CODEWITHSUNDEEP
lua
oroszgy
oroszgy

Reputation: 123

What is the scope of a Lua variable?

I would be really interested why this peace of code works at all and prints the same tables.

t = {

    f = function()
        return t
    end

}

print(t)
print(t.f())

My assumption was that t is only accessible after its definition, thus return t should fail. However this code seems to contradict.

Upvotes: 3

Views: 133

Answers (1)

Mankarse
Mankarse

Reputation: 40613

Since there is no local variable t in your example, your function accesses the global value t (that is: _ENV.t). When your function is called, it accesses the current value of the _ENV variable, then indexes into the "t" index of that _ENV value. Even though the _ENV table does not contain t when the function is defined, when it is later called, it does contain t, and so you are able to access your newly defined table.

local u = {
    f = function()
        -- Refers to the global variable `t` (that is, _ENV.t).
        -- Whenever the function is called, the *current* value of `t` is returned
        return t 
    end
}

print(u) -- prints table: 0xFOOBAR
print(u.f()) -- prints nil

t = u

print(t) --prints table: 0xFOOBAR
print(t.f()) --prints table: 0xFOOBAR

_ENV = {print = print}
print(u.f()) -- prints nil, since _ENV.t is now nil

local _ENV = {print = print, t = u}
print(u.f()) -- still prints nil, because the function definition used the
             -- _ENV variable that was lexically visible at the function definition,
             -- rather than the new _ENV variable that was just defined.

The use of _ENV means that these examples are only valid for Lua 5.2, but the same principle applies for Lua 5.1.

Upvotes: 4

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