Parco is an experimental parser combinator library for PHP inspired by Scala Parser Combinators. See also Wikipedia for general information on parser combinators.
The API documentation is available on parco.nielssp.dk/api.
Requirements:
- PHP 5.4 or newer
Install using composer:
composer require nielssp/parco
See the examples
directory for some examples:
calculator.php
is a simple calculator made usingRegexParsers
based on this example in the Scala Parser Combinators documentation.json.php
is a JSON parser usingRegexParsers
.lexer.php
is a lexer/scanner for a small expression language based on the λ-calculus.tokens.php
is a parser usingPositionalParsers
to convert the token sequence produced bylexer.php
into an abstract syntax tree.
To write a parser using Parco simply use one of the combinator traits in a class. There are currently three traits:
- Parsers for generic parser combinators (the user must provide an input sequence implementation),
- RegexParsers (extends
Parsers
) for parsing strings, and - PositionalParsers (extends
Parsers
) for parsing arrays of objects that implement thePositional
interface (e.g. a list of tokens from a lexer).
class Myparser
{
use \Parco\Combinator\RegexParsers;
}
To implement a parser you may define multiple subparsers and combine them using combinators.
Each subparser is implemented as a parameterless method returning a Parser
object. It usually makes sense to have a method for each production rule in your language grammar, so for a grammar such as:
expr ::= term {"+" term | "-" term}
term ::= factor {"*" factor | "/" factor}
factor ::= "(" expr ")"
| number
number ::= digit {digit} ["." digit {digit}]
our parser class may have the following structure:
class Myparser
{
use \Parco\Combinator\RegexParsers;
public function expr()
{
return // a parser for expressions
}
public function term()
{
return // a parser for terms
}
public function factor()
{
return // a parser for factors
}
public function number()
{
return // a parser for numbers
}
}
You may also want to add a method for invoking the parser:
class MyParser
{
use \Parco\Combinator\RegexParsers;
// ...
public function __invoke($string)
{
return $this->parseAll($this->expr, $string)->get();
}
}
The parseAll
method is provided by the RegexParsers
trait. It converts the input string to a sequence of characters and makes sure that there is no leftover input after the given parser has been applied. Now we can use the parser by constructing and then invoking it:
$myParser = new MyParser();
echo $myParser('1 + 5 - 7 / 2');
If the parser fails, a ParseException
is thrown. The "Error handling" section below explains how to handle parse errors.
The follwing parsers can be used to parse one or more input sequence elements:
$this->elem('a') // exact element
$this->acceptIf(function ($elem) { return $elem instanceof NumberToken; }) // predicate
$this->char('a') // character (RegexParsers)
$this->string('goto') // string (RegexParsers)
Additionally RegexParsers
provides a method for parsing input sequence elements using regular expressions, e.g.:
$this->regex('/[a-z][a-z0-9]*/i')
The above parsers serve as the basic building blocks for constructing more advanced parsers. The following section shows how to combine them.
Some basic combinators provided by Parsers
:
- Sequencing:
a b c
(a
followed byb
followed byc
):
$this->seq($this->a, $this->b, $this->c)
If you want to parse a
followed by b
, but only want to keep the result of a
, the method seqL
can be used:
$this->a->seqL($this->b)
Similarily, seqR
can be used to keep the result of b
instead.
- Alternation:
a | b | c
(a
,b
, orc
):
$this->alt($this->a, $this->b, $this->c)
- Repetition:
{a}
(zero or more repetitions ofa
):
$this->rep($this->a)
- Option:
[a]
(zero or onea
):
$this->opt($this->a)
More combinators are described on the API documentation page.
The abstract Parser
class provides some methods for manipulating the result of a parser. The most important one is the map
-method, which converts the result of a parser using a function, e.g.:
$this->regex('/\d+/')->map(function ($digits) {
return intval($digits);
});
The above parser uses a regular expression to parse one or more digits, then converts the result to and integer.
Two other useful methods are:
withResult
replace the result of a parser:
$this->string('true')->withResult(true);
withFailure
set a custom failure message:
$this->regex('/\d+/')->withFailure('expected an integer');
The Parsers
trait provides a magic getter that converts parameterless parsers into lazy parsers. This can be used to implement recursive grammars such as the follwing:
expr ::= term "-" term
term ::= "(" expr ")"
| number
To use this feature simply refererence your parser functions without parentheses (e.g. $this->expr
instead of $this->expr()
):
public function expr()
{
return $this->seq($this->term, $this->char("-"), $this->term);
}
public function term()
{
return $this->alt(
$this->seq($this->char("("), $this->expr, $this->char(")")),
$this->number
);
}
Some left-recursive grammars (e.g. left-associative operators) such as
expr ::= expr "-" term
| term
can be implemented using the chainl
-combinator:
public function expr()
{
return $this->chainl(
$this->term,
$this->char('-')->withResult(function ($left, $right) {
return $left - $right;
})
);
}
The second parameter to chainl
is a parser that parses the separator (i.e. the '-'
terminal) and returns a function that combines parse result from left to right.
Thus the result of parsing 8 - 4 - 1 - 3
with the above parser is ((8 - 4) - 1) - 3 = 0
.
A similar combinator, chainr
, can be used for right-associative operators.
The result of a parser includes information about the line number and column number. This can be used to produce helpful error messages.
An example of a parser error handler:
$result = $myParser($input);
if (! $result->successful) {
$lines = explode("\n", $input);
$line = $result->getInputLine($lines);
$column = $result->getInputColumn($lines);
echo 'Syntax Error: ' . $result->message
. ' on line ' . $line
. ' column ' . $column . PHP_EOL;
if ($line > 0) {
echo $lines[$line - 1] . PHP_EOL;
echo str_repeat('-', $column - 1) . '^';
}
}
Which produces output such as:
Syntax Error: expected "->" on line 1 column 17
let x = 5 in \y - x + y
----------------^
Parco also has an exception class, ParseException
, that can be used to wrap parse errors. It is thrown automatically when calling get()
on an unsuccessful parser result.
Copyright (C) 2015 Niels Sonnich Poulsen (http://nielssp.dk)
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
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