lua.lua/manual.tex

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\begin{document}

\title{Reference Manual of the Programming Language Lua 2.2}

\author{%
Roberto Ierusalimschy\quad
Luiz Henrique de Figueiredo\quad
Waldemar Celes Filho
\vspace{1.0ex}\\
%\small \tecgraf \ --- PUC-Rio\\
\smallskip
\small\tt roberto,lhf,[email protected]
\vspace{2.0ex}\\
%MCC 08/95 ---
Departamento de Inform\'atica --- PUC-Rio
}

\date{November, 1995}

\maketitle


\begin{abstract}
\noindent
Lua is an extension programming language designed to be used
as a configuration language for any program that needs one.
This document describes version 2.2 of the Lua programming language and the
API that allows interaction between Lua programs and its host C program.
It also presents some examples of using the main features of the system.
\end{abstract}

\vspace{4ex}
\begin{quotation}
\small
\begin{center}{\bf Sum\'ario}\end{center}
\vspace{1ex}
\noindent
Lua \'e uma linguagem de extens\~ao projetada para ser usada como
linguagem de configura\c{c}\~ao em qualquer programa que precise de
uma.
Este documento descreve a vers\~ao 2.2 da linguagem de programa\c{c}\~ao Lua e a
Interface de Programa\c{c}\~ao que permite a intera\c{c}\~ao entre programas Lua
e o programa C hospedeiro.
O documento tamb\'em apresenta alguns exemplos de uso das principais
ca\-racte\-r\'{\i}sticas do sistema.
\end{quotation}


\section{Introduction}

Lua is an extension programming language designed to support
general procedural programming features with data description
facilities.
It is supposed to be used as a configuration language for any
program that needs one.
Its main extensions are related to object-oriented facilities,
and fallbacks,
but it has some other minor contributions.
Lua has been designed and implemented by
W.~Celes~F., L.~H.~de Figueiredo and R.~Ierusalimschy.

Lua is implemented as a library, written in C.
Being an extension language, Lua has no notion of a ``main'' program:
it only works {\em embedded} in a host client,
called the {\em embedding} program.
This host program can invoke functions to execute a piece of
code in Lua, can write and read Lua variables,
and can register C functions to be called by Lua code.
Through the use of C functions, Lua can be augmented to cope with
rather different domains,
thus creating customized programming languages sharing a syntactical framework.

Lua is free distribution software,
and provided as usual with no guarantees.
The implementation described in this manual is available
by anonymous ftp from
\begin{verbatim}
   ftp.icad.puc-rio.br:/pub/lua/lua-2.2.tar.gz
\end{verbatim}
or by WWW (World Wide Web) from
\begin{verbatim}
   http://www.inf.puc-rio.br/~roberto/lua.html
\end{verbatim}


\section{Environment and Modules}

All statements in Lua are executed in a \Def{global environment}.
This environment, which keeps all global variables and functions,
is initialized at the beginning of the embedding program and
persists until its end.

The global environment can be manipulated by Lua code or
by the embedding program,
which can read and write global variables
using functions in the library that implements Lua.

\Index{Global variables} do not need declaration.
Any variable is assumed to be global unless explicitly declared local
(see local declarations, Section~\ref{localvar}).
Before the first assignment, the value of a global variable is \nil.

The unit of execution of Lua is called a \Def{chunk}.
The syntax for chunks is:%
\footnote{As usual, \rep{{\em a}} means 0 or more {\em a\/}'s,
\opt{{\em a}} means an optional {\em a} and \oneormore{{\em a}} means
one or more {\em a\/}'s.}
\begin{Produc}
\produc{chunk}{\rep{statement \Or function}}
\end{Produc}%
A chunk may contain statements and function definitions,
and may be in a file or in a string inside the host program.
When a chunk is executed, first all its functions and statements are compiled,
then the statements are executed in sequential order.
All modifications a chunk effects on the global environment persist
after its end.
Those include modifications to global variables and definitions
of new functions%
\footnote{Actually, a function definition is an
assignment to a global variable; \see{TypesSec}.}.



\section{\Index{Types}} \label{TypesSec}

Lua is a dynamically typed language.
Variables do not have types; only values do.
All values carry their own type.
Therefore, there are no type definitions in the language.

There are seven \Index{basic types} in Lua: \Def{nil}, \Def{number},
\Def{string}, \Def{function}, \Def{CFunction}, \Def{userdata},
and \Def{table}.
{\em Nil} is the type of the value \nil,
whose main property is to be different from any other value.
{\em Number} represents real (floating point) numbers,
while {\em string} has the usual meaning.

Functions are considered first-class values in Lua.
This means that functions can be stored in variables,
passed as arguments to other functions and returned as results.
When a function is defined in Lua, its body is compiled and stored
in a given variable.
Lua can call (and manipulate) functions written in Lua and
functions written in C; the latter have type {\em CFunction\/}.

The type {\em userdata} is provided to allow
arbitrary \Index{C pointers} to be stored in Lua variables.
It corresponds to \verb'void*' and has no pre-defined operations in Lua,
besides assignment and equality test.
However, by using fallbacks, the programmer may define operations
for {\em userdata} values; \see{fallback}.

The type {\em table} implements \Index{associative arrays},
that is, \Index{arrays} which can be indexed not only with numbers,
but with any value (except \nil).
Therefore, this type may be used not only to represent ordinary arrays,
but also symbol tables, sets, records, etc.
To represent \Index{records}, Lua uses the field name as an index.
The language supports this representation by
providing \verb'a.name' as syntactic sugar for \verb'a["name"]'.
Tables may also carry methods.
Because functions are first class values,
table fields may contain functions.
The form \verb't:f(x)' is syntactic sugar for \verb't.f(t,x)',
which calls the method \verb'f' from the table \verb't' passing
itself as the first parameter.

It is important to notice that tables are objects, and not values.
Variables cannot contain tables, only references to them.
Assignment, parameter passing and returns always manipulate references
to tables, and do not imply any kind of copy.
Moreover, tables must be explicitly created before used;
\see{tableconstructor}.



\section{The Language}

This section describes the lexis, syntax and semantics of Lua.


\subsection{Lexical Conventions} \label{lexical}

Lua is a case sensitive language.
\Index{Identifiers} can be any string of letters, digits, and underscores,
not beginning with a digit.
The following words are reserved, and cannot be used as identifiers:
\index{reserved words}
\begin{verbatim}
         and      do        else      elseif    end
         function if        local     nil       not
         or       repeat    return    until     then    while
\end{verbatim}

The following strings denote other \Index{tokens}:
\begin{verbatim}
         ~=  <=  >=  <   >   ==  =   ..  +   -   *   /
         %   (   )   {   }   [   ]   ;   ,   .
\end{verbatim}

\Index{Literal strings} can be delimited by matching single or double quotes,
and can contain the C-like escape sequences
\verb-'\n'-, \verb-'\t'- and \verb-'\r'-.
Literal strings can also be delimited by matching \verb'[[ ... ]]'.
Literals in this last form may run for several lines,
may contain nested \verb'[[ ... ]]',
and do not interpret escape sequences.

\Index{Comments} start anywhere outside a string with a
double hyphen (\verb'--') and run until the end of the line.

