data/pandoc.lua: enable table-like behavior of attributes (#4080)

Attribute lists are represented as associative lists in Lua. Pure
associative lists are awkward to work with. A metatable is attached to
attribute lists, allowing to access and use the associative list as if
the attributes were stored in as normal key-value pair in table.

Note that this changes the way `pairs` works on attribute lists. Instead
of producing integer keys and two-element tables, the resulting iterator
function now returns the key and value of those pairs.  Use `ipairs` to
get the old behavior.

Warning: the new iteration mechanism only works if pandoc has been
compiled with Lua 5.2 or later (current default: 5.3).

The `pandoc.Attr` function is altered to allow passing attributes as
key-values in a normal table. This is more convenient than having to
construct the associative list which is used internally.

Closes #4071

1 files changed, 6 insertions, 0 deletions

diff --git a/test/lua/attr-test.lua b/test/lua/attr-test.lua
new file mode 100644
index 000000000..68dc0012d
--- /dev/null
+++ b/test/lua/attr-test.lua
@@ -0,0 +1,6 @@
+function Div (div)
+  div.attributes.five = ("%d"):format(div.attributes.two + div.attributes.three)
+  div.attributes.two = nil
+  div.attributes.one = "eins"
+  return div
+end