---

name: lsp-implementation

prettier-ignore

description: Use when implementing Language Server Protocol features - covers completions, hover, diagnostics, and navigation

Language Server Protocol Implementation

Quick Start

Copyimport {
  createConnection,
  TextDocuments,
  ProposedFeatures,
} from "vscode-languageserver/node";
import { TextDocument } from "vscode-languageserver-textdocument";

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

connection.onInitialize(() => ({
  capabilities: {
    textDocumentSync: TextDocumentSyncKind.Incremental,
    completionProvider: { triggerCharacters: [".", "/"] },
    hoverProvider: true,
    definitionProvider: true,
  }
}));

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

Core Features

Completions

Copyconnection.onCompletion((params): CompletionItem[] => {
  const document = documents.get(params.textDocument.uri);
  const position = params.position;

  // Get context at cursor
  const line = document?.getText({
    start: { line: position.line, character: 0 },
    end: position
  });

  // Return builtins
  return [
    { label: "map", kind: CompletionItemKind.Function, detail: "(fn) -> List" },
    { label: "filter", kind: CompletionItemKind.Function, detail: "(fn) -> List" },
    { label: "reduce", kind: CompletionItemKind.Function, detail: "(init, fn) -> Value" },
  ];
});

Hover Information

Copyconnection.onHover((params): Hover | null => {
  const document = documents.get(params.textDocument.uri);
  const word = getWordAtPosition(document, params.position);

  const builtin = BUILTINS[word];
  if (builtin) {
    return {
      contents: {
        kind: "markdown",
        value: `**${word}**\n\n${builtin.description}\n\n\`\`\`lea\n${builtin.signature}\n\`\`\``
      }
    };
  }
  return null;
});

Diagnostics

Copydocuments.onDidChangeContent((change) => {
  const document = change.document;
  const diagnostics: Diagnostic[] = [];

  try {
    parse(document.getText());
  } catch (error) {
    if (error instanceof ParseError) {
      diagnostics.push({
        severity: DiagnosticSeverity.Error,
        range: error.range,
        message: error.message,
        source: "lea"
      });
    }
  }

  connection.sendDiagnostics({ uri: document.uri, diagnostics });
});

Go to Definition

Copyconnection.onDefinition((params): Definition | null => {
  const document = documents.get(params.textDocument.uri);
  const word = getWordAtPosition(document, params.position);

  // Find definition in AST
  const definition = findDefinition(document.getText(), word);
  if (definition) {
    return {
      uri: params.textDocument.uri,
      range: definition.range
    };
  }
  return null;
});

Protocol Messages

• textDocument/completion - Code completions
• textDocument/hover - Hover information
• textDocument/definition - Go to definition
• textDocument/references - Find all references
• textDocument/documentSymbol - Document outline
• textDocument/formatting - Code formatting

Reference Files

• references/capabilities.md - Server capabilities
• references/diagnostics.md - Error reporting