\Index{Numerical constants} may be written with an optional decimal part,
and an optional decimal exponent.
Examples of valid numerical constants are:
\begin{verbatim}
       4     4.     .4     4.57e-3     .3e12
\end{verbatim}


\subsection{\Index{Coercion}} \label{coercion}

Lua provides some automatic conversions.
Any arithmetic operation applied to a string tries to convert
that string to a number, following the usual rules.
Conversely, whenever a number is used when a string is expected,
that number is converted to a string, according to the following rule:
if the number is an integer, it is written without exponent or decimal point;
otherwise, it is formatted following the ``\verb'%g'''
conversion specification of the standard \verb'printf' C function.



\subsection{\Index{Adjustment}} \label{adjust}

Functions in Lua can return many values.
Because there are no type declarations,
the system does not know how many values a function will return,
or how many parameters it needs.
Therefore, sometimes, a list of values must be {\em adjusted\/}, at run time,
to a given length.
If there are more values than are needed, the last values are thrown away.
If there are more needs than values, the list is extended with as
many  \nil's as needed.
Adjustment occurs in multiple assignment and function calls.


\subsection{Statements}

Lua supports an almost conventional set of \Index{statements}.
The conventional commands include
assignment, control structures and procedure calls.
Non-conventional commands include table constructors,
explained in Section \ref{tableconstructor},
and local variable declarations.

\subsubsection{Blocks}
A \Index{block} is a list of statements, executed sequentially.
Any statement can be optionally followed by a semicolon.
\begin{Produc}
\produc{block}{\rep{stat sc} \opt{ret sc}}
\produc{sc}{\opt{\ter{;}}}
\end{Produc}%
For syntactic reasons, a \Index{return statement} can only be written
as the last statement of a block.
This restriction also avoids some ``statement not reached'' errors.

\subsubsection{\Index{Assignment}} \label{assignment}
The language allows \Index{multiple assignment}.
Therefore, the syntax defines a list of variables on the left side,
and a list of expressions on the right side.
Both lists have their elements separated by commas.
\begin{Produc}
\produc{stat}{varlist1 \ter{=} explist1}
\produc{varlist1}{var \rep{\ter{,} var}}
\end{Produc}%
This statement first evaluates all values on the right side
and eventual indices on the left side,
and then makes the assignments.
Therefore, it can be used to exchange two values, as in
\begin{verbatim}
   x, y = y, x
\end{verbatim}
Before the assignment, the list of values is {\em adjusted} to
the length of the list of variables; \see{adjust}.

\begin{Produc}
\produc{var}{name}
\end{Produc}%
A single name can denote a global or a local variable,
or a formal parameter.
\begin{Produc}
\produc{var}{var \ter{[} exp1 \ter{]}}
\end{Produc}%
Square brackets are used to index a table.
If \verb'var' results in a table value,
the field indexed by the expression value gets the assigned value.
Otherwise, the fallback {\em settable} is called,
with three parameters: the value of \verb'var',
the value of expression, and the value being assigned to it;
\see{fallback}.
\begin{Produc}
\produc{var}{var \ter{.} name}
\end{Produc}%
The syntax \verb'var.NAME' is just syntactic sugar for
\verb'var["NAME"]'.

\subsubsection{Control Structures}
The \Index{condition expression} of a control structure can return any value.
All values different from \nil\ are considered true,
while \nil\ is considered false.
{\tt if}'s, {\tt while}'s and {\tt repeat}'s have the usual meaning.

\index{while-do}\index{repeat-until}\index{if-then-else}
\begin{Produc}
\produc{stat}{\rwd{while} exp1 \rwd{do} block \rwd{end} \OrNL
\rwd{repeat} block \rwd{until} exp1 \OrNL
\rwd{if} exp1 \rwd{then} block \rep{elseif}
   \opt{\rwd{else} block} \rwd{end}}
\produc{elseif}{\rwd{elseif} exp1 \rwd{then} block}
\end{Produc}

A {\tt return} is used to return values from a function. \label{return}
Because a function may return more than one value,
the syntax for a \Index{return statement} is:
\begin{Produc}
\produc{ret}{\rwd{return} explist}
\end{Produc}

\subsubsection{Expressions as Statements} \label{statexp}
All expressions with possible side-effects can be
executed as statements.
These include function calls and table constructors:
\begin{Produc}
\produc{stat}{functioncall}
\produc{stat}{tableconstructor}
\end{Produc}%
Eventual returned values are thrown away.
Function calls are explained in Section \ref{functioncall};
constructors are the subject of Section \ref{tableconstructor}.

\subsubsection{Local Declarations} \label{localvar}
\Index{Local variables} can be declared anywhere inside a block.
Their scope begins after the declaration and lasts until the
end of the block.
The declaration may include an initial assignment:
\begin{Produc}
\produc{stat}{\rwd{local} declist \opt{init}}
\produc{declist}{name \rep{\ter{,} name}}
\produc{init}{\ter{=} explist1}
\end{Produc}%
If there is an initial assignment, it has the same semantics
of a multiple assignment.
Otherwise, all variables are initialized with \nil.


\subsection{\Index{Expressions}}

\subsubsection{\Index{Simple Expressions}}
Simple expressions are:
\begin{Produc}
\produc{exp}{\ter{(} exp \ter{)}}
\produc{exp}{\rwd{nil}}
\produc{exp}{\ter{number}}
\produc{exp}{\ter{literal}}
\produc{exp}{var}
\end{Produc}%
Numbers (numerical constants) and
string literals are explained in Section~\ref{lexical}.
Variables are explained in Section~\ref{assignment}.

\subsubsection{Arithmetic Operators}
Lua supports the usual \Index{arithmetic operators}.
These operators are the binary
\verb'+', \verb'-', \verb'*', \verb'/' and \verb'^' (exponentiation),
and the unary \verb'-'.
If the operands are numbers, or strings that can be converted to
numbers, according to the rules given in Section \ref{coercion},
all operations but exponentiation have the usual meaning.
Otherwise, the fallback ``arith'' is called; \see{fallback}.
An exponentiation always calls this fallback.
The standard mathematical library redefines this fallback,
giving the expected meaning to \Index{exponentiation};
\see{mathlib}.

\subsubsection{Relational Operators}
Lua offers  the following \Index{relational operators}:
\begin{verbatim}
       <   >   <=  >=  ~=  ==
\end{verbatim}
All return \nil\ as false and 1 as true.

Equality first compares the types of its operands.
If they are different, the result is \nil.
Otherwise, their values are compared.
Numbers and strings are compared in the usual way.
Tables, CFunctions, and functions are compared by reference,
that is, two tables are considered equal only if they are the same table.
The operator \verb'~=' is exactly the negation of equality (\verb'=').

The other operators work as follows.
If both arguments are numbers, they are compared as such.
Otherwise, if both arguments can be converted to strings,
their values are compared using lexicographical order.
Otherwise, the fallback ``order'' is called; \see{fallback}.

\subsubsection{Logical Operators}
All logical operators, like control structures,
consider \nil\ as false and anything else as true.
The \Index{logical operators} are:
\index{and}\index{or}\index{not}
\begin{verbatim}
             and   or   not
\end{verbatim}
The operators \verb'and' and \verb'or' use \Index{short-cut evaluation},
that is,
the second operand is evaluated only if necessary.

