Close #2281 - Separate cwd commands from commands

Working directories are now set seperately from the commands so they
can later be swapped out when running linters over projects is
supported, and also better support filename mapping for running linters
on other machines in future.

ale_linters/elm/make.vim

@@ -186,20 +186,19 @@ function! ale_linters#elm#make#IsTest(buffer) abort
     endif
 endfunction
 
+function! ale_linters#elm#make#GetCwd(buffer) abort
+    let l:root_dir = ale_linters#elm#make#GetRootDir(a:buffer)
+
+    return !empty(l:root_dir) ? l:root_dir : ''
+endfunction
+
 " Return the command to execute the linter in the projects directory.
 " If it doesn't, then this will fail when imports are needed.
 function! ale_linters#elm#make#GetCommand(buffer) abort
     let l:executable = ale_linters#elm#make#GetExecutable(a:buffer)
-    let l:root_dir = ale_linters#elm#make#GetRootDir(a:buffer)
     let l:is_v19 = ale_linters#elm#make#IsVersionGte19(a:buffer)
     let l:is_using_elm_test = l:executable =~# 'elm-test$'
 
-    if empty(l:root_dir)
-        let l:dir_set_cmd = ''
-    else
-        let l:dir_set_cmd = 'cd ' . ale#Escape(l:root_dir) . ' && '
-    endif
-
     " elm-test needs to know the path of elm-make if elm isn't installed globally.
     " https://github.com/rtfeldman/node-test-runner/blob/57728f10668f2d2ab3179e7e3208bcfa9a1f19aa/README.md#--compiler
     if l:is_v19 && l:is_using_elm_test
@@ -213,7 +212,9 @@ function! ale_linters#elm#make#GetCommand(buffer) abort
     " a sort of flag to tell the compiler not to generate an output file,
     " which is why this is hard coded here.
     " Source: https://github.com/elm-lang/elm-compiler/blob/19d5a769b30ec0b2fc4475985abb4cd94cd1d6c3/builder/src/Generate/Output.hs#L253
-    return l:dir_set_cmd . '%e make --report=json --output=/dev/null' . l:elm_test_compiler_flag . '%t'
+    return '%e make --report=json --output=/dev/null'
+    \   . l:elm_test_compiler_flag
+    \   . '%t'
 endfunction
 
 function! ale_linters#elm#make#GetExecutable(buffer) abort
@@ -235,6 +236,7 @@ call ale#linter#Define('elm', {
 \   'name': 'make',
 \   'executable': function('ale_linters#elm#make#GetExecutable'),
 \   'output_stream': 'both',
+\   'cwd': function('ale_linters#elm#make#GetCwd'),
 \   'command': function('ale_linters#elm#make#GetCommand'),
 \   'callback': 'ale_linters#elm#make#Handle'
 \})