Lua: marshal Attr values as userdata

- Adds a new `pandoc.AttributeList()` constructor, which creates the
  associative attribute list that is used as the third component of
  `Attr` values. Values of this type can often be passed to constructors
  instead of `Attr` values.

- `AttributeList` values can no longer be indexed numerically.

data/pandoc.lua

@@ -281,33 +281,12 @@ local function ensureDefinitionPairs (pair)
   return {inlines, blocks}
 end
 
---- Split a string into it's words, using whitespace as separators.
-local function words (str)
-  local ws = {}
-  for w in str:gmatch("([^%s]+)") do ws[#ws + 1] = w end
-  return ws
-end
-
 --- Try hard to turn the arguments into an Attr object.
 local function ensureAttr(attr)
-  if type(attr) == 'table' then
-    if #attr > 0 then return M.Attr(table.unpack(attr)) end
-
-    -- assume HTML-like key-value pairs
-    local ident = attr.id or ''
-    local classes = words(attr.class or '')
-    local attributes = attr
-    attributes.id = nil
-    attributes.class = nil
-    return M.Attr(ident, classes, attributes)
-  elseif attr == nil then
-    return M.Attr()
-  elseif type(attr) == 'string' then
-    -- treat argument as ID
-    return M.Attr(attr)
+  if type(attr) == 'userdata' then
+    return attr
   end
-  -- print(arg, ...)
-  error('Could not convert to Attr')
+  return M.Attr(attr)
 end
 
 ------------------------------------------------------------------------
@@ -831,134 +810,6 @@ M.Underline = M.Inline:create_constructor(
 -- Element components
 -- @section components
 
---- Check if the first element of a pair matches the given value.
--- @param x  key value to be checked
--- @return function returning true iff first element of its argument matches x
--- @local
-local function assoc_key_equals (x)
-  return function (y) return y[1] == x end
-end
-
---- Lookup a value in an associative list
--- @function lookup
--- @local
--- @tparam {{key, value},...} alist associative list
--- @param key key for which the associated value is to be looked up
-local function lookup(alist, key)
-  return (List.find_if(alist, assoc_key_equals(key)) or {})[2]
-end
-
---- Return an iterator which returns key-value pairs of an associative list.
--- @function apairs
--- @local
--- @tparam {{key, value},...} alist associative list
-local apairs = function (alist)
-  local i = 1
-  local cur
-  function nxt ()
-    cur = rawget(alist, i)
-    if cur then
-      i = i + 1
-      return cur[1], cur[2]
-    end
-    return nil
-  end
-  return nxt, nil, nil
-end
-
---- AttributeList, a metatable to allow table-like access to attribute lists
--- represented by associative lists.
--- @local
-local AttributeList = {
-  __index = function (t, k)
-    if type(k) == "number" then
-      return rawget(t, k)
-    else
-      return lookup(t, k)
-    end
-  end,
-
-  __newindex = function (t, k, v)
-    local cur, idx = List.find_if(t, assoc_key_equals(k))
-    if v == nil and not cur then
-      -- deleted key does not exists in list
-      return
-    elseif v == nil then
-      table.remove(t, idx)
-    elseif cur then
-      cur[2] = v
-    elseif type(k) == "number" then
-      rawset(t, k, v)
-    else
-      rawset(t, #t + 1, {k, v})
-    end
-  end,
-
-  __pairs = apairs
-}
-
---- Convert a table to an associative list. The order of key-value pairs in the
--- alist is undefined. The table should either contain no numeric keys or
--- already be an associative list.
--- @local
--- @tparam table tbl associative list or table without numeric keys.
--- @treturn table associative list
-local to_alist = function (tbl)
-  if #tbl ~= 0 or next(tbl) == nil then
-    -- probably already an alist
-    return tbl
-  end
-  local alist = {}
-  local i = 1
-  for k, v in pairs(tbl) do
-    alist[i] = {k, v}
-    i = i + 1
-  end
-  return alist
-end
-
--- Attr
-
---- Create a new set of attributes (Attr).
--- @function Attr
--- @tparam[opt] string       identifier element identifier
--- @tparam[opt] {string,...} classes    element classes
--- @tparam[opt] table        attributes table containing string keys and values
--- @return element attributes
-M.Attr = AstElement:make_subtype'Attr'
-function M.Attr:new (identifier, classes, attributes)
-  identifier = identifier or ''
-  classes = ensureList(classes or {})
-  attributes = setmetatable(to_alist(attributes or {}), AttributeList)
-  return setmetatable({identifier, classes, attributes}, self.behavior)
-end
-M.Attr.behavior.clone = M.types.clone.Attr
-M.Attr.behavior.tag = 'Attr'
-M.Attr.behavior._field_names = {identifier = 1, classes = 2, attributes = 3}
-M.Attr.behavior.__eq = utils.equals
-M.Attr.behavior.__index = function(t, k)
-  return (k == 't' and t.tag) or
-    rawget(t, getmetatable(t)._field_names[k]) or
-    getmetatable(t)[k]
-end
-M.Attr.behavior.__newindex = function(t, k, v)
-  if k == 'attributes' then
-    rawset(t, 3, setmetatable(to_alist(v or {}), AttributeList))
-  elseif getmetatable(t)._field_names[k] then
-    rawset(t, getmetatable(t)._field_names[k], v)
-  else
-    rawset(t, k, v)
-  end
-end
-M.Attr.behavior.__pairs = function(t)
-  local field_names = M.Attr.behavior._field_names
-  local fields = {}
-  for name, i in pairs(field_names) do
-    fields[i] = name
-  end
-  return make_next_function(fields), t, nil
-end
-
 -- Monkey-patch setters for `attr` fields to be more forgiving in the input that
 -- results in a valid Attr value.
 function augment_attr_setter (setters)