\subsubsection{Concatenation}
Lua offers a string \Index{concatenation} operator,
denoted by ``\IndexVerb{..}''.
If operands are strings or numbers, they are converted to
strings according to the rules in Section \ref{coercion}.
Otherwise, the fallback ``concat'' is called; \see{fallback}.

\subsubsection{Precedence}
\Index{Operator precedence} follows the table below,
from the lower to the higher priority:
\begin{verbatim}
             and   or
             <   >   <=  >=  ~=  =
             ..
             +   -
             *   /
             not  - (unary)
             ^
\end{verbatim}
All binary operators are left associative, except for \verb'^',
which is right associative.

\subsubsection{Table Constructors} \label{tableconstructor}
Table \Index{constructors} are expressions that create tables;
every time a constructor is evaluated, a new table is created.
Constructors can be used to create empty tables,
or to create a table and initialize some fields.

The general syntax for constructors is:
\begin{Produc}
\produc{tableconstructor}{\ter{\{} fieldlist \ter{\}}}
\produc{fieldlist}{lfieldlist \Or ffieldlist \Or lfieldlist \ter{;} ffieldlist}
\produc{lfieldlist}{\opt{lfieldlist1}}
\produc{ffieldlist}{\opt{ffieldlist1}}
\end{Produc}

The form {\em lfieldlist1} is used to initialize lists.
\begin{Produc}
\produc{lfieldlist1}{exp \rep{\ter{,} exp} \opt{\ter{,}}}
\end{Produc}%
The expressions in the list are assigned to consecutive numerical indexes,
starting with 1.
As an example:
\begin{verbatim}
   a = {"v1", "v2", 34}
\end{verbatim}
is equivalent to:
\begin{verbatim}
   temp = {}
   temp[1] = "v1"
   temp[2] = "v2"
   temp[3] = 34
   a = temp
\end{verbatim}

The next form initializes named fields in a table.
\begin{Produc}
\produc{ffieldlist1}{ffield \rep{\ter{,} ffield} \opt{\ter{,}}}
\produc{ffield}{name \ter{=} exp}
\end{Produc}%
As an example:
\begin{verbatim}
   a = {x = 1, y = 3}
\end{verbatim}
is equivalent to:
\begin{verbatim}
   temp = {}
   temp.x = 1
   temp.y = 3
   a = temp
\end{verbatim}


\subsubsection{Function Calls}  \label{functioncall}
A \Index{function call} has the following syntax:
\begin{Produc}
\produc{functioncall}{var realParams}
\end{Produc}%
Here, \verb'var' can be any variable (global, local, indexed, etc).
If its type is {\em function\/} or {\em CFunction\/},
this function is called.
Otherwise, the fallback ``function'' is called,
having as first parameter the value of \verb'var',
and then the original call parameters.

The form:
\begin{Produc}
\produc{functioncall}{var \ter{:} name realParams}
\end{Produc}%
can be used to call ``methods''.
A call \verb'var:name(...)'
is syntactic sugar for
\begin{verbatim}
  var.name(var, ...)
\end{verbatim}
except that \verb'var' is evaluated only once.

\begin{Produc}
\produc{realParams}{\ter{(} \opt{explist1} \ter{)}}
\produc{realParams}{tableconstructor}
\produc{explist1}{exp1 \rep{\ter{,} exp1}}
\end{Produc}%
All argument expressions are evaluated before the call;
then the list of \Index{arguments} is adjusted to
the length of the list of parameters (\see{adjust});
finally, this list is assigned to the formal parameters.
A call of the form \verb'f{...}' is syntactic sugar for
\verb'f({...})', that is,
the parameter list is a single new table.

Because a function can return any number of results
(\see{return}),
the number of results must be adjusted before used.
If the function is called as an statement (\see{statexp}),
its return list is adjusted to 0.
If the function is called in a place that needs a single value
(syntactically denoted by the non-terminal \verb'exp1'),
its return list is adjusted to 1.
If the function is called in a place that can hold many values
(syntactically denoted by the non-terminal \verb'exp'),
no adjustment is done.


\subsection{\Index{Function Definitions}}

Functions in Lua can be defined anywhere in the global level of a module.
The syntax for function definition is:
\begin{Produc}
\produc{function}{\rwd{function} var \ter{(} \opt{parlist1} \ter{)}
  block \rwd{end}}
\end{Produc}

When Lua pre-compiles a chunk,
all its function bodies are pre-compiled, too.
Then, when Lua ``executes'' the function definition,
its body is stored, with type {\em function},
into the variable \verb'var'.

Parameters act as local variables,
initialized with the argument values.
\begin{Produc}
\produc{parlist1}{'name' \rep{\ter{,} name}}
\end{Produc}

Results are returned using the \verb'return' statement (\see{return}).
If control reaches the end of a function without a return instruction,
the function returns with no results.

There is a special syntax for definition of \Index{methods},
that is, functions which have an extra parameter \Def{self}.
\begin{Produc}
\produc{function}{\rwd{function} var \ter{:} name \ter{(} \opt{parlist1}
  \ter{)} block \rwd{end}}
\end{Produc}%
A declaration like
\begin{verbatim}
function v:f (...)
  ...
end
\end{verbatim}
is equivalent to
\begin{verbatim}
function v.f (self, ...)
  ...
end
\end{verbatim}
that is, the function gets an extra formal parameter called \verb'self'.
Notice that
the variable \verb'v' must be previously initialized with a table value.


\subsection{Fallbacks} \label{fallback}

Lua provides a powerful mechanism to extend its semantics,
called \Def{fallbacks}.
Basically, a fallback is a programmer defined function
which is called whenever Lua does not know how to proceed.

Lua supports the following fallbacks,
identified by the given strings:
\begin{description}
\item[``arith'']\index{arithmetic fallback}
called when an arithmetic operation is applied to non numerical operands,
or when the binary \verb'^' operation is called.
It receives three arguments:
the two operands (the second one is nil when the operation is unary minus)
and one of the following strings describing the offended operator:
\begin{verbatim}
  add  sub  mul  div  pow  unm
\end{verbatim}
Its return value is the final result of the arithmetic operation.
The default function issues an error.
\item[``order'']\index{order fallback}
called when an order comparison is applied to non numerical or
non string operands.
It receives three arguments:
the two operands and
one of the following strings describing the offended operator:
\begin{verbatim}
  lt gt le ge
\end{verbatim}
Its return value is the final result of the comparison operation.
The default function issues an error.
\item[``concat'']\index{concatenation fallback}
called when a concatenation is applied to non string operands.
It receives the two operands as arguments.
Its return value is the final result of the concatenation operation.
The default function issues an error.
\item[``index'']\index{index fallback}
called when Lua tries to retrieve the value of an index
not present in a table.
It receives as arguments the table and the index.
Its return value is the final result of the indexing operation.
The default function returns nil.
\item[``gettable'']\index{gettable fallback}
called when Lua tries to index a non table value.
It receives as arguments the non table value and the index.
Its return value is the final result of the indexing operation.
The default function issues an error.
\item[``settable'']\index{settable fallback}
called when Lua tries to assign indexed a non table value.
It receives as arguments the non table value,
the index, and the assigned value.
The default function issues an error.
\item[``function'']\index{function falback}
called when Lua tries to call a non function value.
It receives as arguments the non function value and the
arguments given in the original call.
Its return values are the final results of the call operation.
The default function issues an error.
\item[``gc'']
called during garbage collection.
It receives as argument the table being collected.
After each run of the collector this function is called with argument nil.
Because this function operates during garbage collection,
it must be used with great care,
and programmers should avoid the creation of new objects
(tables or strings) in this function.
The default function does nothing.
\item[``error'']\index{error fallback}
called when an error occurs.
It receives as argument a string describing the error.
The default function prints the message on the standard error output.
\end{description}

The function \IndexVerb{setfallback} is used to change a fallback action.
Its first argument is a string describing the fallback,
and the second the new function to be called.
It returns the old function for the given fallback.

