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The __gc closure you create inside tryThis will have inherited the global environment from tryThis at the time it was executed, so it's going to have a reference to that environment, but I'm not sure that would be a reachable reference. 

Daniel.

Matthew Armstrong wrote:
I had already tried the setfenv(tryThis, {}) idea, and it didn't work. After your mail, I also tried tryThis = nil, which also didn't work. Here is my non-trimmed code: 

function tryThis()
    myVar = GfcTimer()
    local mt = getmetatable(myVar)
    mt.__gc = function()
        print("!!! cleanup")
        ldb()
    end

    --myVar = nil
end

function yup()
    local env = {}
    setmetatable(env, { __index = _G })
    setfenv(tryThis, env)

    tryThis()

    setfenv(tryThis, {})
    tryThis = nil
    env = nil

    collectgarbage()
end

yup()
Note: there is one external function, GfcTimer, which generates a userdata object from our library. 

If I uncomment the line:

-- myVar = nil

... then I get a confirmed call to the __gc method.

Can you see anything else which would maintain a reference?
On Tue, Mar 18, 2008 at 5:55 PM, Roberto Ierusalimschy <roberto@inf.puc-rio.br <mailto:roberto@inf.puc-rio.br>> wrote: 

    > So let's say I say:
    >
    > function tryThis()
    >     myVar = { foo="baz" }
    > end
    >
    > local env = {}
    > setfenv(tryThis, env)
    > tryThis()
    >
    > My question is: when does myVar get collected?  With my current
    testing, it
    > seems that myVar never gets collected.  If so, how do I
    "convince" it to be
    > collected?

    'env' is the environment of 'tryThis' (which is global), so it
    cannot be
    collected until tryThis is garbage or until you change its environment
    again. You may try "tryThis = nil" or "setfenv(tryThis, {})".

    -- Roberto