pandoc.cabal

@@ -776,6 +776,7 @@ library
                    Text.Pandoc.Lua.Init,
                    Text.Pandoc.Lua.Marshaling,
                    Text.Pandoc.Lua.Marshaling.AST,
+                   Text.Pandoc.Lua.Marshaling.Attr,
                    Text.Pandoc.Lua.Marshaling.CommonState,
                    Text.Pandoc.Lua.Marshaling.Context,
                    Text.Pandoc.Lua.Marshaling.List,

src/Text/Pandoc/Lua/Init.hs

@@ -89,7 +89,6 @@ putConstructorsInRegistry = liftPandocLua $ do
   constrsToReg $ Pandoc.Meta mempty
   constrsToReg $ Pandoc.MetaList mempty
   constrsToReg $ Pandoc.Citation mempty mempty mempty Pandoc.AuthorInText 0 0
-  putInReg "Attr"  -- used for Attr type alias
   putInReg "ListAttributes"  -- used for ListAttributes type alias
   putInReg "List"  -- pandoc.List
   putInReg "SimpleTable"  -- helper for backward-compatible table handling

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

@@ -1,4 +1,5 @@
 {-# OPTIONS_GHC -fno-warn-orphans #-}
+{-# LANGUAGE FlexibleInstances    #-}
 {-# LANGUAGE LambdaCase           #-}
 {-# LANGUAGE OverloadedStrings    #-}
 {-# LANGUAGE ScopedTypeVariables  #-}
@@ -40,6 +41,7 @@ import Control.Monad ((<$!>), (>=>))
 import HsLua hiding (Operation (Div))
 import Text.Pandoc.Definition
 import Text.Pandoc.Lua.Util (pushViaConstr', pushViaConstructor)
+import Text.Pandoc.Lua.Marshaling.Attr (peekAttr, pushAttr)
 import Text.Pandoc.Lua.Marshaling.List (pushPandocList)
 
 import qualified HsLua as Lua
@@ -413,19 +415,6 @@ peekInline = retrieving "Inline" . \idx -> do
     "Superscript"-> mkBlock Superscript peekInlines
     Name tag -> Lua.failPeek ("Unknown inline type: " <> tag)
 
-pushAttr :: forall e. LuaError e => Attr -> LuaE e ()
-pushAttr (id', classes, kv) = pushViaConstr' @e "Attr"
-  [ pushText id'
-  , pushList pushText classes
-  , pushList (pushPair pushText pushText) kv
-  ]
-
-peekAttr :: LuaError e => Peeker e Attr
-peekAttr = retrieving "Attr" . peekTriple
-  peekText
-  (peekList peekText)
-  (peekList (peekPair peekText peekText))
-
 pushListAttributes :: forall e. LuaError e => ListAttributes -> LuaE e ()
 pushListAttributes (start, style, delimiter) =
     pushViaConstr' "ListAttributes"
@@ -450,3 +439,6 @@ instance Peekable Meta where
 
 instance Peekable Pandoc where
   peek = forcePeek . peekPandoc
+
+instance {-# OVERLAPPING #-} Peekable Attr where
+  peek = forcePeek . peekAttr

