Programming languages are crafted with precision, but building a parser from scratch can feel like navigating a labyrinth without a map. Fear not! PetitParser is an open-source gem that simplifies parser creation in Dart. In this blog post, we’ll walk you through the process of writing your first parser, including writing grammars, parsing inputs, and troubleshooting common issues. Let’s get started!

Getting Started with PetitParser

First and foremost, ensure that you have PetitParser installed. Visit dart packages for installation instructions. Once installed, you can import PetitParser into your Dart code with:

import 'package:petitparser/petitparser.dart';

This library utilizes static extension methods extensively; thus, be cautious when applying library prefixes or selectively importing classes.

Crafting a Simple Grammar

Writing grammars with PetitParser is akin to constructing sentences in natural language: simple yet structured. Here’s an example of how to create a basic parser that recognizes identifiers (a letter followed by letters or digits):

final id = letter() & (letter() | digit()).star();

An Analogy to Understand the Code

Imagine you are building a toy factory. Each component of your toy represents different parts of the parser:

• letter(): The raw materials (i.e., letters) for building the toys.
• digit(): Additional materials (i.e., digits) to enhance the toys.
• star(): The conveyor belt that allows for assembly of any number of parts.
• & and |: The ways to combine the materials, either sequentially or as an option.

So, you create toys (identifiers) by combining materials (letters and digits) through assembly lines (parsers).

Parsing Input

Now that you have your identifier parser ready, let’s see how to use it to parse some inputs:

final result1 = id.parse('yeah');
final result2 = id.parse('f12');

When you parse a string, the returned result will indicate success or failure:

if (result1 is Success) {
  print(result1.value);
}

Common Parsers and Their Usage

PetitParser provides various kinds of parsers to compose and consume languages:

• Terminal Parsers: Basic units like any() or char('a').
• Combinators: Combine parsers, like sequences and choices.
• Transforming Parsers: Apply transformations on parsed results using actions.

final parser = term.end();