A language server that provides smart code completion, diagnostics, hover, document symbols, definition lookup, method signature help and more for Kotlin.

Any editor conforming to LSP is supported, including VSCode and Atom.

• See BUILDING.md for build instructions
• See Editor Integration for editor-specific instructions
• See Roadmap for features, planned additions, bugfixes and changes
• See Kotlin Quick Start for a sample project
• See Kotlin Debug Adapter for debugging support on JVM
• See tree-sitter-kotlin for an experimental Tree-Sitter grammar

The original author created this project while he was considering using Kotlin in his work. He ended up deciding not to and is not really using Kotlin these days though this is a pretty fully-functional language server that just needs someone to use it every day for a while and iron out the last few pesky bugs.

There are two hard parts of implementing a language server:

• Figuring out the dependencies
• Incrementally re-compiling as the user types

The project uses the internal APIs of the Kotlin compiler.

Dependencies are determined by the findClassPath function, which invokes Maven or Gradle and tells it to output a list of dependencies. Currently, both Maven and Gradle projects are supported.

I get incremental compilation at the file-level by keeping the same KotlinCoreEnvironment alive between compilations in Compiler.kt. There is a performance benchmark in OneFilePerformance.kt that verifies this works.

Getting incremental compilation at the expression level is a bit more complicated:

• Fully compile a file and store in CompiledFile:
  • val content: String A snapshot of the source code
  • val parse: KtFile The parsed AST
  • val compile: BindingContext Additional information about the AST from typechecking
• After the user edits the file:
  • Find the smallest section the encompasses all the user changes
  • Get the LexicalScope encompassing this region from the BindingContext that was generated by the full-compile
  • Create a fake, in-memory .kt file with just the expression we want to re-compile
    • Add space at the top of the file so the line numbers match up
  • Re-compile this tiny fake file

The incremental expression compilation logic is all in CompiledFile.kt. The Kotlin AST has a built-in repair API, which seems to be how IntelliJ works, but as far as I can tell this API does not work if the surrounding IntelliJ machinery is not present. Hence I created the "fake tiny file" incremental-compilation mechanism, which seems to be quite fast and predictable.

There is an extensive suite of behavioral tests, which are all implemented in terms of the language server protocol, so you should be able to refactor the code any way you like and the tests should still work.

The Kotlin language server supports some non-standard requests through LSP. See KotlinProtocolExtensions for a description of the interface. The general syntax for these methods is kotlin/someCustomMethod.

• georgewfraser
• fwcd