Originally published on my blog.

TL;DR: vim-ale is awesome and you should give a try!

Introduction: the rise of linters

I've already blogged about linters and how I use them when I write Python code.

Let's recap:

I have a shell script call lint.sh which calls several linters. One for the style only, and several for static analysis.

#!/bin/bash -xe

pycodestyle .
python bin/run-pyflakes.py
python bin/run-mccabe.py 10
pylint mymodule

Note that the script runs the fast linters first (pycodestyle and pyflakes are pretty fast), and ends by running pylint, which can get pretty slow on big code bases.

I use: :set makeprg=./lint.sh
I type: :make when I'm done writing code.
I wait for script to finish (Neovim can't be used for editing while the makeprg program is running)
I use the quickfix window to fix lint errors.

This works quite well for me when I'm using Python because:

I know the code style pretty well so I rarely have style errors
For me running pylint is as slow as running tests, and I'm used to have tests running synchronously too.
I like using the quickfix window (thanks to the very nice vim-qf plug-in by the way)

But some time ago at work, we all start working on some Javascript code.

We use eslint for code style, flow for type checking, and mocha plus chai for tests.

And since we're using GitLab for hosting our source code, we of course have a .gitlab-ci.yml file to run all of these for each merge request.

Shortcomings of the makeprg approach

At first I tried using the same work flow as with Python, but I quickly face some issues.

Finding the binaries to run

Knowing which binary to use for the linting program can be hard. Sometimes you can use the one that is in your $PATH, but sometimes you want the binary that is in node_modules/.bin

Learning the style convention

Our eslint configuration is quite strict. (It's based on the one from Air B&B if you're curious), and I'm not used to Javascript yet, so I constantly had style issues and it was frustrating to have my merge requests blocked because I forgot to run :make and had a bunch of style problems.

The errorformat

For Neovim to populate the quickfix window, it has to parse the output of the linters programs.

By default, eslint output looks like this

$ eslint foo.js
/path/to/foo.js
  1:5   error  'foo' is assigned a value but never used          no-unused-vars
  3:10  error  'bar' is defined but never used                   no-unused-vars
  5:1   error  Expected indentation of 1 tab but found 2 spaces  indent

✖ 3 problems (3 errors, 0 warnings)
  2 errors, 0 warnings potentially fixable with the `--fix` option.

It's very pretty (and I did not even show you the nice colors!), but you can see it's a bit hard to parse for other programs. ¹

The solution is to use the --format option like this:

$ eslint --format compact foo.js
/path/to/foo.js: line 1, col 5, Error - 'foo' is assigned a value but never used. (no-unused-vars)
/path/to/foo.js: line 3, col 10, Error - 'bar' is defined but never used. (no-unused-vars)
/path/to/foo.js: line 5, col 1, Error - Expected indentation of 1 tab but found 2 spaces. (indent)

3 problems

But by default Neovim expects an output exactly like gcc²

$ gcc foo.c
foo.c:3:3: error: ‘a’ undeclared (first use in this function)
foo.c:4:3: warning: implicit declaration of function ‘foo’ [-Wimplicit-function-declaration]

(No 'line' and 'col' , and no commas)

So in order for the line numbers and columns to be parsed correctly you have to use a variable called errorformat

let errorformat =
        \ '%f:%l:%c: %trror: %m,' .
        \ '%f:%l:%c: %tarning: %m,' .
        \ '%f:%l:%c: %tote: %m'

It's pretty hard to get right, even if you read the documentation (chapter 7, the "The Error Format")

Running synchronously

Last but not least, all the linters ran quite fast (even flow, because it uses a background server under the hood).

So it was also painful to have the editor unresponsive for the few seconds it took for all the linters to run.

Switching to vim-ale

After hearing good things about vim-ale, I decided to give it a try, and I was very pleased to notice it was able to run eslint and flow in our Javascript project with zero configuration!

This is how vim-ale works by default:

First, get the current filetype (This is done by Neovim itself ³), in our case javascript
Then, look for files in the matching ale_linters folder to get a list of potential linters.
Then, after a small time of inactivity (200ms by default), run all the linters found in the previous step, concurrently and in the background.
Then, parse the output of all of them and use the sign gutter to display problematic lines, like this:

Notice the red angle brackets in the column on the left, and the detailed message on the bottom.

Also, by default the whole gutter will disappear when there’s nothing found by the linters, which allows you to instantly know you’re done fixing lint errors.

Simple customization

After reading vim-ale documentation, I was able to tweak a few details:

I prefer having the linters run only after I save the file, not continuously, otherwise I find it too distracting:

let g:ale_lint_on_text_changed = 'never'

I also don't like the linters to run right after I open a file:

let g:ale_lint_on_enter = 0

The location list

By default, vim-ale uses the so called location list. It looks pretty much like the quickfix window, but:

There's one location list per window, whereas the quickfix is global.
The location list is destroyed as soon as the window is closed
You can use the same bindings to navigate the location list, using a l prefix instead of a c. (For instance lprevious and lnext instead of cprevious and cnext)

At first I was tempted to configure vim-ale to use the quickfix window instead (which can be done by setting the g:ale_set_quickfix variable), as I was doing for Python, but I've decided to try and let vim-ale use the location list for several reasons:

I've heard about the location list for quite some time, but I'd never actually use it, so I was curious and wanted to know if it could fit in my workflow.
I already used the quickfix window with :grep and other tools and I did not like having it replaced by vim-ale all the time.

Custom mappings

I was already used to type :cp, :cn and :cr (:cprevious, :cnext and :crewind) to navigate the quickfix list, but it turned out it was harder to use their location list counterparts:

:ln cannot be used for :lnext because it actually is used as a shortcut for :lnoremap
I use the Dvorak layout on my keyboard, and typing ln, lp or lr is much harder to type than cn, cp or cr there.