Section \ref{exfallback} shows an example of the use of fallbacks.


\subsection{Error Handling} \label{error}

Because Lua is an extension language,
all Lua actions start from C code calling a function from the Lua library.
Whenever an error occurs during Lua compilation or execution,
an error fallback function is called,
and then the corresponding function from the library
(\verb'lua_dofile', \verb'lua_dostring',
\verb'lua_call', and \verb'lua_callfunction')
is terminated returning an error condition.

The only argument to the error fallback function is a string describing
the error and some extra informations,
like current line (when the error is at compilation)
or current function (when the error is at execution).
For more information about an error,
the Lua program can include the compilation pragma \verb'$debug'.
\index{debug pragma}
This pragma must be written in a line by itself.
When an error occurs in a program compiled with this option,
the error message includes extra information showing the stack of calls.

The standard error routine only prints the error message
to \verb'stderr'.
If needed, it is possible to change the error fallback routine;
\see{fallback}.

Lua code can generate an error by calling the function \verb'error'.
Its optional parameter is a string,
which is used as the error message.


\section{The Application Program Interface}

This section describes the API for Lua, that is,
the set of C functions available to the host program to communicate
with the library.
The API functions can be classified in the following categories:
\begin{enumerate}
\item executing Lua code;
\item converting values between C and Lua;
\item manipulating (reading and writing) Lua objects;
\item calling Lua functions;
\item C functions to be called by Lua;
\item locking Lua Objects.
\end{enumerate}
All API functions are declared in the file \verb'lua.h'.

\subsection{Executing Lua Code}
A host program can execute Lua programs written in a file or in a string,
using the following functions:
\Deffunc{lua_dofile}\Deffunc{lua_dostring}
\begin{verbatim}
int            lua_dofile               (char *filename);
int            lua_dostring             (char *string);
\end{verbatim}
Both functions return an error code:
0, in case of success; non zero, in case of errors.
The function \verb'lua_dofile', if called with argument NULL (0),
executes the ``file'' {\tt stdin}.

\subsection{Converting Values between C and Lua} \label{valuesCLua}
Because Lua has no static type system,
all values passed between Lua and C have type \IndexVerb{lua\_Object},
which works like an abstract type in C that can hold any Lua value.

Lua has automatic memory management, and garbage collection.
Because of that, a \verb'lua_Object' has a limited scope,
and is only valid inside the {\em block\/} where it was created.
A C function called from Lua is a block,
and its parameters are valid only until its end.
A good programming practice is to convert Lua objects to C values
as soon as they are available,
and never to store \verb'lua_Object's in C global variables.

When C code calls Lua repeatedly, as in a loop,
objects returned by these calls accumulate,
and may create a memory problem.
To avoid this,
nested blocks can be defined with the functions:
\begin{verbatim}
void           lua_beginblock           (void);
void           lua_endblock             (void);
\end{verbatim}
After the end of the block,
all \verb'lua_Object''s created inside it are released.

To check the type of a \verb'lua_Object',
the following function is available:
\Deffunc{lua_type}
\begin{verbatim}
int            lua_type                 (lua_Object object);
\end{verbatim}
plus the following macros:
\Deffunc{lua_isnil}\Deffunc{lua_isnumber}\Deffunc{lua_isstring}
\Deffunc{lua_istable}\Deffunc{lua_iscfunction}\Deffunc{lua_isuserdata}
\begin{verbatim}
int            lua_isnil                (lua_Object object);
int            lua_isnumber             (lua_Object object);
int            lua_isstring             (lua_Object object);
int            lua_istable              (lua_Object object);
int            lua_iscfunction          (lua_Object object);
int            lua_isuserdata           (lua_Object object);
\end{verbatim}
All macros return 1 if the object has the given type,
and 0 otherwise.

The function \verb'lua_type' can be used to distinguish between
different kinds of user data; see below.

To translate a value from type \verb'lua_Object' to a specific C type,
the programmer can use:
\Deffunc{lua_getnumber}\Deffunc{lua_getstring}
\Deffunc{lua_getcfunction}\Deffunc{lua_getuserdata}
\begin{verbatim}
double         lua_getnumber            (lua_Object object);
char          *lua_getstring            (lua_Object object);
lua_CFunction  lua_getcfunction         (lua_Object object);
void          *lua_getuserdata          (lua_Object object);
\end{verbatim}
\verb'lua_getnumber' converts a \verb'lua_Object' to a float.
This \verb'lua_Object' must be a number or a string convertible to number
(\see{coercion}); otherwise, the function returns 0.

\verb'lua_getstring' converts a \verb'lua_Object' to a string (\verb'char *').
This \verb'lua_Object' must be a string or a number;
otherwise, the function returns 0 (the null pointer).
This function does not create a new string, but returns a pointer to
a string inside the Lua environment.
Because Lua has garbage collection, there is no guarantee that such
pointer will be valid after the block ends.

\verb'lua_getcfunction' converts a \verb'lua_Object' to a C function.
This \verb'lua_Object' must have type {\em CFunction\/};
otherwise, the function returns 0 (the null pointer).
The type \verb'lua_CFunction' is explained in Section~\ref{LuacallC}.

\verb'lua_getuserdata' converts a \verb'lua_Object' to \verb'void*'.
This \verb'lua_Object' must have type {\em userdata\/};
otherwise, the function returns 0 (the null pointer).

The reverse process, that is, passing a specific C value to Lua,
is done by using the following functions:
\Deffunc{lua_pushnumber}\Deffunc{lua_pushstring}\Deffunc{lua_pushliteral}
\Deffunc{lua_pushcfunction}\Deffunc{lua_pushusertag}\Deffunc{lua_pushuserdata}
\begin{verbatim}
void           lua_pushnumber           (double n);
void           lua_pushstring           (char *s);
void           lua_pushliteral          (char *s);
void           lua_pushcfunction        (lua_CFunction f);
void           lua_pushusertag          (void *u, int tag);
\end{verbatim}
plus the macro:
\begin{verbatim}
void           lua_pushuserdata         (void *u);
\end{verbatim}
All of them receive a C value,
convert it to a correspondent \verb'lua_Object',
and leave the result on the top of the Lua stack,
where it can be assigned to a Lua variable,
passed as paramenter to a Lua function, etc (see below). \label{pushing}
\verb'lua_pushliteral' is like \verb'lua_pushstring',
but also puts the string in the Lua literal table.
This avoids the string to be garbage collected,
and therefore has a better overall performance.
As a rule, when the string to be pushed is a literal,
\verb'lua_pushliteral' should be used.

