Alternative approach: calling mix directly

Instead of having a modified version of the xref mix task, we can just
call `mix` and parse the results

after/plugin/alchemist.vim

@@ -191,6 +191,24 @@ function! alchemist#exdoc(...)
     call s:open_doc_window(query, "new", "split")
 endfunction
 
+function! alchemist#exref(...)
+    let query = ''
+    if empty(a:000)
+        let query = alchemist#lookup_name_under_cursor()
+    else
+        let query = a:000[0]
+    endif
+    let result = alchemist#alchemist_client('XREF ' . query)
+
+    let errorformat = &errorformat
+    try
+        let &errorformat = '%A%f:%l: %m'
+        cex result
+    finally
+        let &errorformat = errorformat
+    endtry
+endfunction
+
 function! alchemist#exdef(...)
     let query = ''
     if empty(a:000)
@@ -439,6 +457,9 @@ command! -nargs=? -complete=customlist,elixircomplete#ExDocComplete ExDoc
 command! -nargs=? -complete=customlist,elixircomplete#ExDocComplete ExDef
       \ call alchemist#exdef(<f-args>)
 
+command! -nargs=? -complete=customlist,elixircomplete#ExDocComplete ExRef
+      \ call alchemist#exref(<f-args>)
+
 if !exists(':Mix')
   command! -bar -nargs=? -complete=custom,alchemist#mix_complete Mix
         \ call alchemist#mix(<q-args>)

alchemist.py

@@ -30,6 +30,22 @@ def __init__(self, **kw):
     def process_command(self, cmd, cmd_type=None):
         if cmd_type == None:
             cmd_type = cmd.split(" ")[0]
+
+        if cmd_type == 'XREF':
+            x = shlex.split('mix xref callers '+cmd.split(" ")[1])
+            return subprocess.check_output(x, cwd=self._cwd, universal_newlines=True)
+
+        sock = self._sock()
+        if cmd_type == 'COMPX':
+            result = self._send_compx(sock, cmd)
+        elif cmd_type == 'DEFLX':
+            result = self._send_deflx(sock, cmd)
+        else:
+            result = self._send_command(sock, cmd_type, cmd)
+
+        return result
+
+    def _sock(self):
         server_log = self._get_running_server_log()
         if server_log == None:
             server_log = self._create_server_log()
@@ -45,15 +61,7 @@ def process_command(self, cmd, cmd_type=None):
             self._run_alchemist_server(server_log)
             connection = self._extract_connection_settings(server_log)
             sock = self._connect(connection)
-
-        if cmd_type == 'COMPX':
-            result = self._send_compx(sock, cmd)
-        elif cmd_type == 'DEFLX':
-            result = self._send_deflx(sock, cmd)
-        else:
-            result = self._send_command(sock, cmd_type, cmd)
-
-        return result
+        return sock
 
     def _log(self, text):
         if self._debug == False: