Running Tests for Parsing DSL in TypeScript Using ANTLR/Grammars-v4

Mastering TypeScript Parsing for Custom DSLs Using ANTLR

Working with bespoke domain-specific languages (DSLs) that resemble TypeScript grammar necessitates powerful parsing tools. In this case, ANTLR, a strong parser generator, can help produce lexer and parser components, allowing such DSLs to be converted into TypeScript Abstract Syntax Trees (ASTs). However, implementing this in TypeScript presents some complications.

Using the grammars in the ANTLR/Grammars-v4 repository, developers can create parsers and lexers from.g4 files like TypeScriptLexer.g4 and TypeScriptParser.g4. These files are required to generate a pure TypeScript AST node, particularly when working with type definitions. Despite its usefulness, parsing complicated strings—such as type declarations—can be difficult.

Using an ANTLR-based lexer and parser to parse a string like typeStorage= {todos:Todo[];} may result in unexpected failures. When compiling TypeScript tests, developers may encounter typical errors such mismatched types or missing properties in their TokenSource, resulting in TSError messages during compilation.

In this article, we'll look at how to fix these problems and run tests using the ANTLR/Grammars-v4 repository examples. Finally, you will be able to correctly parse TypeScript-like DSLs.

Understanding TypeScript Parsing with ANTLR for Custom DSLs

In the given scripts, we use ANTLR to develop a lexer and parser for a bespoke DSL (Domain-Specific Language) that replicates TypeScript's type system. The initial stage is to define grammatical rules in TypeScriptLexer.g4 and TypeScriptParser.g4 files, which aid in tokenization and parsing the input. The command 'npx antlr4ts TypeScriptLexer.g4 TypeScriptParser.g4' generates the required TypeScript files, including the lexer and parser. These files parse strings like 'typeStorage = {todos: Todo[];}' as a structured AST (Abstract Syntax Tree), a key step in converting human-readable code into machine-readable format.

The created lexer turns input strings into a stream of tokens, which the parser then interprets using the grammatical rules specified in the '.g4' files. In our script, we utilize 'CharStreams.fromString()' to turn the input string into a character stream for the lexer. The lexer output is then used to create a CommonTokenStream, which the parser will use. This combination of a lexer and a token stream enables the parser to correctly comprehend the structure of the input using grammar rules, such as recognizing type declarations.

In the second script, we fix an issue where the 'TypeScriptLexer' does not fully implement the 'TokenSource' interface. By extending the lexer class and introducing missing methods such as 'nextToken()', we verify that the lexer can operate as a token source. This step is critical because without these methods, TypeScript will throw an error, as shown in the error message 'Type 'TypeScriptLexer' is not assignable to parameter of type 'TokenSource''. Overriding these functions in the custom lexer addresses the compilation problem, allowing the proper flow from input string to AST.

Finally, the final option introduces unit tests using the Mocha testing framework. These tests ensure that the parser accurately translates various DSL strings. For instance, a test examines if the string 'typeTodo = { title: string; completed: boolean; }' is processed correctly and if the produced AST matches the anticipated structure. This strategy ensures that the parser behaves correctly in all contexts, making the solution more resilient and trustworthy. By covering many use cases, we ensure that our parser is effective for a wide range of TypeScript-like DSL strings.

// Solution 1: Building a Lexer and Parser in TypeScript Using ANTLR
// Step 1: Install ANTLR TypeScript tools and dependencies
npm install antlr4ts ts-node @types/node
// Step 2: Generate TypeScript lexer and parser from TypeScriptLexer.g4 and TypeScriptParser.g4
npx antlr4ts TypeScriptLexer.g4 TypeScriptParser.g4
// Step 3: Create a parser script (test-parser.ts) to parse custom DSL strings
import { CharStreams, CommonTokenStream } from 'antlr4ts';
import { TypeScriptLexer } from './TypeScriptLexer';
import { TypeScriptParser } from './TypeScriptParser';
const input = 'typeStorage = {todos:Todo[];}';
const lexer = new TypeScriptLexer(CharStreams.fromString(input));
const tokens = new CommonTokenStream(lexer);
const parser = new TypeScriptParser(tokens);
parser.startRule();  // Start parsing
// Test parsing logic with additional DSL strings

// Solution 2: Fixing the TokenSource Issue in TypeScriptLexer
// Ensure TypeScriptLexer implements the necessary methods for TokenSource
import { TokenSource, CharStream, Token } from 'antlr4ts';
class MyLexer extends TypeScriptLexer implements TokenSource {
   nextToken(): Token {
       return super.nextToken();  // Use base class token generation
   }
}
// Create a new instance of MyLexer to bypass the compilation error
const lexer = new MyLexer(CharStreams.fromString(input));
const tokens = new CommonTokenStream(lexer);
const parser = new TypeScriptParser(tokens);
parser.startRule();
// This resolves the missing TokenSource properties issue

// Solution 3: Writing Unit Tests for the TypeScript Parser
import { CharStreams, CommonTokenStream } from 'antlr4ts';
import { TypeScriptLexer } from './TypeScriptLexer';
import { TypeScriptParser } from './TypeScriptParser';
import { expect } from 'chai';
describe('DSL Parser Tests', () => {
   it('should parse type definitions correctly', () => {
       const input = 'typeTodo = { title: string; completed: boolean; }';
       const lexer = new TypeScriptLexer(CharStreams.fromString(input));
       const tokens = new CommonTokenStream(lexer);
       const parser = new TypeScriptParser(tokens);
       const result = parser.startRule();  // Call the start rule of the grammar
       expect(result).to.not.be.null;  // Ensure result is not null
   });
});
// Run the test with Mocha: npx mocha test-parser.ts

Building and Testing TypeScript Parsers with ANTLR: Advanced Concepts

When developing a parser for TypeScript-like DSLs, properly processing complicated type definitions necessitates understanding not only ANTLR's grammar design, but also how to integrate the produced parser with recent TypeScript tools. In addition to generating lexer and parser files from .g4 files, developers must ensure that these components work seamlessly in their development environments, especially when parsing sophisticated structures like type declarations with nested elements. One often ignored component is the effective debugging of parsing failures.

Mismatches between grammar rules and the actual structure of the input text are common causes of parsing errors. If the lexer generates incorrect tokens due to incomplete or erroneous grammar rules, the parser will not produce the right AST. Parsing a DSL that incorporates object-like structures, such as TypeScript's 'type' definition, can fail if the language does not support highly nested structures. Using ANTLR's debug tools, such as the ANTLRWorks plugin, can assist in visualizing the token stream and determining where the problem exists. This enables faster correction of grammar problems.

Another major aspect when using ANTLR in TypeScript is maintaining compatibility with the TypeScript ecosystem. The previously described error handling and token source concerns are prevalent when combining the resulting parser with additional TypeScript tools like ts-node. By extending and properly implementing the lexer's missing methods (as previously explained), you ensure that these tools interface correctly with the resulting parser. Testing with unit test frameworks, such as Mocha, helps validate that the solution works in a variety of edge circumstances.