User data can have different tags,
whose semantics are defined by the host program.
Any positive integer can be used to tag a user data.
When a user data is retrieved,
the function \verb'lua_type' can be used to get its tag.

To complete the set,
the value \nil\ or a \verb'lua_Object' can also be pushed onto the stack,
with:
\Deffunc{lua_pushnil}\Deffunc{lua_pushobject}
\begin{verbatim}
void           lua_pushnil              (void);
void           lua_pushobject           (lua_Object object);
\end{verbatim}


\subsection{Manipulating Lua Objects}
To read the value of any global Lua variable,
one can use the function:
\Deffunc{lua_getglobal}
\begin{verbatim}
lua_Object     lua_getglobal            (char *varname);
\end{verbatim}
To store a value previously pushed onto the stack in a global variable,
there is the function:
\Deffunc{lua_storeglobal}
\begin{verbatim}
void           lua_storeglobal          (char *varname);
\end{verbatim}

Tables can also be manipulated via the API.
The function
\Deffunc{lua_getsubscript}
\begin{verbatim}
lua_Object     lua_getsubscript         (void);
\end{verbatim}
expects on the stack a table and an index,
and returns the contents of the table at that index.
As in Lua, if the first object is not a table,
or the index is not present in the table,
the correspondent fallback is called.

For compatibility with previous versions of the API,
the following macros are supported:
\Deffunc{lua_getindexed}\Deffunc{lua_getfield}
\begin{verbatim}
lua_Object     lua_getindexed           (lua_Object table, float index);
lua_Object     lua_getfield             (lua_Object table, char *field);
\end{verbatim}
The first one is used for numeric indices,
while the second can be used for any string index.

To store a value in an index,
the program must push onto the stack the table, the index,
and the value,
and then call the function:
\Deffunc{lua_storesubscript}
\begin{verbatim}
void lua_storesubscript (void);
\end{verbatim}
Again, the correspondent fallback is called if needed.

Finally, the function
\Deffunc{lua_createtable}
\begin{verbatim}
lua_Object     lua_createtable          (void);
\end{verbatim}
creates a new table.

{\em Please Notice:\/}
Most functions from the Lua library receive parameters through the stack.
Because other functions also use the stack,
it is important that these
parameters be pushed just before the correspondent call,
without intermediate calls to the Lua library.
For instance, suppose the user wants the value of \verb'a[i]'.
A simplistic solution would be:
\begin{verbatim}
  /* Warning: WRONG CODE */
  lua_Object result;
  lua_pushobject(lua_getglobal("a"));  /* push table */
  lua_pushobject(lua_getglobal("i"));  /* push index */
  result = lua_getsubscript();
\end{verbatim}
However, the call \verb'lua_getglobal("i")' modifies the stack,
and invalidates the previous pushed value.
A correct solution could be:
\begin{verbatim}
  lua_Object result;
  lua_Object index = lua_getglobal("i");
  lua_pushobject(lua_getglobal("a"));  /* push table */
  lua_pushobject(index);               /* push index */
  result = lua_getsubscript();
\end{verbatim}

\subsection{Calling Lua Functions}
Functions defined in Lua by a chunk executed with
\verb'dofile' or \verb'dostring' can be called from the host program.
This is done using the following protocol:
first, the arguments to the function are pushed onto the Lua stack
(\see{pushing}), in direct order, i.e., the first argument is pushed first.
Again, it is important to emphasize that, during this phase,
no other Lua function can be called.

Then, the function is called using
\Deffunc{lua_call}\Deffunc{lua_callfunction}
\begin{verbatim}
int            lua_call                 (char *functionname);
\end{verbatim}
or
\begin{verbatim}
int            lua_callfunction         (lua_Object function);
\end{verbatim}
Both functions return an error code:
0, in case of success; non zero, in case of errors.
Finally, the returned values (a Lua function may return many values)
can be retrieved with the macro
\Deffunc{lua_getresult}
\begin{verbatim}
lua_Object     lua_getresult             (int number);
\end{verbatim}
where \verb'number' is the order of the result, starting with 1.
When called with a number larger than the actual number of results,
this function returns \verb'LUA_NOOBJECT'.

Two special Lua functions have exclusive interfaces:
\verb'error' and \verb'setfallback'.
A C function can generate a Lua error calling the function
\Deffunc{lua_error}
\begin{verbatim}
void lua_error (char *message);
\end{verbatim}
This function never returns.
If the C function has been called from Lua,
the corresponding Lua execution terminates,
as if an error had occurred inside Lua code.
Otherwise, the whole program terminates.

Fallbacks can be changed with:
\Deffunc{lua_setfallback}
\begin{verbatim}
lua_Object lua_setfallback (char *name, lua_CFunction fallback);
\end{verbatim}
The first parameter is the fallback name,
and the second a CFunction to be used as the new fallback.
This function returns a \verb'lua_Object',
which is the old fallback value,
or nil on fail (invalid fallback name).
This old value can be used for chaining fallbacks.

An example of C code calling a Lua function is shown in
Section~\ref{exLuacall}.


\subsection{C Functions} \label{LuacallC}
To register a C function to Lua,
there is the following macro:
\Deffunc{lua_register}
\begin{verbatim}
#define lua_register(n,f)       (lua_pushcfunction(f), lua_storeglobal(n))
/* char *n;         */
/* lua_CFunction f; */
\end{verbatim}
which receives the name the function will have in Lua,
and a pointer to the function.
This pointer must have type \verb'lua_CFunction',
which is defined as
\Deffunc{lua_CFunction}
\begin{verbatim}
typedef void (*lua_CFunction) (void);
\end{verbatim}
that is, a pointer to a function with no parameters and no results.

In order to communicate properly with Lua,
a C function must follow a protocol,
which defines the way parameters and results are passed.

To access its arguments, a C function calls:
\Deffunc{lua_getparam}
\begin{verbatim}
lua_Object     lua_getparam             (int number);
\end{verbatim}
where \verb'number' starts with 1 to get the first argument.
When called with a number larger than the actual number of arguments,
this function returns \IndexVerb{LUA\_NOOBJECT}.
In this way, it is possible to write functions that work with
a variable number of parameters.

To return values, a C function just pushes them onto the stack,
in direct order; \see{valuesCLua}.
Like a Lua function, a C function called by Lua can also return
many results.

Section~\ref{exCFunction} presents an example of a CFunction.


\subsection{Locking Lua Objects}

As already noted, \verb'lua_Object's are volatile.
If the C code needs to keep a \verb'lua_Object'
outside block boundaries,
it has to {\em lock} the object.
The routines to manipulate locking are the following:
\Deffunc{lua_lock}\Deffunc{lua_getlocked}
\Deffunc{lua_pushlocked}\Deffunc{lua_unlock}
\begin{verbatim}
int        lua_lock (void);
lua_Object lua_getlocked  (int ref);
void       lua_pushlocked (int ref);
void       lua_unlock (int ref);
\end{verbatim}
The function \verb'lua_lock' locks the object
which is on the top of the stack,
and returns a reference to it.
Whenever the locked object is needed,
a call to \verb'lua_getlocked'
returns a handle to it,
while \verb'lua_pushlocked' pushes the handle on the stack.
When a locked object is no longer needed,
it can be unlocked with a call to \verb'lua_unlock'.



