Lua: allow to compare, show Citation values

Comparisons of Citation values are performed in Haskell; values are
equal if they represent the same Haskell value. Converting a Citation
value to a string now yields its native Haskell string representation.

src/Text/Pandoc/Lua/Marshaling/AST.hs

@@ -111,7 +111,18 @@ instance Pushable Block where
   push = pushBlock
 
 typeCitation :: LuaError e => DocumentedType e Citation
-typeCitation = deftype "Citation" []
+typeCitation = deftype "Citation"
+  [ operation Eq $ lambda
+    ### liftPure2 (==)
+    <#> parameter (optional . peekCitation) "Citation" "a" ""
+    <#> parameter (optional . peekCitation) "Citation" "b" ""
+    =#> functionResult pushBool "boolean" "true iff the citations are equal"
+
+  , operation Tostring $ lambda
+    ### liftPure show
+    <#> parameter peekCitation "Citation" "citation" ""
+    =#> functionResult pushString "string" "native Haskell representation"
+  ]
   [ property "id" "citation ID / key"
       (pushText, citationId)
       (peekText, \citation cid -> citation{ citationId = cid })

test/lua/module/pandoc.lua

@@ -481,6 +481,22 @@ return {
     end)
   },
   group 'Other types' {
+    group 'Citation' {
+      test('checks equality by comparing Haskell values', function()
+        assert.are_equal(
+          pandoc.Citation('a', pandoc.NormalCitation),
+          pandoc.Citation('a', pandoc.NormalCitation)
+        )
+        assert.is_falsy(
+          pandoc.Citation('a', pandoc.NormalCitation) ==
+          pandoc.Citation('a', pandoc.AuthorInText)
+        )
+        assert.is_falsy(
+          pandoc.Citation('a', pandoc.NormalCitation) ==
+          pandoc.Citation('b', pandoc.NormalCitation)
+        )
+      end),
+    },
     group 'SimpleTable' {
       test('can access properties', function ()
         local spc = pandoc.Space()