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

@@ -0,0 +1,225 @@
+{-# LANGUAGE BangPatterns         #-}
+{-# LANGUAGE LambdaCase           #-}
+{-# LANGUAGE OverloadedStrings    #-}
+{-# LANGUAGE ScopedTypeVariables  #-}
+{-# LANGUAGE TupleSections        #-}
+{-# LANGUAGE TypeApplications     #-}
+{- |
+Module      : Text.Pandoc.Lua.Marshaling.Attr
+Copyright   : © 2012-2021 John MacFarlane
+              © 2017-2021 Albert Krewinkel
+License     : GNU GPL, version 2 or above
+
+Maintainer  : Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
+Stability   : alpha
+
+Marshaling/unmarshaling instances for document AST elements.
+-}
+module Text.Pandoc.Lua.Marshaling.Attr
+  ( typeAttr
+  , peekAttr
+  , pushAttr
+  , mkAttr
+  , mkAttributeList
+  ) where
+
+import Control.Applicative ((<|>), optional)
+import Control.Monad ((<$!>))
+import Data.Maybe (fromMaybe)
+import Data.Text (Text)
+import HsLua
+import HsLua.Marshalling.Peekers (peekIndexRaw)
+import Safe (atMay)
+import Text.Pandoc.Definition (Attr, nullAttr)
+import Text.Pandoc.Lua.Marshaling.List (pushPandocList)
+
+import qualified Data.Text as T
+
+typeAttr :: LuaError e => DocumentedType e Attr
+typeAttr = deftype "Attr"
+  [ operation Eq $ lambda
+    ### liftPure2 (==)
+    <#> parameter peekAttr "a1" "Attr" ""
+    <#> parameter peekAttr "a2" "Attr" ""
+    =#> functionResult pushBool "boolean" "whether the two are equal"
+  ]
+  [ property "identifier" "element identifier"
+      (pushText, \(ident,_,_) -> ident)
+      (peekText, \(_,cls,kv) -> (,cls,kv))
+  , property "classes" "element classes"
+      (pushPandocList pushText, \(_,classes,_) -> classes)
+      (peekList peekText, \(ident,_,kv) -> (ident,,kv))
+  , property "attributes" "various element attributes"
+      (pushAttribs, \(_,_,attribs) -> attribs)
+      (peekAttribs, \(ident,cls,_) -> (ident,cls,))
+  , method $ defun "clone"
+    ### return
+    <#> parameter peekAttr "attr" "Attr" ""
+    =#> functionResult pushAttr "Attr" "new Attr element"
+  ]
+
+pushAttr :: LuaError e => Pusher e Attr
+pushAttr = pushUD typeAttr
+
+peekAttribs :: LuaError e => Peeker e [(Text,Text)]
+peekAttribs idx = liftLua (ltype idx) >>= \case
+  TypeUserdata -> peekUD typeAttributeList idx
+  TypeTable    -> liftLua (rawlen idx) >>= \case
+    0 -> peekKeyValuePairs peekText peekText idx
+    _ -> peekList (peekPair peekText peekText) idx
+  _            -> fail "unsupported type"
+
+pushAttribs :: LuaError e => Pusher e [(Text, Text)]
+pushAttribs = pushUD typeAttributeList
+
+typeAttributeList :: LuaError e => DocumentedType e [(Text, Text)]
+typeAttributeList = deftype "AttributeList"
+  [ operation Eq $ lambda
+    ### liftPure2 (==)
+    <#> parameter peekAttribs "a1" "AttributeList" ""
+    <#> parameter peekAttribs "a2" "AttributeList" ""
+    =#> functionResult pushBool "boolean" "whether the two are equal"
+
+  , operation Index $ lambda
+    ### liftPure2 lookupKey
+    <#> udparam typeAttributeList "t" "attributes list"
+    <#> parameter peekKey "string|integer" "key" "lookup key"
+    =#> functionResult (maybe pushnil pushAttribute) "string|table"
+          "attribute value"
+
+  , operation Newindex $ lambda
+    ### setKey
+    <#> udparam typeAttributeList "t" "attributes list"
+    <#> parameter peekKey "string|integer" "key" "lookup key"
+    <#> optionalParameter peekAttribute "string|nil" "value" "new value"
+    =#> []
+
+  , operation Len $ lambda
+    ### liftPure length
+    <#> udparam typeAttributeList "t" "attributes list"
+    =#> functionResult pushIntegral "integer" "number of attributes in list"
+
+  , operation Pairs $ lambda
+    ### pushIterator (\(k, v) -> 2 <$ pushText k <* pushText v)
+    <#> udparam typeAttributeList "t" "attributes list"
+    =?> "iterator triple"
+
+  , operation Tostring $ lambda
+    ### liftPure show
+    <#> udparam typeAttributeList "t" "attributes list"
+    =#> functionResult pushString "string" ""
+  ]
+  []
+
+data Key = StringKey Text | IntKey Int
+
+peekKey :: LuaError e => Peeker e (Maybe Key)
+peekKey idx = liftLua (ltype idx) >>= \case
+  TypeNumber -> Just . IntKey <$!> peekIntegral idx
+  TypeString -> Just . StringKey <$!> peekText idx
+  _          -> return Nothing
+
