Goto in LuaJIT

Lua 在 5.2 之后的版本，加入了 goto 这个关键字，用来控制程序跳转到指定 label。我们可以利用这个特性，来模拟 continue 的实现。需要注意的是 goto 只能跳转到 label，而 ::name:: 的格式就可以设置一个 label。

for i=1,5 do
    if i == 3 then
        goto continue
    end
    print(i)
    ::continue::
end

这样就简单实现了 continue。但是有些同学可能会有疑问，这是 Lua 5.2 的特性，我们都知道 OR 中使用的 Lua 5.1，那如何使用？

其实我们只需要使用 LuaJIT 就可以解决这个问题了。在 LuaJIT 的主页上有这个介绍：

LuaJIT supports some language and library extensions from Lua 5.2. Features that are unlikely to break existing code are unconditionally enabled:

goto and ::labels::.

也就是说 LuaJIT 把 5.2 的这个特性拿过来了。那我们来验证下：

[root@master:~]# cd /usr/local/openresty/luajit/bin/
[root@master:bin]# ./luajit
LuaJIT 2.1.0-beta1 -- Copyright (C) 2005-2015 Mike Pall. http://luajit.org/
JIT: ON SSE2 SSE3 SSE4.1 fold cse dce fwd dse narrow loop abc sink fuse
>
> for i=1,5 do
    if i == 3 then
        goto continue
    end
    print(i)
    ::continue::
end>> >> >> >> >> >>
1
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5
>

继续验证 OR ：

[root@master:demo]# nl app/app.lua
     1	local lor = require("lor.index")
     2	local app = lor()
 <span class="mi">3</span>	<span class="n">app</span><span class="p">:</span><span class="n">get</span><span class="p">(</span><span class="s2">&#34;/index&#34;</span><span class="p">,</span> <span class="k">function</span><span class="p">(</span><span class="n">req</span><span class="p">,</span><span class="n">res</span><span class="p">,</span><span class="nb">next</span><span class="p">)</span>
 <span class="mi">4</span>	    <span class="n">res</span><span class="p">:</span><span class="n">send</span><span class="p">(</span><span class="s2">&#34;hello&#34;</span><span class="p">)</span>
 <span class="mi">5</span>	<span class="k">end</span><span class="p">)</span>

 <span class="mi">6</span>	<span class="n">app</span><span class="p">:</span><span class="n">get</span><span class="p">(</span><span class="s2">&#34;goto&#34;</span><span class="p">,</span> <span class="k">function</span><span class="p">(</span><span class="n">req</span><span class="p">,</span><span class="n">res</span><span class="p">,</span><span class="nb">next</span><span class="p">)</span>
 <span class="mi">7</span>	    <span class="k">for</span> <span class="n">i</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="mi">5</span> <span class="k">do</span>
 <span class="mi">8</span>	        <span class="k">if</span> <span class="n">i</span> <span class="o">==</span> <span class="mi">3</span> <span class="k">then</span>
 <span class="mi">9</span>	            <span class="n">goto</span> <span class="n">continue</span>
<span class="mi">10</span>	        <span class="k">end</span>
<span class="mi">11</span>	        <span class="n">ngx</span><span class="p">.</span><span class="n">say</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="mi">12</span>	        <span class="p">::</span><span class="n">continue</span><span class="p">::</span>
<span class="mi">13</span>	    <span class="k">end</span>
<span class="mi">14</span>	<span class="k">end</span><span class="p">)</span>

<span class="mi">15</span>	<span class="k">return</span> <span class="n">app</span>

[root@master:demo]#

[root@master:demo]# http :8888/goto

HTTP/1.1 200 OK

Connection: keep-alive

Content-Type: text/plain

Date: Fri, 02 Dec 2016 02:55:43 GMT

Server: openresty/1.9.7.4

Transfer-Encoding: chunked

X-Powered-By: Lor Framework
1

2

4

5
[root@master:demo]#

那么 goto 还有哪些玩法？尾递归优化使用 goto 来实现：

do
    local function fact(n)
        return fact_iter(n, 1)
    end
<span class="k">function</span> <span class="nf">fact_iter</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="n">now</span><span class="p">)</span>
    <span class="p">::</span><span class="n">call</span><span class="p">::</span>
    <span class="k">if</span> <span class="n">n</span> <span class="o">&lt;=</span> <span class="mi">1</span> <span class="k">then</span>
        <span class="k">return</span> <span class="n">now</span>
    <span class="k">else</span>
        <span class="c1">-- return fact_iter(n-1, now*n)</span>
        <span class="n">n</span><span class="p">,</span> <span class="n">now</span> <span class="o">=</span> <span class="n">n</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="n">now</span><span class="o">*</span><span class="n">n</span>
        <span class="n">goto</span> <span class="n">call</span>
    <span class="k">end</span>
<span class="k">end</span>

<span class="nb">print</span><span class="p">(</span><span class="n">fact</span><span class="p">(</span><span class="mi">5</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">fact</span><span class="p">(</span><span class="mi">10000</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">fact</span><span class="p">(</span><span class="mi">10000000</span><span class="p">))</span>

end

然而感觉并没什么卵用。其他玩法需要各位同学自己去挖掘了。。不过需要注意的是 Lua 也有对 goto 的滥用限制：

A label is visible in the entire block where it is defined (including nested blocks, but not nested functions).
A goto may jump to any visible label as long as it does not enter into the scope of a local variable.

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