manual.html

采用C语言写的Lua的解释器的代码！Lua不用介绍了吧

<p>
All global variables live as fields in ordinary Lua tables,
called <em>environment tables</em> or simply
<em>environments</em> (see <a href="#2.9">&sect;2.9</a>).
Each function has its own reference to an environment,
so that all global variables in this function
will refer to this environment table.
When a function is created,
it inherits the environment from the function that created it.
To get the environment table of a Lua function,
you call <a href="#pdf-getfenv"><code>getfenv</code></a>.
To replace it,
you call <a href="#pdf-setfenv"><code>setfenv</code></a>.
(You can only manipulate the environment of C&nbsp;functions
through the debug library; (see <a href="#5.9">&sect;5.9</a>).)


<p>
An access to a global variable <code>x</code>
is equivalent to <code>_env.x</code>,
which in turn is equivalent to

<pre>
     gettable_event(_env, "x")
</pre><p>
where <code>_env</code> is the environment of the running function.
(See <a href="#2.8">&sect;2.8</a> for a complete description of the
<code>gettable_event</code> function.
This function is not defined or callable in Lua.
Similarly, the <code>_env</code> variable is not defined in Lua.
We use them here only for explanatory purposes.)





<h2>2.4 - <a name="2.4">Statements</a></h2>

<p>
Lua supports an almost conventional set of statements,
similar to those in Pascal or C.
This set includes
assignment, control structures, function calls,
and variable declarations.



<h3>2.4.1 - <a name="2.4.1">Chunks</a></h3>

<p>
The unit of execution of Lua is called a <em>chunk</em>.
A chunk is simply a sequence of statements,
which are executed sequentially.
Each statement can be optionally followed by a semicolon:

<pre>
	chunk ::= {stat [`<b>;</b>&acute;]}
</pre><p>
There are no empty statements and thus '<code>;;</code>' is not legal.


<p>
Lua handles a chunk as the body of an anonymous function 
with a variable number of arguments
(see <a href="#2.5.9">&sect;2.5.9</a>).
As such, chunks can define local variables,
receive arguments, and return values.


<p>
A chunk may be stored in a file or in a string inside the host program.
When a chunk is executed, first it is pre-compiled into instructions for
a virtual machine,
and then the compiled code is executed
by an interpreter for the virtual machine.


<p>
Chunks may also be pre-compiled into binary form;
see program <code>luac</code> for details.
Programs in source and compiled forms are interchangeable;
Lua automatically detects the file type and acts accordingly.






<h3>2.4.2 - <a name="2.4.2">Blocks</a></h3><p>
A block is a list of statements;
syntactically, a block is the same as a chunk:

<pre>
	block ::= chunk
</pre>

<p>
A block may be explicitly delimited to produce a single statement:

<pre>
	stat ::= <b>do</b> block <b>end</b>
</pre><p>
Explicit blocks are useful
to control the scope of variable declarations.
Explicit blocks are also sometimes used to
add a <b>return</b> or <b>break</b> statement in the middle
of another block (see <a href="#2.4.4">&sect;2.4.4</a>).





<h3>2.4.3 - <a name="2.4.3">Assignment</a></h3>

<p>
Lua allows multiple assignment.
Therefore, the syntax for assignment
defines a list of variables on the left side
and a list of expressions on the right side.
The elements in both lists are separated by commas:

<pre>
	stat ::= varlist `<b>=</b>&acute; explist
	varlist ::= var {`<b>,</b>&acute; var}
	explist ::= exp {`<b>,</b>&acute; exp}
</pre><p>
Expressions are discussed in <a href="#2.5">&sect;2.5</a>.


<p>
Before the assignment,
the list of values is <em>adjusted</em> to the length of
the list of variables.
If there are more values than needed,
the excess values are thrown away.
If there are fewer values than needed,
the list is extended with as many  <b>nil</b>'s as needed.
If the list of expressions ends with a function call,
then all values returned by this call enter in the list of values,
before the adjustment
(except when the call is enclosed in parentheses; see <a href="#2.5">&sect;2.5</a>).


