Unmarshal empty strings as empty Docs

Author: tarleb

src/Foreign/Lua/Module/DocLayout.hs

@@ -236,7 +236,10 @@ docTypeName = "HsLua DocLayout.Doc"
 -- converted to plain @'Doc'@ values.
 peekDoc :: StackIndex -> Lua (Doc Text)
 peekDoc idx = Lua.ltype idx >>= \case
-  Lua.TypeString   -> Doc.literal <$> Lua.peek idx
+  Lua.TypeString   -> let stringToDoc s = if T.null s
+                                          then Doc.empty
+                                          else Doc.literal s
+                      in stringToDoc <$> Lua.peek idx
   Lua.TypeNumber   -> Doc.literal <$> Lua.peek idx
   _                -> Lua.reportValueOnFailure docTypeName
                         (`Lua.toAnyWithName` docTypeName)

test/test-doclayout.lua

@@ -34,6 +34,10 @@ return {
   },
 
   group 'Doc type' {
+    test('empty strings equal the empty Doc', function ()
+      assert.are_equal(doclayout.empty .. '', doclayout.empty)
+    end),
+
     test('strings can be used as Doc values', function ()
       assert.are_equal(doclayout.render('hello world'), 'hello world')
     end),