function! s:getValaProjectRoot(buffer) abort
    let l:cur_path_abs = fnamemodify(a:buffer, ':p:h')
    let l:maybe_doap_file = expand(l:cur_path_abs . s:sep . '*.doap')
    if !empty(l:maybe_doap_file)
        return l:cur_path_abs
    endif

    for l:path in ale#path#Upwards(expand('#' . a:buffer . ':p:h'))
        if filereadable(expand(l:path . '*.doap'))
            return l:path
        endif
    endfor

    return ''
endfunction

function! s:curPath(buffer) abort
    let l:cur_path = fnamemodify(a:buffer, ':p:h')
    return l:cur_path
endfunction

call ale#linter#Define('vala', {
\   'name': 'vala-language-server',
\   'lsp': 'stdio',
\   'output_stream': 'both',
\   'executable': '/usr/bin/vala-language-server',
\   'command': '%e',
\   'project_root': function('s:getValaProjectRoot'),
\   'callback': 'ale#handlers#unix#HandleAsWarning',
\})

call ale#linter#Define('promql', {
\   'name': 'promql-langserver',
\   'lsp': 'stdio',
\   'output_stream': 'both',
\   'executable': '/usr/local/bin/promql-langserver',
\   'command': '%e',
\   'project_root': function('s:curPath'),
\   'callback': 'ale#handlers#unix#HandleAsWarning',
\})