Contenu connexe Similaire à Recursive descent parsing Similaire à Recursive descent parsing (20) Recursive descent parsing2. Plan for the next 40 mins
1. Walk through creating a Parsing Expression Grammar
and scannerless predictive recursive descent parser
for a subset of print_r output.
2. Talk about why anyone would want to do such a
thing.
Source Code: https://bit.ly/dpc14rdp
4. I am Boy Baukema
Senior Software Engineer @ ibuildings.nl
10. –Martin Fowler
“…it’s a technique that isn't as widely known as it
should be. Many people are under the impression
that using it is quite hard. I think that this fear often
comes from the fact that Syntax- Directed
Translation is usually described in the context of
parsing a general-purpose language—which
introduces a lot of complexities that you don't face
with a DSL.”
11. V1 - An empty array
Source Code: https://bit.ly/dpc14rdp
15. PrintRLang V1
RecursiveDescentParser
- $content : string
+ __construct ( string $content )
+ consume ( string $terminal )
+ lookAhead ( string $terminal )
Source Code: https://bit.ly/dpc14rdp
16. PrintRLang V1
ArrayParser
- $parser : RecursiveDescentParser
+ __construct(RecursiveDescentParser $parser)
+ parse(): array
+ arrayStart()
+ lf()
+ braceOpen()
+ braceClose()
Source Code: https://bit.ly/dpc14rdp
17. !
$parser = new PrintRLang ArrayParser(
new PrintRLang RecursiveDescentParser(
"Arrayn(n)n"
)
);
$parser->parse();
18. public function parse() {
$this->arrayStart();
$this->lf();
$this->braceOpen();
$this->lf()
$this->braceClose();
$this->lf();
return array();
}
19. A r r a y n ( n ) n
public function arrayStart() {
$this->parser->consume('Array');
}
n ( n ) n
20. n ( n ) n
public function lf() {
$this->parser->consume("n");
}
( n ) n
21. ( n ) n
public function braceOpen() {
$this->parser->consume('(');
}
n ) n
22. n ) n
public function lf() {
$this->parser->consume("n");
}
) n
25. V2 - Array of strings
Source Code: https://bit.ly/dpc14rdp
34. PrintRLang V2
RecursiveDescentParser
- $content : string
+ __construct ( string $content )
+ consume ( string $terminal )
+ consumeRegex( string $regex )
+ lookAhead ( string $terminal )
+ lookAheadRegex( string $regex
Source Code: https://bit.ly/dpc14rdp
35. PrintRLang V2
ArrayParser
- $parser : RecursiveDescentParser
...
+ arrayAssign( array $result )
+ arrayKey() : string
+ arrayValue() : string
+ space()
+ fatArrow
...
40. PrintRLang V3
ArrayParser
- $parser : RecursiveDescentParser
...
+ string()
...
42. – Steve Yegge, Rich Programmer Food
“If you don't know how parsing works, you'll do it
badly with regular expressions, or if you don't know
those, then with hand-rolled state machines that are
thousands of lines of incomprehensible code that
doesn't actually work.”
43. Mail::RFC822::Address
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48. Useful applications I’ve seen
REST API with CQL querying (MediaMosa.org)
Migrating wiki content
Parsing log files
Parsing obscure specifications (ARF)
Concise configuration files
Domain Specific Languages
49. –Martin Fowler, Domain Specific Languages
“a DSL is a front-end to a library providing a different
style of manipulation to the command-query API.
”
50. Rules for building a parser
Consider using an existing parser.
Consider porting one from another language.
Consider XML or the new XMLs: JSON / YAML
Consider working around it.
Then and only then consider building your own
parser
52. Let’s build a parser!
http://protalk.me/dpcradio-lets-build-a-parser
53. Thank you for your time and attention!
Questions?
Tweet to @relaxnow
Rate @ https://joind.in/10859
Slides @ https://joind.in/10859
Code @ https://bit.ly/dpc14rdp