flix.nvim

• Neovim support for Flix

Requirements

• Neovim 0.11+ — uses the native vim.lsp.config / vim.lsp.enable API introduced in 0.11.
• Java 21+ in your PATH (Flix runs on the JVM).
• flix.jar in your project root (or set an absolute path via jar, see Configuration).

Run :checkhealth flix to verify your setup.

Functionality

Flix leverages LSP for syntax highlighting via semantic tokens, which Neovim enables automatically when the server attaches — no extra configuration needed.

This plugin serves three main purposes:

1. create an LSP configuration for Flix
2. set the flix filetype and language defaults (see ftplugin/flix.lua)
3. Lua functions for interacting with the Flix CLI

Installation

Install with your plugin manager of choice using the repo flix/nvim.

Neovim 0.12 ships a built-in plugin manager, so no third party is required:

-- Neovim 0.12+ (vim.pack)
vim.pack.add({ "https://github.com/flix/nvim" })

require("flix").setup()
vim.lsp.enable("flix")

With lazy.nvim:

{
  "flix/nvim",
  ft = "flix",
  config = function()
    require("flix").setup()
    vim.lsp.enable("flix")
  end,
}

LSP

Call require("flix").setup() to register the config, then enable the server:

-- create the Flix LSP config
require("flix").setup()
-- enable the server
vim.lsp.enable("flix")

Configuration

setup() accepts an options table (all fields optional):

require("flix").setup({
  java = "java",              -- executable used to launch the jar
  jar = "flix.jar",           -- relative paths resolve from the project root
  root_markers = { "flix.toml" },
  features = {
    codelens = true,          -- refresh code lenses while editing
    completion = false,       -- native autocompletion (see below)
  },
  lsp = {                     -- merged into vim.lsp.config("flix", ...)
    -- settings = { ... },
  },
})

Native completion (Neovim 0.11+)

Set features.completion = true to get insert-mode autocompletion straight from the LSP — no nvim-cmp / blink.cmp required. Leave it false (the default) if you already drive completion with one of those plugins to avoid duplicate menus.

In Neovim 0.12 the built-in LSP client also supports inlineCompletion, onTypeFormatting, and linkedEditingRange. They will activate automatically when the Flix server advertises them (:help lsp for details).

Commands

Set local Flix keybindings with an autocommand or by creating ftplugin/flix.lua in your own config. The example below maps run/test for a Flix project:

-- import the `flix_cmd` function
local flix_cmd = require("flix.commands").flix_cmd
-- setting `buffer` keeps the mappings scoped to Flix buffers
local bufnr = vim.api.nvim_get_current_buf()

vim.keymap.set("n", "<Space>br", function() flix_cmd("run") end,
  { noremap = true, silent = true, buffer = bufnr, desc = "run flix project" })
vim.keymap.set("n", "<Space>bt", function() flix_cmd("test") end,
  { noremap = true, silent = true, buffer = bufnr, desc = "test flix project" })

flix_cmd finds the project root from root_markers, so it works even when the LSP is not attached, and reports an error if no flix.toml is found.

Full setup example (for lazy.nvim)

{
  "flix/nvim",
  ft = "flix",
  config = function()
    -- show lsp diagnostics by highlighting line numbers
    vim.diagnostic.config({
      signs = {
        text = {
          [vim.diagnostic.severity.ERROR] = "",
          [vim.diagnostic.severity.WARN] = "",
        },
        numhl = {
          [vim.diagnostic.severity.ERROR] = "ErrorMsg",
          [vim.diagnostic.severity.WARN] = "WarningMsg",
        },
      },
      severity_sort = true,
    })

    -- buffer-local LSP mappings
    vim.api.nvim_create_autocmd("LspAttach", {
      group = vim.api.nvim_create_augroup("my.lsp", {}),
      callback = function(args)
        local function get_opts(desc)
          return { desc = desc, buffer = args.buf, noremap = true, silent = true }
        end
        local client = assert(vim.lsp.get_client_by_id(args.data.client_id))
        if client:supports_method("textDocument/formatting") then
          vim.keymap.set("n", "<space>=", vim.lsp.buf.format, get_opts("format buffer"))
        end
        if client:supports_method("textDocument/rename") then
          vim.keymap.set("n", "<space>rn", vim.lsp.buf.rename, get_opts("rename"))
        end
        vim.keymap.set("n", "<space>ee", vim.diagnostic.open_float, get_opts("diagnostic open float"))
      end,
    })

    -- register the flix lsp config and enable it
    require("flix").setup()
    vim.lsp.enable("flix")
  end,
}