<p>
The assignment statement first evaluates all its expressions
and only then are the assignments performed.
Thus the code

<pre>
     i = 3
     i, a[i] = i+1, 20
</pre><p>
sets <code>a[3]</code> to 20, without affecting <code>a[4]</code>
because the <code>i</code> in <code>a[i]</code> is evaluated (to 3)
before it is assigned&nbsp;4.
Similarly, the line

<pre>
     x, y = y, x
</pre><p>
exchanges the values of <code>x</code> and <code>y</code>.


<p>
The meaning of assignments to global variables
and table fields can be changed via metatables.
An assignment to an indexed variable <code>t[i] = val</code> is equivalent to
<code>settable_event(t,i,val)</code>.
(See <a href="#2.8">&sect;2.8</a> for a complete description of the
<code>settable_event</code> function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)


<p>
An assignment to a global variable <code>x = val</code>
is equivalent to the assignment
<code>_env.x = val</code>,
which in turn is equivalent to

<pre>
     settable_event(_env, "x", val)
</pre><p>
where <code>_env</code> is the environment of the running function.
(The <code>_env</code> variable is not defined in Lua.
We use it here only for explanatory purposes.)





<h3>2.4.4 - <a name="2.4.4">Control Structures</a></h3><p>
The control structures
<b>if</b>, <b>while</b>, and <b>repeat</b> have the usual meaning and
familiar syntax:




<pre>
	stat ::= <b>while</b> exp <b>do</b> block <b>end</b>
	stat ::= <b>repeat</b> block <b>until</b> exp
	stat ::= <b>if</b> exp <b>then</b> block {<b>elseif</b> exp <b>then</b> block} [<b>else</b> block] <b>end</b>
</pre><p>
Lua also has a <b>for</b> statement, in two flavors (see <a href="#2.4.5">&sect;2.4.5</a>).


<p>
The condition expression of a
control structure may return any value.
Both <b>false</b> and <b>nil</b> are considered false.
All values different from <b>nil</b> and <b>false</b> are considered true
(in particular, the number 0 and the empty string are also true).


<p>
In the <b>repeat</b>&ndash;<b>until</b> loop,
the inner block does not end at the <b>until</b> keyword,
but only after the condition.
So, the condition can refer to local variables
declared inside the loop block.


<p>
The <b>return</b> statement is used to return values
from a function or a chunk (which is just a function).

Functions and chunks may return more than one value,
so the syntax for the <b>return</b> statement is

<pre>
	stat ::= <b>return</b> [explist]
</pre>

<p>
The <b>break</b> statement is used to terminate the execution of a
<b>while</b>, <b>repeat</b>, or <b>for</b> loop,
skipping to the next statement after the loop:


<pre>
	stat ::= <b>break</b>
</pre><p>
A <b>break</b> ends the innermost enclosing loop.


<p>
The <b>return</b> and <b>break</b>
statements can only be written as the <em>last</em> statement of a block.
If it is really necessary to <b>return</b> or <b>break</b> in the
middle of a block,
then an explicit inner block can be used,
as in the idioms
<code>do return end</code> and <code>do break end</code>,
because now <b>return</b> and <b>break</b> are the last statements in
their (inner) blocks.





<h3>2.4.5 - <a name="2.4.5">For Statement</a></h3>

<p>

The <b>for</b> statement has two forms:
one numeric and one generic.


<p>
The numeric <b>for</b> loop repeats a block of code while a
control variable runs through an arithmetic progression.
It has the following syntax:

<pre>
	stat ::= <b>for</b> Name `<b>=</b>&acute; exp `<b>,</b>&acute; exp [`<b>,</b>&acute; exp] <b>do</b> block <b>end</b>
</pre><p>
The <em>block</em> is repeated for <em>name</em> starting at the value of
the first <em>exp</em>, until it passes the second <em>exp</em> by steps of the
third <em>exp</em>.
More precisely, a <b>for</b> statement like

<pre>
     for v = <em>e1</em>, <em>e2</em>, <em>e3</em> do <em>block</em> end
</pre><p>
is equivalent to the code:

