data/pandoc.lua: make all types subtypes of AstElement

*Pandoc*, *Meta*, and *Citation* were just plain functions and did not
set a metatable on the returned value, which made it difficult to amend
objects of these types with new behavior. They are now subtypes of
AstElement, meaning that all their objects can gain new features when a
method is added to the behavior object (e.g., `pandoc.Pandoc.behavior`).

data/pandoc.lua

@@ -76,6 +76,14 @@ end
 -- @type AstElement
 -- @local
 local AstElement = Type:make_subtype 'AstElement'
+AstElement.__call = function(t, ...)
+  local success, ret = pcall(t.constructor, ...)
+  if success then
+    return setmetatable(ret, t.behavior)
+  else
+    error(string.format('Constructor for %s failed: %s\n', t.name, ret))
+  end
+end
 
 --- Create a new constructor
 -- @local
@@ -144,7 +152,7 @@ end
 --- Create a new element given its tag and arguments
 -- @local
 function AstElement.new(constr, ...)
-  local element = { t = constr.__type.name }
+  local element = {}
   local content = {...}
   -- special case for unary constructors
   if #content == 1 then
@@ -167,7 +175,8 @@ end
 -- @function Pandoc
 -- @tparam      {Block,...} blocks      document content
 -- @tparam[opt] Meta        meta        document meta data
-function M.Pandoc(blocks, meta)
+M.Pandoc = AstElement:make_subtype'Pandoc'
+function M.Pandoc.constructor (blocks, meta)
   meta = meta or {}
   return {
     ["blocks"] = List:new(blocks),
@@ -187,20 +196,18 @@ M.Doc = M.Pandoc
 --- `Meta`.
 -- @function Meta
 -- @tparam meta table table containing document meta information
-M.Meta = {}
-M.Meta.__call = function(t, meta)
-  return setmetatable(meta, t)
-end
-setmetatable(M.Meta, M.Meta)
+M.Meta = AstElement:make_subtype'Meta'
+M.Meta.constructor = function (meta) return meta end
 
 
 ------------------------------------------------------------------------
 -- MetaValue
 -- @section MetaValue
-M.MetaValue = AstElement:make_subtype{}
-M.MetaValue.__call = function(t, ...)
+M.MetaValue = AstElement:make_subtype('MetaValue')
+M.MetaValue.__call = function (t, ...)
   return t:new(...)
 end
+
 --- Meta blocks
 -- @function MetaBlocks
 -- @tparam {Block,...} blocks blocks
@@ -256,7 +263,7 @@ end
 
 --- Block elements
 -- @type Block
-M.Block = AstElement:make_subtype{}
+M.Block = AstElement:make_subtype'Block'
 M.Block.__call = function (t, ...)
   return t:new(...)
 end
@@ -431,7 +438,7 @@ M.Table = M.Block:create_constructor(
 
 --- Inline element class
 -- @type Inline
-M.Inline = AstElement:make_subtype{}
+M.Inline = AstElement:make_subtype'Inline'
 M.Inline.__call = function (t, ...)
   return t:new(...)
 end
@@ -794,7 +801,7 @@ end
 setmetatable(M.Attr, M.Attr)
 
 -- Citation
-M.Citation = {}
+M.Citation = AstElement:make_subtype'Citation'
 
 --- Creates a single citation.
 -- @function Citation
@@ -804,20 +811,16 @@ M.Citation = {}
 -- @tparam[opt] {Inline,...} suffix   citation suffix
 -- @tparam[opt] int          note_num note number
 -- @tparam[opt] int          hash  hash number
-M.Citation.__call = function(t, id, mode, prefix, suffix, note_num, hash)
-  return setmetatable(
-    {
-      id = id,
-      mode = mode,
-      prefix = prefix or {},
-      suffix = suffix or {},
-      note_num = note_num or 0,
-      hash = hash or 0,
-    },
-    t
-  )
+function M.Citation.constructor (id, mode, prefix, suffix, note_num, hash)
+  return {
+    id = id,
+    mode = mode,
+    prefix = prefix or {},
+    suffix = suffix or {},
+    note_num = note_num or 0,
+    hash = hash or 0,
+  }
 end
-setmetatable(M.Citation, M.Citation)
 
 
 ------------------------------------------------------------------------