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On Sat, Feb 14, 2009 at 2:02 PM, Peter Hickman <> wrote:
I have some code that I want to use to add an invert method to tables.

table.invert = function (t)
       local i = {}
       for k in pairs(t) do
               i[t[k]] = true
       return i

When I call it as "table.invert(a)" it works fine, but it errors when I call it as "a:invert()"

lua: invert.lua:15: attempt to call method 'invert' (a nil value)

My understanding was that a:invert() was just sugar on table.invert(a), what am I doing wrong?

a:invert() is actually sugar for a.invert(a), which will only be equivalent to table.invert(a) if you have a metatable set on 'a' which looks up the 'table' library when __index is called.

The situation is different with strings because all string values share a common metatable, while tables each individually have their own metatable, which by default is nil. Therefore tables will never inherit any methods as soon as they are created, something has to be done to provide them.