+data Attribute
+  = AttributePair (Text, Text)
+  | AttributeValue Text
+
+pushAttribute :: LuaError e => Pusher e Attribute
+pushAttribute = \case
+  (AttributePair kv) -> pushPair pushText pushText kv
+  (AttributeValue v) -> pushText v
+
+-- | Retrieve an 'Attribute'.
+peekAttribute :: LuaError e => Peeker e Attribute
+peekAttribute idx = (AttributeValue <$!> peekText idx)
+  <|> (AttributePair <$!> peekPair peekText peekText idx)
+
+lookupKey :: [(Text,Text)] -> Maybe Key -> Maybe Attribute
+lookupKey !kvs = \case
+  Just (StringKey str) -> AttributeValue <$> lookup str kvs
+  Just (IntKey n)      -> AttributePair <$!> atMay kvs (n - 1)
+  Nothing              -> Nothing
+
+setKey :: forall e. LuaError e
+       => [(Text, Text)] -> Maybe Key -> Maybe Attribute
+       -> LuaE e ()
+setKey kvs mbKey mbValue = case mbKey of
+  Just (StringKey str) ->
+    case break ((== str) . fst) kvs of
+      (prefix, _:suffix) -> case mbValue of
+        Nothing -> setNew $ prefix ++ suffix
+        Just (AttributeValue value) -> setNew $ prefix ++ (str, value):suffix
+        _ -> failLua "invalid attribute value"
+      _  -> case mbValue of
+        Nothing -> return ()
+        Just (AttributeValue value) -> setNew (kvs ++ [(str, value)])
+        _ -> failLua "invalid attribute value"
+  Just (IntKey idx) ->
+    case splitAt (idx - 1) kvs of
+      (prefix, (k,_):suffix) -> setNew $ case mbValue of
+        Nothing -> prefix ++ suffix
+        Just (AttributePair kv) -> prefix ++ kv : suffix
+        Just (AttributeValue v) -> prefix ++ (k, v) : suffix
+      (prefix, []) -> case mbValue of
+        Nothing -> setNew prefix
+        Just (AttributePair kv) -> setNew $ prefix ++ [kv]
+        _ -> failLua $ "trying to set an attribute key-value pair, "
+             ++ "but got a single string instead."
+
+  _  -> failLua "invalid attribute key"
+  where
+    setNew :: [(Text, Text)] -> LuaE e ()
+    setNew new =
+      putuserdata (nthBottom 1) (udName @e typeAttributeList) new >>= \case
+        True -> return ()
+        False -> failLua "failed to modify attributes list"
+
+peekAttr :: LuaError e => Peeker e Attr
+peekAttr idx = retrieving "Attr" $ liftLua (ltype idx) >>= \case
+  TypeString -> (,[],[]) <$!> peekText idx -- treat string as ID
+  TypeUserdata -> peekUD typeAttr idx
+  TypeTable -> peekAttrTable idx
+  x -> liftLua . failLua $ "Cannot get Attr from " ++ show x
+
+-- | Helper function which gets an Attr from a Lua table.
+peekAttrTable :: LuaError e => Peeker e Attr
+peekAttrTable idx = do
+  len' <- liftLua $ rawlen idx
+  let peekClasses = peekList peekText
+  if len' > 0
+    then do
+      ident <- peekIndexRaw 1 peekText idx
+      classes <- fromMaybe [] <$!> optional (peekIndexRaw 2 peekClasses idx)
+      attribs <- fromMaybe [] <$!> optional (peekIndexRaw 3 peekAttribs idx)
+      return $ ident `seq` classes `seq` attribs `seq`
+        (ident, classes, attribs)
+    else retrieving "HTML-like attributes" $ do
+      kvs <- peekKeyValuePairs peekText peekText idx
+      let ident = fromMaybe "" $ lookup "id" kvs
+      let classes = maybe [] T.words $ lookup "class" kvs
+      let attribs = filter ((`notElem` ["id", "class"]) . fst) kvs
+      return $ ident `seq` classes `seq` attribs `seq`
+        (ident, classes, attribs)
+
+mkAttr :: LuaError e => LuaE e NumResults
+mkAttr = do
+  attr <- ltype (nthBottom 1) >>= \case
+    TypeString -> forcePeek $ do
+      mident <- optional (peekText (nthBottom 1))
+      mclass <- optional (peekList peekText (nthBottom 2))
+      mattribs <- optional (peekAttribs (nthBottom 3))
+      return (fromMaybe "" mident, fromMaybe [] mclass, fromMaybe [] mattribs)
+    TypeTable  -> forcePeek $ peekAttrTable (nthBottom 1)
+    TypeUserdata -> forcePeek $ peekUD typeAttr (nthBottom 1) <|> do
+      attrList <- peekUD typeAttributeList (nthBottom 1)
+      return ("", [], attrList)
+    TypeNil    -> pure nullAttr
+    TypeNone   -> pure nullAttr
+    x          -> failLua $ "Cannot create Attr from " ++ show x
+  pushAttr attr
+  return 1
+
+mkAttributeList :: LuaError e => LuaE e NumResults
+mkAttributeList = do
+  attribs <- forcePeek $ peekAttribs (nthBottom 1)
+  pushUD typeAttributeList attribs
+  return 1

