Langoustine - write Language Servers in Scala 3

... and use them with Javascript, JVM, or even native libraries

Status as of April 15th, 2026: updated to latest and greatest, API is stabilising

• API documentation
• SBT: libraryDependencies += "tech.neander" %%% "langoustine-app" % "0.1.0"
• Mill: ivy"tech.neander::langoustine-app::0.1.0"
• Scala CLI //> using lib "tech.neander::langoustine-app::0.1.0"

What is it?

It's a clean room implementation of the LSP protocol definitions.

By "clean room" we mean

1. Using only Scala libraries
2. Idiomatic Scala code
3. Using Scala 3 features

Most of the code is generated directly from the recently published LSP specification in JSON format.

Is there a simple example?

Introducing Quickmaffs, a primitive language with a LSP, REPL, and an interpreter, designed specifically to demonstrate how easy it is to build Language Servers with Langoustine.

What can I use it for?

Writing a language server for:

1. your own toy language

2. already existing language but with specific requirements

   For example, see Grammar.js LSP - written specifically for the grammar.js files in the Tree Sitter grammars.

   It uses the Scala.js artifact of this project, because it's easier to parse JavaScript using a JavaScript library and package the whole server as a Node.js application.

3. markup languages and protocol files, think

   1. Certain YAML files (LSP with verification for Github Actions YAML files!)
   2. Avro files
   3. Protobuf files
   4. Smithy files (jk a great one already exists)
   5. LLVM IR text files (for all those compiler engineers!)
   6. Scala Native's NIR files
   7. loads and loads more

Even basic Go To Definition implementation for the files you work with for hours on a daily basis can have an immeasurable impact on your productivity.

Do I need to write a custom extension for my editor to talk to my LSP server ?

Depends on how amazing you want the UX to be really, but if you're a lazy sloth like we are, head over the following list :

• neovim
• vscode
• intellij (no commits in a few years, will need a brave soul to step-up)
• emacs
• sublime