\section{Predefined Functions and Libraries}

The set of \Index{predefined functions} in Lua is small but powerful.
Most of them provide features that allows some degree of
\Index{reflexivity} in the language.
Many of these features cannot be simulated with the rest of the
Language nor with the standard API.

The libraries, on the other hand, provide useful routines
that are implemented directly through the standard API.
Therefore, they are not necessary to the language,
and are provided as separated C modules.
Currently there are three standard libraries:
\begin{itemize}
\item string manipulation;
\item mathematical functions (sin, cos, etc);
\item input and output.
\end{itemize}
In order to have access to these libraries,
the host program must call the functions
\verb-strlib_open-, \verb-mathlib_open-, and \verb-iolib_open-,
declared in \verb-lualib.h-.


\subsection{Predefined Functions}

\subsubsection*{{\tt dofile (filename)}}\Deffunc{dofile}
This function receives a file name,
opens it and executes its contents as a Lua chunk.
It returns 1 if there are no errors, \nil\ otherwise.

\subsubsection*{{\tt dostring (string)}}\Deffunc{dostring}
This function executes a given string as a Lua chunk.
It returns 1 if there are no errors, \nil\ otherwise.

\subsubsection*{{\tt next (table, index)}}\Deffunc{next}
This function allows a program to traverse all fields of a table.
Its first argument is a table and its second argument
is an index in this table.
It returns the next index of the table and the
value associated with the index.
When called with \nil\ as its second argument,
the function returns the first index
of the table (and its associated value).
When called with the last index, or with \nil\ in an empty table,
it returns \nil.

In Lua there is no declaration of fields;
semantically, there is no difference between a
field not present in a table or a field with value \nil.
Therefore, the function only considers fields with non nil values.
The order the indices are enumerated is not specified,
{\em even for numeric indices}.

See Section \ref{exnext} for an example of the use of this function.

\subsubsection*{{\tt nextvar (name)}}\Deffunc{nextvar}
This function is similar to the function \verb'next',
but it iterates over the global variables.
Its single argument is the name of a global variable,
or \nil\ to get a first name.
Similarly to \verb'next', it returns the name of another variable
and its value,
or \nil\ if there are no more variables.
See Section \ref{exnext} for an example of the use of this function.

\subsubsection*{{\tt print (e1, e2, ...)}}\Deffunc{print}
This function receives any number of arguments,
and prints their values in a reasonable format.
Each value is printed in a new line.
This function is not intended for formatted output,
but as a quick way to show a value,
for instance for error messages or debugging.
See Section~\ref{libio} for functions for formatted output.

\subsubsection*{{\tt tonumber (e)}}\Deffunc{tonumber}
This function receives one argument,
and tries to convert it to a number.
If the argument is already a number or a string convertible
to a number (\see{coercion}), it returns that number;
otherwise, it returns \nil.

\subsubsection*{{\tt type (v)}}\Deffunc{type}
This function allows Lua to test the type of a value.
It receives one argument, and returns its type, coded as a string.
The possible results of this function are
\verb'"nil"' (a string, not the value \nil),
\verb'"number"',
\verb'"string"',
\verb'"table"',
\verb'"function"' (returned both for C functions and Lua functions),
and \verb'"userdata"'.

Besides this string, the function returns a second result,
which is the \Def{tag} of the value.
This tag can be used to distinguish between user
data with different tags,
and between C functions and Lua functions.

\subsubsection*{{\tt error (message)}}\Deffunc{error}
This function issues an error message and terminates
the last called function from the library
(\verb'lua_dofile', \verb'lua_dostring', \ldots).
It never returns.

\subsubsection*{{\tt setglobal (name, value)}}\Deffunc{setglobal}
This function assigns the given value to a global variable.
The string \verb'name' does not need to be a syntactically valid variable name.
Therefore, this function can set global variables with strange names like
\verb'm v 1' or \verb'34'.

\subsubsection*{{\tt getglobal (name)}}\Deffunc{getglobal}
This function retrieves the value of a global variable.
The string \verb'name' does not need to be a syntactically valid variable name.

\subsubsection*{{\tt setfallback (fallbackname, newfallback)}}
\Deffunc{setfallback}
This function sets a new fallback function to the given fallback.
It returns the old fallback function.

\subsection{String Manipulation}
This library provides generic functions for string manipulation,
such as finding and extracting substrings.
When indexing a string, the first character has position 1.
See Section \ref{exstring} for some examples on string manipulation
in Lua.

\subsubsection*{{\tt strfind (str, substr, [init, [end]])}}
\Deffunc{strfind}
Receives two string arguments,
and returns a number.
This number indicates the first position where the second argument appears
in the first argument.
If the second argument is not a substring of the first one,
then \verb'strfind' returns \nil.
A third optional numerical argument specifies where to start the search.
Another optional numerical argument specifies where to stop it.

\subsubsection*{{\tt strlen (s)}}\Deffunc{strlen}
Receives a string and returns its length.

\subsubsection*{{\tt strsub (s, i, [j])}}\Deffunc{strsub}
Returns another string, which is a substring of \verb's',
starting at \verb'i'  and runing until \verb'j'.
If \verb'j' is absent,
it is assumed to be equal to the length of \verb's'.
Particularly, the call \verb'strsub(s,1,j)' returns a prefix of \verb's'
with length \verb'j',
while the call \verb'strsub(s,i)' returns a suffix of \verb's',
starting at \verb'i'.

\subsubsection*{{\tt strlower (s)}}\Deffunc{strlower}
Receives a string and returns a copy of that string with all
upper case letters changed to lower case.
All other characters are left unchanged.

\subsubsection*{{\tt strupper (s)}}\Deffunc{strupper}
Receives a string and returns a copy of that string with all
lower case letters changed to upper case.
All other characters are left unchanged.

\subsubsection*{{\tt ascii (s, [i])}}\Deffunc{ascii}
Returns the ascii code of the character \verb's[i]'.
If \verb'i' is absent, it is assumed to be 1.

\subsubsection*{{\tt int2str (\{i\})}}\Deffunc{int2str}
Receives 0 or more numbers.
Returns a string with length equal to the number of arguments,
wherein each character has ascii value equal
to its correspondent argument.

\subsection{Mathematical Functions} \label{mathlib}

This library is an interface to some functions of the standard C math library.
Moreover, it registers a fallback for the binary operator \verb'^' which,
when applied to numbers \verb'x^y', returns $x^y$.

The library provides the following functions:
\Deffunc{abs}\Deffunc{acos}\Deffunc{asin}\Deffunc{atan}
\Deffunc{atan2}\Deffunc{ceil}\Deffunc{cos}\Deffunc{floor}
\Deffunc{log}\Deffunc{log10}\Deffunc{max}\Deffunc{min}
\Deffunc{mod}\Deffunc{sin}\Deffunc{sqrt}\Deffunc{tan}
\begin{verbatim}
abs acos asin atan atan2 ceil cos floor
log log10 max min  mod  sin  sqrt tan
\end{verbatim}
Most of them
are only interfaces to the homonymous functions in the C library,
except that, for the trigonometric functions,
all angles are expressed in degrees.