src/Text/Pandoc/Lua/Module/Pandoc.hs

@@ -29,6 +29,7 @@ import Text.Pandoc.Lua.Filter (SingletonsList (..), walkInlines,
                                walkInlineLists, walkBlocks, walkBlockLists)
 import Text.Pandoc.Lua.Marshaling ()
 import Text.Pandoc.Lua.Marshaling.AST
+import Text.Pandoc.Lua.Marshaling.Attr (mkAttr, mkAttributeList)
 import Text.Pandoc.Lua.Marshaling.List (List (..))
 import Text.Pandoc.Lua.PandocLua (PandocLua, addFunction, liftPandocLua,
                                   loadDefaultModule)
@@ -54,6 +55,8 @@ pushModule = do
   addFunction "walk_inline" (walkElement peekInline pushInline)
   -- Constructors
   addFunction "Pandoc" mkPandoc
+  addFunction "Attr" (liftPandocLua mkAttr)
+  addFunction "AttributeList" (liftPandocLua mkAttributeList)
   return 1
 
 walkElement :: (Walkable (SingletonsList Inline) a,

test/Tests/Lua.hs

@@ -204,7 +204,7 @@ tests = map (localOption (QuickCheckTests 20))
                 [Para [Str "ignored"]])
       Lua.getfield Lua.top "attr"
       Lua.liftIO . assertEqual "no accessor" (("hi", ["moin"], []) :: Attr)
-        =<< Lua.peek Lua.top
+        =<< Lua.peek @Attr Lua.top
 
