Merge pull request #11470 from bfredl/emptytable

metatable for empty dict value

runtime/doc/lua.txt

@@ -717,6 +717,16 @@ vim.NIL								    *vim.NIL*
 	is equivalent to a missing value: `{"foo", nil}` is the same as 
 	`{"foo"}`
 
+vim.empty_dict()					    *vim.empty_dict()*
+	Creates a special table which will be converted to an empty
+	dictionary when converting lua values to vimL or API types. The
+	table is empty, and this property is marked using a metatable. An
+	empty table `{}` without this metatable will default to convert to
+	an array/list.
+
+	Note: if numeric keys are added to the table, the metatable will be
+	ignored and the dict converted to a list/array anyway.
+
 vim.rpcnotify({channel}, {method}[, {args}...])		    *vim.rpcnotify()*
 	Sends {event} to {channel} via |RPC| and returns immediately.
 	If {channel} is 0, the event is broadcast to all channels.

runtime/lua/vim/inspect.lua

@@ -244,6 +244,11 @@ function Inspector:putTable(t)
 
     local nonSequentialKeys, nonSequentialKeysLength, sequenceLength = getNonSequentialKeys(t)
     local mt                = getmetatable(t)
+    if (vim and sequenceLength == 0 and nonSequentialKeysLength == 0
+        and mt == vim._empty_dict_mt) then
+      self:puts(tostring(t))
+      return
+    end
 
     self:puts('{')
     self:down(function()

runtime/lua/vim/shared.lua

@@ -275,9 +275,15 @@ function vim.tbl_flatten(t)
 end
 
 -- Determine whether a Lua table can be treated as an array.
+--
+-- An empty table `{}` will default to being treated as an array.
+-- Use `vim.emtpy_dict()` to create a table treated as an
+-- empty dict. Empty tables returned by `rpcrequest()` and
+-- `vim.fn` functions can be checked using this function
+-- whether they represent empty API arrays and vimL lists.
 ---
 --@params Table
---@returns true: A non-empty array, false: A non-empty table, nil: An empty table
+--@returns true: An array-like table, false: A dict-like or mixed table
 function vim.tbl_islist(t)
   if type(t) ~= 'table' then
     return false
@@ -296,7 +302,12 @@ function vim.tbl_islist(t)
   if count > 0 then
     return true
   else
-    return nil
+    -- TODO(bfredl): in the future, we will always be inside nvim
+    -- then this check can be deleted.
+    if vim._empty_dict_mt == nil then
+      return nil
+    end
+    return getmetatable(t) ~= vim._empty_dict_mt
   end
 end