Phunkie: Building Parser Combinators in PHP (Part 1)

This tutorial, adapted from the Inviqa blog, demonstrates how to create parser combinators using the Phunkie functional library for PHP. We'll focus on the core concepts and build basic parsers, setting the stage for more advanced combinators in a subsequent part.

Why Functional Programming for Parsers?

Functional programming, with its emphasis on pure functions and composability, is ideally suited for building robust and maintainable parsers. The ability to combine smaller, well-defined parsing functions into larger, more complex ones is a key advantage.

The Phunkie library provides the necessary functional structures to simplify this process in PHP.

Understanding Parsers

Parsing is the process of recognizing phrases within a string. We'll use recursive-descent parsing, a straightforward yet powerful technique.

Combinators: Compositional Power

Combinators are reusable patterns for composing functions. In functional programming, they are fundamental for building complex systems from simpler components. We'll implement parsers as functions and use combinators to combine them.

Representing Parsers with Types

A parser takes a string as input and attempts to match a grammatical definition. The result is either a successful match with the remaining string or a failure. We'll use Phunkie's Pair type to represent this result: a pair containing the matched portion and the remaining string. To handle multiple possible matches, we'll use ImmList (immutable list).

The Parser Type in PHP

use Phunkie\Types\Pair;
use Phunkie\Types\ImmList;

class Parser {
    private $run;
    public function __construct(callable $run) { $this->run = $run; }
    public function run(string $toParse): ImmList { return ($this->run)($toParse); }
}

Primitive Parsers

1. result(string $a): Always succeeds, returning the given string $a and the input string unchanged.

   function result(string $a): Parser {
       return new Parser(fn(string $s) => ImmList(Pair($a, $s)));
   }

2. zero(): Always fails, returning an empty list.

   function zero(): Parser {
       return new Parser(fn($s) => Nil());
   }

3. item(): Consumes the first character of the input string. Fails if the string is empty.

   function item(): Parser {
       return new Parser(fn(string $s) => strlen($s) == 0 ? Nil() : ImmList(Pair($s[0], substr($s, 1))));
   }

Parser Combinators: seq

The seq combinator allows sequencing of parsers. It applies parsers one after another, returning the combined results. A naive implementation would be complex and error-prone. Instead, we leverage the power of monads.

The flatMap Method

The flatMap method (also known as bind) is a key component of the monad pattern. It allows chaining of computations, handling the results of one parser and passing them to the next.

class Parser {
    // ... (previous code) ...

    public function flatMap(callable $f): Parser {
        return new Parser(function(string $s) use ($f) {
            return $this->run($s)->flatMap(function(Pair $result) use ($f) {
                return $f($result->_1)->run($result->_2);
            });
        });
    }
    public function map(callable $f) {
        return new Parser(function(string $s) use ($f) {
            return $this->run($s)->map(function(Pair $result) use ($f) {
                return Pair($f($result->_1), $result->_2);
            });
        });
    }
}

seq using flatMap and map

A more elegant seq implementation using flatMap and map:

use Phunkie\Types\Pair;
use Phunkie\Types\ImmList;

class Parser {
    private $run;
    public function __construct(callable $run) { $this->run = $run; }
    public function run(string $toParse): ImmList { return ($this->run)($toParse); }
}

Or, using Phunkie's for comprehension (0.6.0 and later):

function result(string $a): Parser {
    return new Parser(fn(string $s) => ImmList(Pair($a, $s)));
}

This concludes Part 1. Part 2 will explore more advanced combinators and parsing strategies.

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