Lemon Parser-Generator：非终结符不被评估吗？
我尝试学习解析器。因为我的 C 技能相当低，所以我在 google 上搜索了 PHP Lemon 了解解析器生成器。不管怎样，这里的代码对于普通的柠檬朋友来说应该也是可读的。
与往常一样，在处理解析问题时，我首先尝试生成一个简单的计算器。
所以我的第一步很简单：
start(A) ::= expr(B). {echo "======RESULT:".A.":".B.":=========".PHP_EOL;}
解析第一个测试：
include "mysimple.php"; //include the generated Parser
$P = new ParseParser(); //create a Parser object
$P->Parse(ParseParser::VALUE,"13"); // here is the simple test, just understand the Number 13, pls
$P->Parse(0,0); //input is finished, parse!
echo "finished. yeah!".PHP_EOL;
...结果：
======RESULT:13:=========
finished. yeah!
所以，一切都按预期进行。现在我们尝试准备一个最终允许我们处理操作的步骤，即表达式：
start ::= expr(B). {echo "======RESULT:".B.":=========".PHP_EOL;}
expr ::= VALUE(B). {echo "got a value:".B.PHP_EOL;}
当我现在运行相同的测试时，我期望看到相同的输出，加上一行表示got值：13。但我只是明白：
got a value:13
======RESULT::=========
finished. yeah!
嗯，发生了什么事？为什么结果行是空的？显然，expr 的计算结果为 VALUE '13'。 Lemon不关心评价吗？我必须自己做吗？但是，如果我在这一点上什么也没得到呢？
I try to learn parsers. Because my C skills are pretty low, I googled a PHP Lemon to learn sth about Parser Generators. Anyway, the code here should be readable for normal lemon friends, too.
As always when handling parsing questions, I start with trying to produce a simple calculator first.
So my first step is simply this:
start(A) ::= expr(B). {echo "======RESULT:".A.":".B.":=========".PHP_EOL;}
what parses the first test:
include "mysimple.php"; //include the generated Parser
$P = new ParseParser(); //create a Parser object
$P->Parse(ParseParser::VALUE,"13"); // here is the simple test, just understand the Number 13, pls
$P->Parse(0,0); //input is finished, parse!
echo "finished. yeah!".PHP_EOL;
...to the result of:
======RESULT:13:=========
finished. yeah!
So, everything as expected. Now we try to prepare a step that finally will allow us to handle operations, the expression:
start ::= expr(B). {echo "======RESULT:".B.":=========".PHP_EOL;}
expr ::= VALUE(B). {echo "got a value:".B.PHP_EOL;}
When I run the same test now, I expect to see the same output, plus one line saying got a value: 13. But I just get this:
got a value:13
======RESULT::=========
finished. yeah!
Well, what happened? Why is the result line empty? Obviously expr evaluates to a VALUE of `13'. Does Lemon not care about evaluating? Do I have to do that myself somehow? But how, if I get nothing in this point?
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你不想要这样的东西吗：
Do you not want something like: