ale/ale_linters/elixir/mix.vim

" Author: evnu - https://github.com/evnu
" Author: colbydehart - https://github.com/colbydehart

function! ale_linters#elixir#mix#Handle(buffer, lines) abort
    " Matches patterns like the following:
    "
    " Error format
    " ** (CompileError) apps/sim/lib/sim/server.ex:87: undefined function update_in/4
    "
    " TODO: Warning format
    " warning: variable "foobar" does not exist and is being expanded to "foobar()", please use parentheses to remove the ambiguity or change the variable name

    let l:pattern = '\v\(([^\)]+Error)\) ([^:]+):([^:]+): (.+)$'
    let l:output = []

    for l:match in ale#util#GetMatches(a:lines, l:pattern)
        let l:type = 'E'
        let l:text = l:match[4]

        call add(l:output, {
        \   'bufnr': a:buffer,
        \   'lnum': l:match[3] + 0,
        \   'col': 0,
        \   'type': l:type,
        \   'text': l:text,
        \})
    endfor

    return l:output
endfunction

function! ale_linters#elixir#mix#Command(buffer) abort
    let l:project_dir = fnamemodify(ale#path#FindNearestFile(a:buffer, 'mix.exs'), ':h')
    return  'cd ' . l:project_dir . ' && MIX_BUILD_PATH=/tmp/mix mix compile %s'
endfunction

call ale#linter#Define('elixir', {
\   'name': 'mix',
\   'executable': 'mix',
\   'command_callback': 'ale_linters#elixir#mix#Command',
\   'callback': 'ale_linters#elixir#mix#Handle',
\})