<pre>
     do
       local <em>var</em>, <em>limit</em>, <em>step</em> = tonumber(<em>e1</em>), tonumber(<em>e2</em>), tonumber(<em>e3</em>)
       if not (<em>var</em> and <em>limit</em> and <em>step</em>) then error() end
       while (<em>step</em> &gt; 0 and <em>var</em> &lt;= <em>limit</em>) or (<em>step</em> &lt;= 0 and <em>var</em> &gt;= <em>limit</em>) do
         local v = <em>var</em>
         <em>block</em>
         <em>var</em> = <em>var</em> + <em>step</em>
       end
     end
</pre><p>
Note the following:

<ul>

<li>
All three control expressions are evaluated only once,
before the loop starts.
They must all result in numbers.
</li>

<li>
<code><em>var</em></code>, <code><em>limit</em></code>, and <code><em>step</em></code> are invisible variables.
The names are here for explanatory purposes only.
</li>

<li>
If the third expression (the step) is absent,
then a step of&nbsp;1 is used.
</li>

<li>
You can use <b>break</b> to exit a <b>for</b> loop.
</li>

<li>
The loop variable <code>v</code> is local to the loop;
you cannot use its value after the <b>for</b> ends or is broken.
If you need this value,
assign it to another variable before breaking or exiting the loop.
</li>

</ul>

<p>
The generic <b>for</b> statement works over functions,
called <em>iterators</em>.
On each iteration, the iterator function is called to produce a new value,
stopping when this new value is <b>nil</b>.
The generic <b>for</b> loop has the following syntax:

<pre>
	stat ::= <b>for</b> namelist <b>in</b> explist <b>do</b> block <b>end</b>
	namelist ::= Name {`<b>,</b>&acute; Name}
</pre><p>
A <b>for</b> statement like

<pre>
     for <em>var_1</em>, &middot;&middot;&middot;, <em>var_n</em> in <em>explist</em> do <em>block</em> end
</pre><p>
is equivalent to the code:

<pre>
     do
       local <em>f</em>, <em>s</em>, <em>var</em> = <em>explist</em>
       while true do
         local <em>var_1</em>, &middot;&middot;&middot;, <em>var_n</em> = <em>f</em>(<em>s</em>, <em>var</em>)
         <em>var</em> = <em>var_1</em>
         if <em>var</em> == nil then break end
         <em>block</em>
       end
     end
</pre><p>
Note the following:

<ul>

<li>
<code><em>explist</em></code> is evaluated only once.
Its results are an <em>iterator</em> function,
a <em>state</em>,
and an initial value for the first <em>iterator variable</em>.
</li>

<li>
<code><em>f</em></code>, <code><em>s</em></code>, and <code><em>var</em></code> are invisible variables.
The names are here for explanatory purposes only.
</li>

<li>
You can use <b>break</b> to exit a <b>for</b> loop.
</li>

<li>
The loop variables <code><em>var_i</em></code> are local to the loop;
you cannot use their values after the <b>for</b> ends.
If you need these values,
then assign them to other variables before breaking or exiting the loop.
</li>

</ul>




<h3>2.4.6 - <a name="2.4.6">Function Calls as Statements</a></h3><p>
To allow possible side-effects,
function calls can be executed as statements:

<pre>
	stat ::= functioncall
</pre><p>
In this case, all returned values are thrown away.
Function calls are explained in <a href="#2.5.8">&sect;2.5.8</a>.





<h3>2.4.7 - <a name="2.4.7">Local Declarations</a></h3><p>
Local variables may be declared anywhere inside a block.
The declaration may include an initial assignment:

<pre>
	stat ::= <b>local</b> namelist [`<b>=</b>&acute; explist]
</pre><p>
If present, an initial assignment has the same semantics
of a multiple assignment (see <a href="#2.4.3">&sect;2.4.3</a>).
Otherwise, all variables are initialized with <b>nil</b>.


<p>
A chunk is also a block (see <a href="#2.4.1">&sect;2.4.1</a>),
and so local variables can be declared in a chunk outside any explicit block.
The scope of such local variables extends until the end of the chunk.


<p>
The visibility rules for local variables are explained in <a href="#2.6">&sect;2.6</a>.







<h2>2.5 - <a name="2.5">Expressions</a></h2>