   , testCase "module `pandoc.system` is present" . runLuaTest $ do
       Lua.getglobal' "pandoc.system"

test/lua/module/pandoc.lua

@@ -11,34 +11,32 @@ end
 return {
   group 'Attr' {
     group 'Constructor' {
+      test('pandoc.Attr is a function', function ()
+        assert.are_equal(type(pandoc.Attr), 'function')
+      end),
       test('returns null-Attr if no arguments are given', function ()
         local attr = pandoc.Attr()
         assert.are_equal(attr.identifier, '')
         assert.are_same(attr.classes, {})
-        assert.are_same(attr.attributes, {})
+        assert.are_same(#attr.attributes, 0)
       end),
       test(
         'accepts string-indexed table or list of pairs as attributes',
         function ()
-          local attributes_list = pandoc.List:new {{'one', '1'}, {'two', '2'}}
-          local attr_from_list = pandoc.Attr('', {}, attributes_list:clone())
+          local attributes_list = {{'one', '1'}, {'two', '2'}}
+          local attr_from_list = pandoc.Attr('', {}, attributes_list)
 
-          assert.are_same(
-            pandoc.List:new(attr_from_list.attributes),
-            attributes_list
-          )
+          assert.are_equal(attr_from_list.attributes.one, '1')
+          assert.are_equal(attr_from_list.attributes.two, '2')
 
           local attributes_table = {one = '1', two = '2'}
           local attr_from_table = pandoc.Attr('', {}, attributes_table)
-
-          local assoc_list_from_table =
-            pandoc.List:new(attr_from_table.attributes)
-          -- won't work in general, but does in this special case
-          table.sort(assoc_list_from_table, function(x, y) return x[1]<y[1] end)
-          assert.are_same(
-            assoc_list_from_table,
-            attributes_list
+          assert.are_equal(
+            attr_from_table.attributes,
+            pandoc.AttributeList(attributes_table)
           )
+          assert.are_equal(attr_from_table.attributes.one, '1')
+          assert.are_equal(attr_from_table.attributes.two, '2')
         end
       )
     },
@@ -53,27 +51,41 @@ return {
         assert.are_same(attributes[1], {'a', '1'})
         assert.are_same(attributes[2], {'b', '2'})
       end),
+      test('allows replacing a pair', function ()
+        local attributes = pandoc.AttributeList{{'a', '1'}, {'b', '2'}}
+        attributes[1] = {'t','five'}
+        assert.are_same(attributes[1], {'t', 'five'})
+        assert.are_same(attributes[2], {'b', '2'})
+      end),
+      test('allows to remove a pair', function ()
+         local attributes = pandoc.AttributeList{{'a', '1'}, {'b', '2'}}
+         attributes[1] = nil
+         assert.are_equal(#attributes, 1)
+      end),
       test('adds entries by field name', function ()
         local attributes = pandoc.Attr('',{}, {{'c', '1'}, {'d', '2'}}).attributes
         attributes.e = '3'
         assert.are_same(
+          attributes,
           -- checking the full AttributeList would "duplicate" entries
-          setmetatable(attributes, nil),
-          {{'c', '1'}, {'d', '2'}, {'e', '3'}}
+          pandoc.AttributeList{{'c', '1'}, {'d', '2'}, {'e', '3'}}
         )
       end),
       test('deletes entries by field name', function ()
         local attributes = pandoc.Attr('',{}, {a = '1', b = '2'}).attributes
         attributes.a = nil
         assert.is_nil(attributes.a)
-        local assoc_list = setmetatable(attributes, nil)
-        assert.are_same(assoc_list, {{'b', '2'}})
+        assert.are_same(attributes, pandoc.AttributeList{{'b', '2'}})
       end),
       test('remains unchanged if deleted key did not exist', function ()
         local assoc_list = pandoc.List:new {{'alpha', 'x'}, {'beta', 'y'}}
-        local attributes = pandoc.Attr('', {}, assoc_list:clone()).attributes
+        local attributes = pandoc.Attr('', {}, assoc_list).attributes
         attributes.a = nil
-        assert.are_same(pandoc.List:new(attributes), assoc_list)
+        local new_assoc_list = pandoc.List()
+        for k, v in pairs(attributes) do
+          new_assoc_list:insert({k, v})
+        end
+        assert.are_same(new_assoc_list, assoc_list)
       end),
       test('gives key-value pairs when iterated-over', function ()
         local attributes = {width = '11', height = '22', name = 'test'}
@@ -110,6 +122,7 @@ return {
         }
         local span = pandoc.Span 'test'
         span.attr = html_attributes
+        span = span:clone() -- normalize
         assert.are_equal(span.attr.identifier, 'the-id')
         assert.are_equal(span.attr.classes[1], 'class1')
         assert.are_equal(span.attr.classes[2], 'class2')