Now, Neovim has this concept of "leader" key you can use to prefix all your custom mappings.

I'm using "comma" for this, instead of the default (backslash), that I find hard to type.

So of course I could have used a <leader> mapping for the location list command, but it unfortunately <leader>p, <leader>n, and <leader>r were already taken.

That's when I remember someone suggesting using "space" as a leader key, and so I used:

nnoremap <space>l :lnext<CR>
nnoremap <space>p :lprevious<CR>
nnoremap <space>r :lrewind<CR>

Note: this means I can no longer easily use the space bar in normal mode, but by default all the space key does in move the cursor forward, so it's not a big deal.

Back to Python

As I describer earlier, vim-ale is pretty smart, and comes pre-configured with the pylint and pyflake linters.

This means that every time I was saving code in a Python file, vim-ale would find pylint and flake in the virtualenv or in the PATH, and run them with the default options.

It's not what I wanted, but it was easy to turn in off:

let g:ale_linters = {
  \ 'python': [] ,
  \ }

(Configuration of the ale_linters dictionary is a bit weird: it assumes you want to run all the linters for every filetype, so you have to define some kind of "blacklist" to turn off the linters you don't want to run)

But by then I started to miss having the style being checked after each save.

(This is what happens when you start using great tools: you wish you can use them all the time ⁴)

Unfortunately, vim-ale did not know about pycodestyle so I had to patch the code of the plug-in.

Adding a vim-ale linter

In our case the pycodestyle output looks like this:

$ pycodestyle foo.py
foo.py:4:1: E302 expected 2 blank lines, found 1

There's the filename, the line, the column, a short code (E302 in the example), and a detailed message (expected 2 blank lines, found 1)

So I started adding a new file in code in ale_linters/python/pycodestyle.vim

Here are the relevant parts:

" By default, use `pycodestyle` from path, unless it is set
" in the `g;ale_python_pycodestyle_executable` variable:
let g:ale_python_pycodestyle_executable =
\   get(g:, 'ale_python_pycodestyle_executable', 'pycodestyle')

" Look for pycodestyle in the virtualenv, and only if not found
" in it, default to the previously set pycodestyle executable:
function! ale_linters#python#pycodestyle#GetExecutable(buffer) abort
  ...
  if !ale#Var(a:buffer, 'python_pycodestyle_use_global')
    let l:virtualenv = ale#python#FindVirtualenv(a:buffer)

    if !empty(l:virtualenv)
      let l:ve_pycodestyle = l:virtualenv . '/bin/pycodestyle'

      if executable(l:ve_pycodestyle)
        return l:ve_pycodestyle
      endif
    endif

  endif

  return ale#Var(a:buffer, 'python_pycodestyle_executable')

endfunction

" Get the full command line to run: append contents from the
" python_pycodestyle_options variable if it exists:
function! ale_linters#python#pycodestyle#GetCommand(buffer) abort
    return fnameescape(ale_linters#python#pycodestyle#GetExecutable(a:buffer))
    \   . ' ' . ale#Var(a:buffer, 'python_pycodestyle_options')
    \   . ' %s'
endfunction

" Register pycodestyle as a linter for the `python` filetype:
call ale#linter#Define('python', {
\   'name': 'pycodestyle',
\   'executable_callback': 'ale_linters#python#pycodestyle#GetExecutable',
\   'command_callback': 'ale_linters#python#pycodestyle#GetCommand',
\   'callback': 'ale#handlers#python#HandlePyCodeStyle',
...
\})

You'll notice everything is done with callbacks to get the path to the
executable, the full command line, and how to parse the output.

And here's the implementation of the pycodestyle output parser.

function! ale#handlers#python#HandlePyCodeStyle(buffer, lines) abort
    " Regular expression to match messages:
    " They look like:
    " <file>:<line>:<col>: <code> <detailed text>
    let l:pattern = '\v^[a-zA-Z]?:?[^:]+:(\d+):?(\d+)?: \[?([[:alnum:]]+)\]? (.*)$'

    " For each match, update the l:output list:
    let l:output = []
    for l:match in ale#util#GetMatches(a:lines, l:pattern)
        let l:code = l:match[3]

        call add(l:output, {
        \   'lnum': l:match[1] + 0,
        \   'col': l:match[2] + 0,
        \   'text': l:code . ': ' . l:match[4],
        \})
    endfor

    return l:output
endfunction

You'll note it's done by writing "real" code (it's still vimscript, though) with helpers functions, instead of trying to set a weird errorformat option, with hard to read syntax.

And that's all there is to it. All I had to do was to set the ale_linters dictionary to only run pycodestyle:

let g:ale_linters = {
  \ 'python': [] ,
  \ }

and then I could keep using :make and the quickfix window for the slower linters like pylint like I used to :)

Conclusion

vim-ale requires very little configuration to work out of the box. The asynchronous features are really nice (and they work both in Neovim and Vim 8).
It has tons of customization options, and last but not least, new linters are easy to add and customize.

I highly recommend you give it a try.

Cheers!

Thanks for reading this far :)

I'd love to hear what you have to say, so please feel free to leave a comment below, or read the feedback page for more ways to get in touch with me.

There's a whole rant to be written about the console output of Javascript tools, but let's save that for another day ... ↩
There's a bunch of code in Vim that assumes you are writing C code, even today (historical reasons, as they say) ↩
It's done in 2793 lines of vim script ↩
Regular readers of this blog will notice this is exactly what happened to me with fzf ↩

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Dimitri Merejkowsky