nobody to return a result to.

- Class LuaDefStack assumes that anything already on the
  stack must be a function argument. Class LuaExtStack
  assumes that anything already on the stack is data that it
  is not supposed to touch in any way: it leaves it alone.
  It allocates stack space above whatever is already there.

- Class LuaExtStack has a destructor that automatically pops
  the stack back to the level where it was when the
  LuaExtStack was constructed. This is convenient.
  LuaDefStack, on the other hand, doesn't restore the stack
  top: it can't, because return values must be left on the
  stack.

Aside from those differences, LuaExtStack and LuaDefStack
are quite similar. For example, all the methods are the
same: LS.set, LS.ckstring, LS.nkeys, and so forth all work
the same with LuaDefStack and LuaExtStack.

## Using Multiple Lua Stacks

In 95% of the code you'll write, there is only one lua
stack. However, there are two big exceptions to this rule.

In the difference-transmission code, you need to compare the
master world model to the synchronous world model. Each
world model has its own instance of the lua interpreter.
Each instance of the lua interpreter has its own lua stack.
So when comparing master to synchronous, you're manipulating
two lua stacks at the same time.

In that situation, it is typical to use two LuaExtStacks:
one for the master model, one for the synchronous model. The
naming convention is as follows:

```cpp
LuaVar mtable; // A variable meant to hold a master model table.
LuaVar stable; // A variable meant to hold a synchronous model table.
LuaExtStack MLS(ML, mtable); // Master world model stack object
LuaExtStack SLS(SL, stable); // Synchronous world model stack object
```

In cases like that, it is essential to be careful - every
LuaSlot must be used with the correct stack. For example,
this will cause a crash:

```cpp
MLS.set(stable, 3);
```

That's because stable contains an index_ on the SLS
stack, not an index_ on the MLS stack. Every LuaSlot
is associated with one and only one lua stack, and you must
pair them up correctly.

The other place we use two stacks is inside World::spawn.
This does some initial calculation using the default stack
of the world model. It creates a LuaExtStack to manipulate
the default stack. Then, after the initial calculations are
done, it actually creates the new thread, which has its own
stack. In the current codebase, the new thread doesn't get
its own LuaExtStack. Instead, it's simpler to just use the
standard API to set up the new thread and move the data onto
it. Therefore, we avoid the risk of using LuaSlot objects
with the wrong LuaExtStack.

## Mixing LuaStack with the Standard API

It is rarely necessary to call functions in the standard API
- the LuaStack API is meant to be a complete replacement.
But every now and then, you encounter a case that LuaStack
doesn't handle, or doesn't handle easily. In cases like
that, it's possible to mix the standard API with the
LuaStack API.

Here's an example. When you type some lua code into the
luprex console, that code has to be compiled into a closure,
after which the closure gets called. This is all handled by
the function World::invoke_lua.

Our LuaStack API does provide a method to compile lua code
into a closure (LS.load), but for historical reasons,
World::invoke_lua uses the standard lua API to compile the
code. This is an excerpt from World::invoke_lua:

```cpp
LuaVar func;
LuaExtStack LS(L, func);
int status = luaL_loadbuffer(L, datapack.data(), datapack.size(), "=invoke");
lua_replace(L, func.index());
```

So the first two lines use the LuaStack API. We've defined a
LuaVar *func* to hold the compiled closure. The LuaExtStack
constructor allocates one space on the lua stack to hold
*func*. At this point, the lua stack contains:

```
stack[1] contains func, whose value is nil
```

Then, we call the function luaL_loadbuffer. This is a
function from the standard lua API, it compiles a source
code string and pushes the compiled closure on the stack.
The C++ variable "datapack" contains the lua source code
that was typed in at the console. After calling
luaL_loadbuffer, the stack contains:

```
stack[1] contains func, whose value is nil
stack[2] contains the compiled closure
```

Finally, our snippet calls lua_replace. This is a function
from the standard lua API, it pops the stack and stores the
popped value at a specified position on the lua stack. We
use this to pop the compiled closure off the stack, and
transfer it into the LuaVar *func*. After calling
lua_replace, the stack contains:

```
stack[1] contains func, whose value is the compiled closure
```

From this point forward, we are done using the standard API.
The rest of World::invoke_lua uses only the LuaStack API. So
as you see, it is often possible to write most of the code
using the LuaStack API, switching to the standard API for
only two or three lines of code.

The standard API generally manipulates values that are on
top of the stack. Generally, when you're done using the
standard API, you want to move the values from the top of
the stack to a LuaSlot. As demonstrated above, that is done
using lua_replace.

There's a mirror image of this function: lua_pushvalue. This
is useful for copying a value from a LuaSlot to the top of
the stack. This is generally useful when you want to take a
value from a LuaSlot, and then manipulate it using the
standard API.

## Metamethod Functions missing from our API

Several API functions from the old Lua API will
automatically call metamethods.  For example, consider the
function lua_less.  This can be used to compare two
strings.  But if you store the right metamethod, lua_less
can end up calling a closure and running lua code.

I feel strongly that this is a *very dangerous* design
decision. Imagine that you're writing code for the
difference transmitter.  You *think* you're just comparing
two strings, but all of a sudden you're running untrusted
lua code. If that code generates a lua error, suddenly
you're throwing a C++ exception from inside the difference
transmitter.

Because of this, LuaStack is *strongly* discouraging the use of
lua API functions that use metamethods: they're not even
exposed through the LuaStack API.  Here is a short list
of API functions that use metamethods and substitutes that
don't:

| Uses Metamethods        | Safe Substitute          |
|-------------------------|--------------------------|
| lua_gettable            | LS.rawget                |
| lua_settable            | LS.rawset                |
| lua_equal               | LS.rawequal              |
| lua_lessthan            | LS.genlt                 |
| lua_len                 | LS.rawlen or LS.nkeys    |

## Our Extensions to Lua

We have made many extensions to the lua interpreter:

* We have created a new lua datatype, the "token".  This is
  a string-like datatype, limited to holding short lua
  identifiers.  The entire string is actually packed in the
  8 bytes of a LUA_LIGHTUSERDATA.  See the markdown
  document, "Tokens-a-new-Lua-Type."  LuaStack is the
  module that implements and supports tokens.

* We have patched the lua runtime in quite a few ways.
  See the markdown "A-Summary-of-our-Lua-Patches."  Some of
  these patches have APIs which are accessible via LuaStack.

* Table Subtypes.  One of our patches allows you to assign
  table subtypes: tables can be general tables, classes,
  tangibles, and several other specialized types.  LuaStack
  provides the enum for all the table types, and the
  accessors to get and set the extended type of a table.

* Classes.  We have added the concept of a "class" to lua.
  The lua registry contains a table of all classes.
  LuaStack implements the functions to create classes,
  lookup classes, get class names, and the like.
  Classes will be documented in another markdown document.

* Tangible basics.  Tangibles are a complex data structure
  which is in class World, which is *way* above LuaStack's
  abstraction level.  However, LuaStack does implement
  one tiny part of the tangible infrastructure: it provides
  the tangibles table in the lua registry, and functions
  to look up tangibles.  The rest of what tangibles are about
  are implemented by the World module.

