incorrect auto-indent behavior

Question

In some very specific scenarios, the auto indenter has almost unexplainable behavior that is very annoying. Example with haskell MultiWayIf syntax (cursor is at the "!"):

test
    | test
    | test = if
        |!

when I press space, this turns into:

test
    | test
    | test = if
                | !

I assume this is because it wants the pipe to be indented visually after the if keyword, because when I press enter here:

    | test = if!

it ends up here:

    | test = if
                !

but I do not want this behavior, and there should be some way to turn it off without turning off auto indentation entirely.

To be clear, the desired behavior would be this:

    | test = if!

(enter is pressed)

    | test = if
        !

to conform with the four spaces for every level indentation style I have set in my .vimrc (set tabstop=8 softtabstop=0 expandtab shiftwidth=4 smarttab ).

Answer 1

There are two options that can be set to run arbitrary programs to format text (code). When they aren't set, vim will work with lots of other (often complex, and often somewhat hacky) options to indent or format your text.

These options are equalprg (trigged by =) and formatprg (triggered by gq). Technically one is for indenting and the other for formatting, but since most formatting programs do both these days you may not care; pick the one you use the most.

It's not specified for equalprg, but the program you use for either (take input on standard in, write formatted output to standard out).

So in the case of haskell-formatter, you would do

" ~/.vim/after/indent/haskell.vim
setlocal equalprg=haskell-formatter
" or
setlocal formatprg=haskell-formatter

Then use = or gq to fix your code. You may find that some programs don't handle single lines too well; in that case, using =ap or gqap tends to work better. gg=G and gggqG take care of the whole file, but at that point it may be easier to run :%!haskell-formatter.

You could technically drop these lines in an ftplugin file, too, but this seems a bit cleaner to me.

An alternative, if you need to always reindent/reformat the whole file:

" ~/.vim/after/indent/haskell.vim
setlocal equalprg=haskell-formatter
" ~/.vim/after/ftplugin/haskell.vim
nnoremap <buffer> = :execute '%!' &l:equalprg<CR>
" or
nnoremap <buffer> == :execute '%!' &l:equalprg<CR>
" or
nnoremap <buffer> = gg=G
" ... you get the idea

You could similarly remap gq (or gqgq + gqq) and use &l:formatprg.

Make sure to set b:undo_ftplugin if you care about such things.

If you want to restore cursor position/etc., I would use winsaveview()/winrestview().