Automagic_Tables.lua

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--   AUTOMAGIC TABLES
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-- There is a well-known "standard" implementation of automagic tables at http://lua-users.org/wiki/AutomagicTables
--
-- Unfortunately, that implementation behaves weirdly in some situations:
--    local a = AutomagicTable()
--    local x = a.b.c
--    local y = a.b.c
--    assert(x == y)  -- assertion fails !
--
-- This version of "AutomagicTable()" improves this behavior by storing weak references to all created objects.
--
-- Please see "usage example" at the end of this file to understand the benefit of new behavior.


do
   local weak, create = {__mode = "kv"}

   local function get(tab, key)
      local c = getmetatable(tab).content
      local v = c[key]
      if v == nil then
         v = create(nil, tab, key)
         c[key] = v
      end
      return v
   end

   local function set(tab, key, val)
      if val ~= nil then
         rawset(tab, key, val)
         local m = getmetatable(tab)
         m.content[key] = nil
         local p = m.parent_tab
         if p then
            p[m.parent_key] = tab
            m.parent_tab, m.parent_key = nil
         end
      end
   end

   function create(tab, parent_tab, parent_key)
      return
         setmetatable(tab or {}, {
            __index = get,
            __newindex = set,
            parent_tab = parent_tab,
            parent_key = parent_key,
            content = setmetatable({}, weak)
         })
   end

   function AutomagicTable(tab)
      return create(tab)
   end
end

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--- USAGE EXAMPLE
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--- This is some sort of DSL config file :-)
--- It is actually a Lua script which runs in "automagic" global environment
AutomagicTable(_G)
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----- (beginning of config file) --------------
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--- The following options are adjustable by user
table.insert(shopping.list, "'Programming in Lua' book")
today = weekdays.Friday                     --<== get a reference to empty object
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--- User should not change anything below this line
weekdays.Monday.beverage    = "beer"
weekdays.Tuesday.beverage   = "beer"
weekdays.Wednesday.beverage = "beer"
weekdays.Thursday.beverage  = "beer"
weekdays.Friday.beverage    = "vodka"       --<== fill the object with some data
-- Please note that the following line does not work as expected
-- if we are using standard implementation of automagic tables
table.insert(shopping.list, today.beverage) --<== access the object using the reference
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----- (end of config file) --------------------
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--- Now let's check the result:
print(table.concat(shopping.list, ", "))
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