Introduction

Language Server Protocol (LSP) is a powerful tool that facilitates the development of custom languages in Visual Studio Code (VSCode). By implementing a language server, you can provide advanced language support, such as syntax highlighting, code completion, and error checking, to users of your custom language within the VSCode environment. This article will guide you through the process of creating your own custom language using LSP in VSCode, with practical examples to illustrate each step.

Understanding Language Server Protocol (LSP)

Language Server Protocol is a standardized communication protocol between an editor (like VSCode) and a language server that analyzes and processes code. It allows the editor to interact with the server to provide advanced language features. LSP allows communication between the editor and the language server. For example, when a user opens a file in VSCode, the editor sends a ‘initialize’ request to the language server, which responds with its capabilities and settings.

Setting Up Your Development Environment

To begin, ensure you have the following installed:

• Visual Studio Code (VSCode)
• Node.js and npm (Node Package Manager)

Creating the Language Server

Step 1: Initialize the project with npm and create necessary folders and files.

npm init -y

Create necessary folders and files:

- my-language-server
  - src
    - server.js
  - package.json

Step 2: Implement the language server using the LSP API. The server will handle various language features like parsing, analyzing, and validating code.

// server.js
const { createConnection, ProposedFeatures, TextDocuments } = require('vscode-languageserver/node');
const { TextDocument } = require('vscode-languageserver-textdocument');

const connection = createConnection(ProposedFeatures.all);
const documents = new TextDocuments(TextDocument);

documents.onDidChangeContent(change => {
  const document = change.document;
  // Implement parsing and analyzing code logic here
  // Send diagnostics (errors) to the editor using connection.sendDiagnostics()
});

connection.onInitialize(() => {
  return {
    capabilities: {
      textDocumentSync: documents.syncKind,
      // Add other capabilities as needed
    },
  };
});

documents.listen(connection);
connection.listen();

Defining Your Language Grammar

Step 1: Create a TextMate grammar file to define the syntax rules of your language.

// my-language.tmLanguage.json
{
  "name": "My Language",
  "scopeName": "source.my-language",
  "patterns": [
    // Define your syntax patterns here
  ]
}

Step 2: Implement the grammar using regular expressions and scopes to define syntax highlighting.

// my-language.tmLanguage.json
{
  "name": "My Language",
  "scopeName": "source.my-language",
  "patterns": [
    {
      "include": "#keywords"
    },
    {
      "include": "#strings"
    }
  ],
  "repository": {
    "keywords": {
      "patterns": [
        {
          "match": "\\b(if|else|while|for)\\b",
          "name": "keyword.control.my-language"
        }
      ]
    },
    "strings": {
      "patterns": [
        {
          "match": "'[^']*'",
          "name": "string.quoted.single.my-language"
        },
        {
          "match": "\"[^\"]*\"",
          "name": "string.quoted.double.my-language"
        }
      ]
    }
  }
}

In this example, we’re creating syntax highlighting rules for a custom language with keywords like “if,” “else,” “while,” and “for,” as well as single-quoted and double-quoted strings.

• The keywords repository uses the patterns field to define a regular expression match for the keywords, and assigns them the scope keyword.control.my-language.
• The strings repository defines patterns for both single-quoted and double-quoted strings, assigning them respective scopes.

These scopes (keyword.control.my-language, string.quoted.single.my-language, and string.quoted.double.my-language) are placeholders for actual scope names that match the conventions of your custom language and the desired syntax highlighting style.

Please note that this is a basic example, and real-world grammar files can be more complex, handling various language constructs and scopes. The patterns and repository fields can be expanded to encompass a wider range of syntax highlighting rules for your custom language.

Registering the Language in VSCode

Step 1: Create a VSCode extension to bundle your custom language server and grammar.

- my-language-extension
  - package.json
  - extension.js
  - syntaxes
    - my-language.tmLanguage.json

Step 2: Register your language with VSCode by providing necessary configurations in the extension.

