Prototype for functions in configuration

autoload/langserver/default.vim

@@ -6,16 +6,19 @@ function! langserver#default#cmd() abort
   let l:bad_cmd = [-1]
 
   if has_key(g:langserver_executables, &filetype)
-      let l:tmp_cmd = g:langserver_executables[&filetype]['cmd']
-
-      if type(l:tmp_cmd) != type([])
-        echoerr 'Make sure your dictionary is structued like: {"filetype": {"cmd": [cmd, list, here]}}'
-        return l:bad_cmd
-      endif
+    let l:tmp_cmd = g:langserver_executables[&filetype]['cmd']
 
+    if type(l:tmp_cmd) == type([])
       return l:tmp_cmd
-    return 
-  else
-    return l:bad_cmd
+    elseif type(l:tmp_cmd) == type(function('tr'))
+      let l:result = l:tmp_cmd()
+      if type(l:result) == type([])
+        return l:result
+      endif
+    endif
   endif
+
+  " If we didn't return anything, there was an error.
+  echoerr 'Please consult the documentation for how to configure the langserver'
+  return l:bad_cmd
 endfunction