The function \verb'max' returns the maximum
value of its numeric arguments.
Similarly, \verb'min' computes the minimum.
Both can be used with an unlimited number of arguments.

The function \verb'mod' is equivalent to the \verb'%' operator in C.


\subsection{I/O Facilities} \label{libio}

All I/O operations in Lua are done over two {\em current} files,
one for reading and one for writing.
Initially, the current input file is \verb'stdin',
and the current output file is \verb'stdout'.

Unless otherwise stated,
all I/O functions return 1 on success and \nil\ on failure.

\subsubsection*{{\tt readfrom (filename)}}\Deffunc{readfrom}

This function opens a file named \verb'filename' and sets it as the
{\em current} input file.
When called without parameters,
this function closes the current input file,
and restores \verb'stdin' as the current input file.

{\em System dependent:} if \verb'filename' starts with a \verb'|',
then a \Index{piped input} is open, via function \IndexVerb{popen}.

\subsubsection*{{\tt writeto (filename)}}\Deffunc{writeto}

This function opens a file named \verb'filename' and sets it as the
{\em current} output file.
Notice that, if the file already exists, it is completely erased with this
operation.
When called without parameters,
this function closes the current output file,
and restores \verb'stdout' as the current output file.
\index{closing a file}

{\em System dependent:} if \verb'filename' starts with a \verb'|',
then a \Index{piped output} is open, via function \IndexVerb{popen}.

\subsubsection*{{\tt appendto (filename)}}\Deffunc{appendto}

This function opens a file named \verb'filename' and sets it as the
{\em current} output file.
Unlike the \verb'writeto' operation,
this function does not erase any previous content of the file.
This function returns 2 if the file already exists,
1 if it creates a new file, and \nil\ on failure.

\subsubsection*{{\tt remove (filename)}}\Deffunc{remove}

This function deletes the file with the given name.

\subsubsection*{{\tt read ([format])}}\Deffunc{read}

This function returns a value read from the current input.
An optional string argument specifies the way the input is interpreted.

Without a format argument, {\tt read} first skips blanks, tabs and newlines.
Then it checks whether the current character is \verb'"' or \verb-'-.
If so, it reads a string up to the ending quotation mark,
and returns this string, without the quotation marks.
Otherwise it reads up to a blank, tab or newline.

The format string can have the following format:
\begin{verbatim}
   ?[n]
\end{verbatim}
where \verb'?' can be:
\begin{description}
\item['s' or 'S'] to read a string;
\item['f' or 'F'] to read a real number;
\item['i' or 'I'] to read an integer.
\end{description}
The optional \verb'n' is a number which specifies how many characters
must be read to compose the input value.
Particularly, the format \verb'"s1"' reads a single character.

\subsubsection*{{\tt readuntil (char)}}\Deffunc{readuntil}

Reads the current input until the first ocurrence of the given character.
Returns the string read.
The character itself is not read.

\subsubsection*{{\tt write (value, [format])}}\Deffunc{write}

This function writes the value of its first argument to the current output.
An optional second argument specifies the format to be used.
This format is given as a string, composed of four parts.
The first part is the only one not optional, and must be one of the
following characters:
\begin{description}
\item['s' or 'S'] to write strings;
\item['f' or 'F'] to write floats;
\item['i' or 'I'] to write integers.
\end{description}
These characters can be followed by
\begin{verbatim}
   [?][m][.n]
\end{verbatim}
where:
\begin{description}
\item[\verb'?'] indicates justification inside the field.
\begin{itemize}
\item['\verb'<''] right justification (default);
\item['\verb'>''] left justification;
\item['\verb'|''] center justification.
\end{itemize}
\item[\verb'm'] Indicates the field size in characters.
\item[\verb'.n'] For reals, indicates the number of digital places.
For integers, it is the minimum number of digits.
This option has no meaning for strings.
\end{description}

When called without a format string,
this function writes numbers using the \verb'%g' format
and strings with \verb'%s'.

% \subsubsection*{{\tt debug ()}}
% This function, when called, repeatedly presents a prompt \verb'lua_debug> '
% in the error output stream (\verb'stderr'),
% reads a line from the standard input,
% and executes (``dostring'') the line.
% The loop ends when the user types \verb'cont' to the prompt.
% This function then returns and the execution of the program continues.


\section{Some Examples}

This section gives examples showing some features of Lua.
It does not intend to cover the whole language,
but only to illustrate some interesting uses of the system.


\subsection{The Functions {\tt next} and {\tt nextvar}} \label{exnext}
\Deffunc{next}\Deffunc{nextvar}
This example shows how to use the function \verb'next' to iterate
over the fields of a table.
Function \Def{clone} receives any table and returns a clone of it.
\begin{verbatim}
function clone (t)           -- t is a table
  local new_t = {}           -- creates a new table
  local i, v = next(t, nil)  -- i is an index of t, v = t[i]
  while i do
    new_t[i] = v
    i, v = next(t, i)        -- get next index
  end
  return new_t
end
\end{verbatim}

The next example prints the names of all global variables
in the system with non nil values:
\begin{verbatim}
function printGlobalVariables ()
  local i, v = nextvar(nil)
  while i do
    print(i)
    i, v = nextvar(i)
  end
end
\end{verbatim}


\subsection{String Manipulation} \label{exstring}

The first example is a function to trim extra blanks at the beginning
and end of a string.
\begin{verbatim}
function trim(s)
  local l = 1
  while strsub(s,l,l) == ' ' do
    l = l+1
  end
  local r = strlen(s)
  while strsub(s,r,r) == ' ' do
    r = r-1
  end
  return strsub(s,l,r)
end
\end{verbatim}

The second example shows a function that eliminates all blanks
of a string.
\begin{verbatim}
function remove_blanks (s)
  local b = strfind(s, ' ')
  while b do
    s = strsub(s, 1, b-1) .. strsub(s, b+1)
    b = strfind(s, ' ')
  end
  return s
end
\end{verbatim}


\subsection{\Index{Persistence}}
Because of its reflexive facilities,
persistence in Lua can be achieved within the language.
This section shows some ways to store and retrieve values in Lua,
using a text file written in the language itself as the storage media.

To store a single value with a name,
the following code is enough:
\begin{verbatim}
function store (name, value)
  write('\n' .. name .. '=')
  write_value(value)
end
\end{verbatim}
\begin{verbatim}
function write_value (value)
  local t = type(value)
      if t == 'nil'    then write('nil')
  elseif t == 'number' then write(value)
  elseif t == 'string' then write('[[' .. value .. ']]')
  end
end
\end{verbatim}
In order to restore this value, a \verb'lua_dofile' suffices.