// extension.js
const { ExtensionContext } = require('vscode');

function activate(context) {
  // Register your language with VSCode here
  const serverModule = context.asAbsolutePath('path-to-your-server.js'); // Provide the correct path
  const debugOptions = { execArgv: ['--nolazy', '--inspect=6009'] };

  const serverOptions = {
    run: { module: serverModule, transport: TransportKind.ipc },
    debug: { module: serverModule, transport: TransportKind.ipc, options: debugOptions },
  };

  const clientOptions = {
    documentSelector: [{ scheme: 'file', language: 'my-language' }],
    synchronize: {
      configurationSection: 'myLanguageServer',
      fileEvents: [
        workspace.createFileSystemWatcher('**/*.my-language'),
      ],
    },
  };

  const disposable = new LanguageClient(
    'myLanguageServer',
    'My Language Server',
    serverOptions,
    clientOptions
  ).start();

  context.subscriptions.push(disposable);
}

function deactivate() {}

module.exports = {
  activate,
  deactivate,
};

In the activate function, we’re registering your custom language with VSCode by creating a LanguageClient instance. Make sure to replace 'path-to-your-server.js' with the correct relative path to your server implementation. Also, ensure that 'my-language' matches the language identifier you provided when defining your language.

Implementing Language Features

Step 1: Enable code completion by responding to ‘textDocument/completion’ requests.
Step 2: Enable signature help by responding to ‘textDocument/signatureHelp’ requests.
Step 3: Enable hover information by responding to ‘textDocument/hover’ requests.
Step 4: Enable error checking and diagnostics by responding to ‘textDocument/publishDiagnostics’ requests.

// server.js
connection.onCompletion(textDocumentPosition => {
  const document = documents.get(textDocumentPosition.textDocument.uri);
  const position = textDocumentPosition.position;
  // Implement code completion logic here and return suggestions
});

connection.onSignatureHelp(textDocumentPosition => {
  const document = documents.get(textDocumentPosition.textDocument.uri);
  const position = textDocumentPosition.position;
  // Implement signature help logic here and return relevant information
});

connection.onHover(textDocumentPosition => {
  const document = documents.get(textDocumentPosition.textDocument.uri);
  const position = textDocumentPosition.position;
  // Implement hover information logic here and return relevant details
});

function validateDocument(document) {
  // Implement code validation logic here
  const diagnostics = []; // Store diagnostics (errors) in this array
  return diagnostics;
}

documents.onDidChangeContent(change => {
  const document = change.document;
  const diagnostics = validateDocument(document);
  connection.sendDiagnostics({ uri: document.uri, diagnostics });
});

Extending Language Support

Step 1: Implement code formatting by handling ‘textDocument/formatting’ and ‘textDocument/rangeFormatting’ requests.
Step 2: Implement code folding by handling ‘textDocument/foldingRange’ requests.
Step 3: Implement find references by handling ‘textDocument/references’ requests.
Step 4: Implement symbol search by handling ‘workspace/symbol’ requests.

// server.js
connection.onDocumentFormatting(formattingParams => {
  const document = documents.get(formattingParams.textDocument.uri);
  // Implement code formatting logic here and return formatted content
});

connection.onFoldingRanges(foldingRangeParams => {
  const document = documents.get(foldingRangeParams.textDocument.uri);
  // Implement code folding logic here and return folding ranges
});

connection.onReferences(referenceParams => {
  const document = documents.get(referenceParams.textDocument.uri);
  const position = referenceParams.position;
  // Implement find references logic here and return references
});

connection.onWorkspaceSymbol(workspaceSymbolParams => {
  const query = workspaceSymbolParams.query;
  // Implement symbol search logic here and return symbol matches
});

Testing Your Custom Language

Step 1: Create test cases to validate the correctness of your language server’s behavior. Create test cases to validate the correctness of your language server’s behavior. For example, using a testing framework like Mocha and Chai.
Step 2: Use the Language Server Protocol Inspector extension in VSCode to debug and test your custom language.

Conclusion

By following this step-by-step guide, you can create your own custom language using Language Server Protocol in Visual Studio Code. Utilizing LSP will enable you to provide a seamless development experience to your users with advanced language features such as syntax highlighting, code completion, and error checking. Happy coding and creating your custom language!