Writing Neovim plugin in Haskell

Paul Meng

What's neovim

  • A fork of vim and pioneered the supports for the long-wanted features. (then we have vim 8.0)
  • Features include async io, RPC architecture, job-control, integrated term.
  • Modernize the C code for vim. Vim has been notorious for legacy and bad code.
  • Unlike the contribution to vim, the initiator of neovim makes the contribution env much more friendly

RPC architecture

  • The main functions of the features run in another process.
  • Using Msgpack binary protocol to communicate with the plugin process.
  • Abstract the communication with "channel"
  • That means you can write the plugin in any programming language you prefer
  • VimL is only for minimal glue.

Neovim API

  • The official promises to maintain the python library binding while the active development
  • However, you can use nvim --api-info to dump the api info in msgpack format
nvim --api-info | python -c 'import msgpack, sys, yaml; print yaml.dump(msgpack.unpackb(sys.stdin.read()))'
  Exception: {id: 0}
  Validation: {id: 1}
- async: false
  can_fail: true
  name: buffer_line_count
  - [Buffer, buffer]
  return_type: Integer

Haskell Wrapper

REPL in ghci

  • Start the nvim by specifying NVIM_LISTEN_ADDRESS
NVIM_LISTEN_ADDRESS=/tmp/nvim nvim
  • Then start ghci by setting the env var
NVIM_LISTEN_ADDRESS=/tmp/nvim stack ghci
λ Right (tids, cfg) <- develMain Nothing

λ runNeovim' cfg $ vim_call_function "getqflist" []
 Right (Right (ObjectArray []))

Launching the plugin

  • Make a shell script and put the stack exec in it

    PATH=`stack path --bin-path` stack exec nvim-hs-emoji-exe -- "$@"
  • Let neovim start the process so that the channel is established
  • Neovim provides rpcstart function call
  • remote#host#Register and remote#host#Reqreui are VimL help function

Launching the plugin

if has('nvim') " This way you can also put it in your vim config file
  call remote#host#Register('fibonacci', "*", rpcstart('/Users/mno2/Develop/haskell/nvim-fib/nvim-hs-devel.sh'))
  let haskellChannel = remote#host#Require('fibonacci')

  if haskellChannel < 1
    echom 'Failure to initialize the haskell channel for remote procedure calls'

Fibonacci Function

  • Get the number N under the cursor, calculate the Nth Fibonacci number.
  • Using Neovim monad, which is data Neovim r st a
module Fibonacci.Plugin (fibonacci) where

import Neovim

fibonacci :: Neovim r st String
fibonacci = do
  cw <- errOnInvalidResult $ vim_call_function "expand" $ [toObject "<cword>"]
  let n = read cw
  return $ show (fibs !! n)
    fibs :: [Integer]
    fibs = 0:1:scanl1 (+) fibs

Autocomplete Emoji

  • Vim has a built-in omnifunc mechanism for autocompletion
  • Type some chars and press Ctrl-X Ctrl-O
  • You implemented a function with the following spec
  1. Two params: findstart and base
  2. It would be first call with findstart as 1 and base as empty
  3. Then it would be call with findstart as 0 and base as the prefix

Autocomplete Emoji

emojicomplete :: Bool -> String -> Neovim r st (Either Int [String])
emojicomplete findstart base = do
  if findstart
     then do
       curr_line :: String <- errOnInvalidResult $ vim_call_function "getline" $ [toObject "."]
       curr_col :: Int <- errOnInvalidResult $ vim_call_function "col" $ [toObject "."]
       let prefix = take (curr_col-1) curr_line
       let reverse_idx = fromMaybe 0 (L.findIndex (== ' ') (reverse prefix))
       return $ Left (curr_col - reverse_idx - 1)
     else do
       let emoji = [":thumbsup:", ":thumbsdown:", ":smile:", ":banana:"]
       let ans = filter (L.isPrefixOf base) emoji
       return $ Right ans

Thank you