racket-langserver

racket-langserver is a Language Server Protocol implementation for Racket. This project seeks to use DrRacket's public APIs to provide functionality that mimics DrRacket's code tools as closely as possible.

Installation and usage

A Racket runtime is a prerequisite, so before using racket-langserver, ensure that a Racket runtime is installed. You can install an from the official download page or install one from your package manager. Currently racket-langserver is compatible with Racket version 7.6 to 9.2.

VSCode

Use the Magic Racket extension.

Other editors and IDEs

First, install an LSP runtime for your editor.

Next, install the package via raco:

raco pkg install racket-langserver

Once it is installed, you can configure your editor to use a custom LSP client for Racket (and all installed module, e.g. Rhombus) files (usually .rkt), and set the command for the custom client to

racket -l racket-langserver

You may need to restart your LSP runtime or your editor for racket-langserver to start.

Language Support

The server recognizes language families and provides different levels of support depending on whether the language uses s-expression syntax.

• Racket - The standard Racket language (#lang racket, #lang racket/*, etc.).
• Typed Racket - (#lang typed/racket, #lang typed/racket/*, etc.).
• Other sexp - Predefined s-expression language families beyond Racket and Typed Racket.
• Scribble - (#lang scribble, #lang scribble/*, etc.).
• Rhombus - (#lang rhombus, #lang rhombus/*, etc.).
• Unknown - Language declaration found and parsed, but not in the predefined list.
• Unrecognized - No language declaration found (missing #lang, #reader, or (module ...) form).

Legend

Mark	Meaning
✅	Feature works well and produces useful results.
⚠️	Partial support - the feature runs but may produce incomplete or imprecise results.
❌	Not implemented or intentionally filtered out for this language family.

The matrix rates expected usefulness for each language family. Expansion-based features are marked supported when they only depend on successful expansion and check-syntax data. Features are marked partial when they have additional syntax-family limits, lexer limits, or intentionally noisy results.

Support Matrix

Feature	Racket	Typed Racket	Other sexp	Scribble	Rhombus	Unknown	Unrecognized
Completion	✅	✅	✅	⚠️	⚠️	⚠️	⚠️
Definition	✅	✅	✅	✅	✅	✅	❌
Hover	✅	✅	✅	✅	✅	✅	❌
Signature Help	✅	✅	✅	❌	❌	⚠️	❌
References	✅	✅	✅	✅	✅	✅	❌
Document Highlight	✅	✅	✅	✅	✅	✅	❌
Rename	✅	✅	✅	✅	✅	✅	❌
Prepare Rename	✅	✅	✅	✅	✅	✅	❌
Code Action	✅	✅	✅	✅	✅	✅	❌
Diagnostics	✅	✅	✅	✅	✅	⚠️	✅
Document Symbols	⚠️	⚠️	⚠️	⚠️	⚠️	⚠️	⚠️
Semantic Tokens, Delta	❌	❌	❌	❌	❌	❌	❌
Semantic Tokens, Full	✅	✅	✅	⚠️	⚠️	⚠️	⚠️
Semantic Tokens, Range	✅	✅	✅	⚠️	⚠️	⚠️	⚠️
Formatting	✅	✅	✅	⚠️	❌	⚠️*	❌
Range Formatting	✅	✅	✅	⚠️	❌	⚠️*	❌
On-Type Formatting	✅	✅	✅	❌	❌	❌	❌
Inlay Hints	✅	✅	❌	❌	❌	❌	❌

* Formatting for an otherwise unknown language uses drracket and requires a usable reader indentation hook; without a hook, it produces no edits.

Features

See features.md for a detailed breakdown of each feature.

Configuration

Set options in your editor's language-server settings under racket-langserver. The server reads that section through ordinary LSP configuration; there is no project configuration file. Ignore vscode warning of Unknown Configuration Setting!

{
  "racket-langserver": {
    "resyntax": {
      "enable": true
    },
    "formatting": {
      "documentFormatter": "fixw", // fixw | drracket | fmt
      "indentationFormatter": "fixw", // fixw | drracket
      // "fmtSettings": {
      //   "width": 91,
      //   "indent": 2,
      //   "maxBlankLines": 1
      // }
    }
  }
}

Settings apply to the whole language-server process. A racket-langserver section is a snapshot: omitted keys use shipped defaults, so removing formatter configuration returns both backends to fixw. Unknown values are ignored.

Resyntax

resyntax.enable turns Resyntax diagnostics and code actions on or off. It defaults to true. Resyntax is an optional package; if it is not installed, the server runs normally and produces no Resyntax suggestions.

Formatting

Name	Description	Allowed values
formatting.documentFormatter	Sets the formatter backend used for whole-document formatting (Format Document).	fixw(default), drracket, fmt
formatting.indentationFormatter	Sets the formatter backend used for range formatting (Format Selection) and on-type indentation.	fixw(default), drracket
formatting.fmtSettings	Optional settings passed to raco fmt when documentFormatter is fmt. Its width, indent, and maxBlankLines keys are nonnegative integers. Omitted keys use fmt's own defaults.	Object containing optional width, indent, and maxBlankLines integer properties

Standard LSP formatting options (tabSize, insertSpaces, and the trim/newline flags) are ignored by all backends.

The two formatter settings are independent, so a document can use fmt to reflow whole files while range and on-type requests still indent with drracket or fixw.

fmt is an optional package; install it with raco pkg install fmt before selecting it. If fmt is selected but unavailable for an eligible document, or if the raco fmt command fails, the request fails with an error instead of falling back to another backend.

fixw and fmt format only recognized s-expression languages. For other languages, document and range formatting automatically use drracket even when fixw or fmt is selected. drracket formats Scribble and languages outside the built-in language table when their reader publishes a usable drracket:indentation or drracket:range-indentation hook. A missing or failing hook produces no edits. Format on type remains limited to recognized s-expression languages, where the server can derive a safe local range.

Logging

Server diagnostics use Racket's built-in logger under the racket-langserver topic. Errors go to stderr by default. Enable debug logging with:

racket -W "error debug@racket-langserver" -l racket-langserver

Alternatively, set PLTSTDERR="error debug@racket-langserver" in the server's environment. Debug messages include document text received from the client. See Racket's logging documentation for details.

Development

See CONTRIBUTING.md.