Storing tables is a little more complex.
Assuming that the table is a tree,
and all indices are identifiers
(that is, the tables are being used as records),
its value can be written directly with table constructors.
First, the function \verb'write_value' is changed to
\begin{verbatim}
function write_value (value)
  local t = type(value)
      if t == 'nil'    then write('nil')
  elseif t == 'number' then write(value)
  elseif t == 'string' then write('"' .. value .. '"')
  elseif t == 'table'  then write_record(value)
  end
end
\end{verbatim}
The function \verb'write_record' is:
\begin{verbatim}
function write_record(t)
  local i, v = next(t, nil)
  write('{')  -- starts constructor
  while i do
    store(i, v)
    write(', ')
    i, v = next(t, i)
  end
  write('}')  -- closes constructor
end
\end{verbatim}


\subsection{Inheritance} \label{exfallback}
The fallback for absent indices can be used to implement many
kinds of \Index{inheritance} in Lua.
As an example,
the following code implements single inheritance:
\begin{verbatim}
function Index (t,f)
  if f == 'parent' then  -- to avoid loop
    return OldIndex(t,f)
  end
  local p = t.parent
  if type(p) == 'table' then
    return p[f]
  else
    return OldIndex(t,f)
  end
end

OldIndex = setfallback("index", Index)
\end{verbatim}
Whenever Lua attempts to access an absent field in a table,
it calls the fallback function \verb'Index'.
If the table has a field \verb'parent' with a table value,
then Lua attempts to access the desired field in this parent object.
This process is repeated ``upwards'' until a value
for the field is found or the object has no parent.
In the latter case, the previous fallback is called to supply a value
for the field.

When better performance is needed,
the same fallback may be implemented in C,
as illustrated in Figure~\ref{Cinher}.
\begin{figure}
\Line
\begin{verbatim}
int lockedParentName;  /* stores the lock index for the string "parent" */
int lockedOldIndex;    /* previous fallback function */

void callOldFallback (lua_Object table, lua_Object index)
{
  lua_Object oldIndex = lua_getlocked(lockedOldIndex);
  lua_pushobject(table);
  lua_pushobject(index);
  lua_callfunction(oldIndex);
}

void Index (void)
{
  lua_Object table = lua_getparam(1);
  lua_Object index = lua_getparam(2);
  lua_Object parent;
  if (lua_isstring(index) && strcmp(lua_getstring(index), "parent") == 0)
  {
    callOldFallback(table, index);
    return;
  }
  lua_pushobject(table);
  lua_pushlocked(lockedParentName);
  parent = lua_getsubscript();
  if (lua_istable(parent))
  {
    lua_pushobject(parent);
    lua_pushobject(index);
    /* return result from getsubscript */
    lua_pushobject(lua_getsubscript());
  }
  else
    callOldFallback(table, index);
}
\end{verbatim}
\caption{Inheritance in C.\label{Cinher}}
\Line
\end{figure}
This code must be registered with:
\begin{verbatim}
  lua_pushliteral("parent");
  lockedParentName = lua_lock();
  lua_pushobject(lua_setfallback("index", Index));
  lockedOldIndex = lua_lock();
\end{verbatim}
Notice how the string \verb'"parent"' is kept
locked in Lua for optimal performance.

\subsection{A CFunction} \label{exCFunction}\index{functions in C}
A CFunction to compute the maximum of a variable number of arguments
is shown in Figure~\ref{Cmax}.
\begin{figure}
\Line
\begin{verbatim}
void math_max (void)
{
 int i=1;   /* number of arguments */
 double d, dmax;
 lua_Object o;
 /* the function must get at least one argument */
 if ((o = lua_getparam(i++)) == LUA_NOOBJECT)
   lua_error ("too few arguments to function `max'");
 /* and this argument must be a number */
 if (!lua_isnumber(o))
   lua_error ("incorrect argument to function `max'");
 dmax = lua_getnumber (o);
 /* loops until there is no more arguments */
 while ((o = lua_getparam(i++)) != LUA_NOOBJECT)
 {
  if (!lua_isnumber(o))
    lua_error ("incorrect argument to function `max'");
  d = lua_getnumber (o);
  if (d > dmax) dmax = d;
 }
 /* push the result to be returned */
 lua_pushnumber (dmax);
}
\end{verbatim}
\caption{C function {\tt math\_max}.\label{Cmax}}
\Line
\end{figure}
After registered with
\begin{verbatim}
lua_register ("max", math_max);
\end{verbatim}
this function is available in Lua, as follows:
\begin{verbatim}
i = max(4, 5, 10, -34)  -- i receives 10
\end{verbatim}


\subsection{Calling Lua Functions} \label{exLuacall}

This example illustrates how a C function can call the Lua function
\verb'remove_blanks' presented in Section~\ref{exstring}.
\begin{verbatim}
void remove_blanks (char *s)
{
  lua_pushstring(s);  /* prepare parameter */
  lua_call("remove_blanks");  /* call Lua function */
  strcpy(s, lua_getstring(lua_getresult(1)));  /* copy result back to 's' */
}
\end{verbatim}


\section*{Acknowledgments}

The authors would like to thank CENPES/PETROBR\'AS which,
jointly with TeCGraf, used extensively early versions of
this system and gave valuable comments.
The authors would also like to thank Carlos Henrique Levy,
who found the name of the game%
\footnote{BTW, Lua means {\em moon} in Portuguese.}.



\appendix

\section{Incompatibilities with Previous Versions}

Although great care has been taken to avoid incompatibilities with
the previous public versions of Lua,
some differences had to be introduced.
Here is a list of all these differences.

\subsection*{Incompatibilities with \Index{version 2.1}}
\begin{itemize}
\item
The function {\tt type} now returns the string {\tt function}
both for C and Lua functions.
Because Lua functions and C functions are compatible,
this behavior is usually more useful.
When needed, the second result of function {\tt type} may be used
to distinguish between Lua and C functions.
\item
A function definition only assigns the function value to the
given variable at execution time.
\end{itemize}

\subsection*{Incompatibilities with \Index{version 1.1}}
\begin{itemize}
\item
The equality test operator now is denoted by \verb'==',
instead of \verb'='.
\item
The syntax for table construction has been greatly simplified.
The old \verb'@(size)' has been substituted by \verb'{}'.
The list constructor (formerly \verb'@[...]') and the record
constructor (formerly \verb'@{...}') now are both coded like
\verb'{...}'.
When the construction involves a function call,
like in \verb'@func{...}',
the new syntax does not use the \verb'@'.
More important, {\em a construction function must now
explicitly return the constructed table}.
\item
The function \verb'lua_call' no longer has the parameter \verb'nparam'.
\item
The function \verb'lua_pop' is no longer available,
since it could lead to strange behavior.
In particular,
to access results returned from a Lua function,
the new macro \verb'lua_getresult' should be used.
\item
The old functions \verb'lua_storefield' and \verb'lua_storeindexed'
have been replaced by
\begin{verbatim}
int lua_storesubscript (void);
\end{verbatim}
with the parameters explicitly pushed on the stack.
\item
The functionality of the function \verb'lua_errorfunction' has been
replaced by the {\em fallback} mechanism; \see{error}.
\item
When calling a function from the Lua library,
parameters passed through the stack
must be pushed just before the correspondent call,
with no intermediate calls to Lua.
Special care should be taken with macros like
\verb'lua_getindexed' and \verb'lua_getfield'.
\end{itemize}

\end{document}