Using Neovim as a Fennel Compiler

3 December 2022

I’ve been doing Advent of Code this year in Fennel. (I’m updating my answers every day in a GitHub repo.) I use a few different computers day to day and I need to setup each computer with the proper Lua libraries and Fennel compiler. For speed, I want to run the Fennel compiler under LuaJIT and I also want to have access to the inspect.lua library for debugging. I got everything working alright on MacOS but on Linux, Fennel wasn’t detecting luarocks libraries. That’s when I realized I don’t actually need an extra program to compile and run Fennel. I already have everything I need built directly into Neovim.

Running Fennel scripts through Neovim gives me access to all of Neovim’s builtin APIs. This is a fantastic environment for debugging and development since a lot of helper functions already exist. For example vim.inspect implements the same functionality as inspect.lua and Neovim even provides a short hand for printing vim.inspect’s results using vim.pretty_print.

I already partly setup Neovim as a Fennel compiler in a previous blog post. The implementation I originally came up with worked well for compiling a file to output but what if I want to write that output to stdout instead? Naively, I initially thought I could just use the print() function.

command("FnlCompile", function(t)
    if debug.traceback ~= fennel.traceback then
        debug.traceback = fennel.traceback

    local in_path, out_path = unpack(vim.fn.split(t.args, " "))
    assert(in_path, "missing input path")

    local stream = open_stream(in_path)
    local out = fnl_compile(stream)

    if out_path ~= nil then
        local file = assert(, "w"))
        -- if no file path is provided lets just print the contents
end, { nargs = 1 })
FENNEL_COMPILE=true nvim --headless -c "FnlCompile test.fnl" +q

But then I ran into an issue: print doesn’t print to stdout when this command is run. No matter how I redirected the output, I couldn’t get the it to print to stdout. A quick search, helped me find this GitHub issue. I found that, you have to explicitly write your output to /dev/stdout. My fix was to create a helper function for printing to stdout.

local function print_stdout(message)
    message = vim.fn.split(message, "\n")
    vim.fn.writefile(message, "/dev/stdout")

Now I can print the results of compilation directly to stdout.

I also need to setup a way to run individual Fennel scripts in the Neovim environment. The Fennel compiler API includes a function for evaluation of Fennel files. I can expose this function using a simple Neovim command.

command("FnlRun", function(t)
    local in_path = unpack(vim.fn.split(t.args, " "))
    assert(in_path, "missing input path")

    local out = fennel.dofile(in_path)
    -- print an extra newline to seperate script output from from the printed return values
end, { nargs = 1 })

Now I can run a Fennel script using:

FENNEL_COMPILE=true nvim --headless -c "FnlRun test.fnl" +q

This provides a very nice DX. My goal for using Fennel in Advent of Code is to practice writing future Neovim plugins. I no longer have to setup Fennel, Lua, or luarocks. I have my entire compile and run cycle going through Neovim which I already have installed on all my systems.