Built a language server for learning purposes

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Educational project

I was always intrigued by how IDEs provide smart completions when you press certain keys, how the diagnostics hit right, and all the mechanics they provide behind the scenes.

So I decided to dive into the process behind it: the Language Server Protocol. I also wanted to learn Elixir and ended up building an educational language server in Elixir.

While this is educational, I have implemented some basic functionalities regarding English writing:

  1. Shows the definition of the word under the cursor by calling vim.lsp.buf.hover() (default to K in Neovim).
  2. Sends an error diagnostic if the document contains VS Code.
  3. Suggests a list of words for completion.
  4. Suggests some code actions: replace VS Code with Neovim or censor it with VS C*de.

Any feedback, especially on the Elixir part, is welcome. I’m looking to get better in that language.

Architecture

  1. There are two GenServer: InputServer and LSPServer. They are both initialized in the module Application by a Supervisor.
                                    ┌──────────────┐ 
                               ┌───►│  InputServer │ 
                               │    └──────────────┘ 
                               │                     
                               │                     
                               │                     
  ┌─────────────┐  initialize  │                     
  │ Application ├──────────────┤                     
  │  Supervisor │              │                     
  └─────────────┘              │                     
                               │                     
                               │                     
                               │    ┌───────────┐    
                               └───►│ LSPServer │    
                                    └───────────┘
  1. This is where the life cycle starts: InputServer listens on stdio.
    2.1. InputServer sends that message to LSPServer.
    2.2. LSPServer processes it and sends back a result to InputServer.
                                     
                                     
           message                   
  ┌──────┐         ┌─────────────┐   
  │client├────────►│ InputServer │   
  └──────┘         └───┬────▲────┘   
                       │    │ result 
                       │    │        
                       │    │        
               convey  │    │        
                    ┌──▼────┴───┐    
                    │ LSPServer │    
                    └───────────┘
  1. InputServer sends the result back to the client and then loops back to listening on stdio.
                                                               
                                                               
                                              ready to listen  
                                            ┌──────────┐       
                                            │          │       
                                            │          │       
                                            ▼          │       
                               ┌───────────────┐       │       
   ┌────────┐  result          │               ├───────┘       
   │ client │◄─────────────────┤  InputServer  │               
   └────────┘                  │               │               
                               └───────────────┘

Notes

I have encountered two challenges that I haven’t solved in the Elixir way.

When and how to properly shut down the application?

I have set two System.stop() at two different occasions, which I found redundant:

  • when the client sends the shutdown request,
  • when :eof is received after listening with IO.read(:stdio, :line).

Process still runs in the background if a crash happens

In the life cycle of InputServer, if a crash happens (example String.split(nil, "\n")),
the process is still alive even after having closed Neovim, probably a zombie?
The only fix I have found was finding the source of the issue by checking the logs and correcting it.

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