title: Using Combinators

Fluent interface

Many combinators come both as a standalone function, and as a method on the Parserobject. They behave the same, and exist as a convenience for writing more readable code. Choosing one or the other will mostly depend on your usecase.

The general rule is that combinator($parserA, $parserB) ≡ $parserA->combinator($parserB), in other words, they are equivalent.

In the example below, the sequence and optional combinators are used as functions and as methods, and both parsers are fully equivalent.

<?php
$parser1 = sequence(
            optional(char('a')),
            char('b')
          );
$parser2 = char('a')->optional()
            ->sequence(char('b'));

Sometimes combinators have different names for the same behaviour: $parserA->or($parserB) ≡ either($parserA, $parserB). In this case, the reason is partially because or is a reserved keyword in PHP, and partially because either reads better in this case. Some combinators have aliases, such as Parser#sequence() and Parser#followedBy(), again these exist purely for convenience.

Sequences

sequence is one of the most basic combinators you'll find. sequence($parser1, $parser2) means "Try the first parser. If it fails, return the failure. If it succeeds, take the remaining input that was not consumed by $parser1, and try $parser2. Return the result of $parser2."

It's important to understand that this drops whatever output $parser1 produced. That's useful when you're only interested in what comes after $parser1. This example extracts a value that is prefixed by a string.

<?php
$parser = sequence(string('My name is '), atLeastOne(alphaChar()));
$result = $parser->tryString("My name is Parsica");
assertEquals("Parsica", $result->output());

Alternatives

@TODO

Appending

@TODO

Folding combinators

There are also combinators that extend the behaviour of others. For example, choice is a left fold over the either combinator, effectively turning it from a combinator that takes two arguments, to one that take n arguments. choice($parser1, $parser2, $parser3, ...) ≡ $parser1->or($parser2)->or($parser3)->or...

The same happens with the assemble combinator, which call appends all its arguments. assemble($parser1, $parser2, $parser3, ...) ≡ $parser1->and($parser2)->and($parser3)->...

In general, you should use the simplest form available, so if you only have two choices, favour or over choice.

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