Add is_empty querying function

Author: tarleb

README.md

@@ -324,6 +324,14 @@ If `a` and `b` are `Doc` elements, then `a / b` puts `a` above `b`.
 If `a` and `b` are `Doc` elements, then `a // b` puts `a` above
 `b`, inserting a blank line between them.
 
+### Document Querying
+
+#### is_empty
+
+`is_empty (doc)`
+
+Returns `true` iff `doc` is the empty document, `false` otherwise.
+
 
 License
 -------

src/Foreign/Lua/Module/DocLayout.hs

@@ -48,9 +48,12 @@ module Foreign.Lua.Module.DocLayout (
   , space
   , vfill
 
-  -- * Functions
+  -- * Rendering
   , render
 
+  -- * Document Querying
+  , is_empty
+
   -- * Marshaling
   , peekDoc
   , pushDoc
@@ -109,6 +112,8 @@ pushModule = do
   Lua.addfunction "prefixed"   prefixed
   Lua.addfunction "rblock"     rblock
   Lua.addfunction "vfill"      vfill
+  -- querying
+  Lua.addfunction "is_empty"   is_empty
   -- rendering
   Lua.addfunction "render" render
   return 1
@@ -124,6 +129,15 @@ preloadModule = flip Lua.preloadhs pushModule
 render :: Doc Text -> Optional Int -> Lua Text
 render doc optLength = return $ Doc.render (Lua.fromOptional optLength) doc
 
+--
+-- Querying
+--
+
+-- | @True@ iff the document is empty.
+is_empty :: Doc Text -> Lua Bool
+is_empty = return . Doc.isEmpty
+
+
 --
 -- Constructors
 --

test/test-doclayout.lua

@@ -53,6 +53,14 @@ return {
     end)
   },
 
+  group 'document querying' {
+    test('is_empty', function ()
+      assert.is_truthy(doclayout.is_empty(doclayout.empty))
+      assert.is_truthy(doclayout.is_empty(''))
+      assert.is_falsy(doclayout.is_empty('non-empty'))
+    end)
+  },
+
   group 'Doc type' {
     test('empty strings equal the empty Doc', function ()
       assert.are_equal(doclayout.empty .. '', doclayout.empty)