mirror of https://github.com/LeOS-GSI/LeOS-Genesis
71 lines
2.6 KiB
Plaintext
71 lines
2.6 KiB
Plaintext
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Lime: An LALR(1) parser generator in and for PHP.
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Interpretter pattern got you down? Time to use a real parser? Welcome to Lime.
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If you're familiar with BISON or YACC, you may want to read the metagrammar.
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It's written in the Lime input language, so you'll get a head-start on
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understanding how to use Lime.
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0. If you're not running Linux on an IA32 box, then you will have to rebuild
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lime_scan_tokens for your system. It should be enough to erase it,
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and then type "CFLAGS=-O2 make lime_scan_tokens" at the bash prompt.
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1. Stare at the file lime/metagrammar to understand the syntax. You're seeing
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slightly modified and tweaked Backus-Naur forms. The main differences
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are that you get to name your components, instead of refering to them
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by numbers the way that BISON demands. This idea was stolen from the
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C-based "Lemon" parser from which Lime derives its name. Incidentally,
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the author of Lemon disclaimed copyright, so you get a copy of the C
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code that taught me LALR(1) parsing better than any book, despite the
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obvious difficulties in understanding it. Oh, and one other thing:
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symbols are terminal if the scanner feeds them to the parser. They
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are non-terminal if they appear on the left side of a production rule.
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Lime names semantic categories using strings instead of the numbers
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that BISON-based parsers use, so you don't have to declare any list of
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terminal symbols anywhere.
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2. Look at the file lime/lime.php to see what pragmas are defined. To be more
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specific, you might look at the method lime::pragma(), which at the
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time of this writing, supports "%left", "%right", "%nonassoc",
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"%start", and "%class". The first three are for operator precedence.
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The last two declare the start symbol and the name of a PHP class to
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generate which will hold all the bottom-up parsing tables.
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3. Write a grammar file.
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4. php /path/to/lime/lime.php list-of-grammar-files > my_parser.php
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5. Read the function parse_lime_grammar() in lime.php to understand
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how to integrate your parser into your program.
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6. Integrate your parser as follows:
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--------------- CUT ---------------
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include_once "lime/parse_engine.php";
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include_once "my_parser.php";
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#
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# Later:
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#
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$parser = new parse_engine(new my_parser());
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#
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# And still later:
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#
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try {
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while (..something..) {
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$parser->eat($type, $val);
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# You figure out how to get the parameters.
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}
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# And after the last token has been eaten:
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$parser->eat_eof();
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} catch (parse_error $e) {
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die($e->getMessage());
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}
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return $parser->semantic;
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--------------- CUT ---------------
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7. You now have the computed semantic value of whatever you parsed. Add salt
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and pepper to taste, and serve.
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