3.5 – Visibility Rules

Lua is a lexically scoped language. The scope of a local variable begins at the first statement after its declaration and lasts until the last non-void statement of the innermost block that includes the declaration. Consider the following example:

x = 10                -- global variable     do                    -- new block       local x = x         -- new 'x', with value 10       print(x)            --> 10       x = x+1       do                  -- another block         local x = x+1     -- another 'x'         print(x)          --> 12       end       print(x)            --> 11     end     print(x)              --> 10  (the global one)

Notice that, in a declaration like local x = x, the new x being declared is not in scope yet, and so the second x refers to the outside variable.

Because of the lexical scoping rules, local variables can be freely accessed by functions defined inside their scope. A local variable used by an inner function is called an upvalue, or external local variable, inside the inner function.

Notice that each execution of a local statement defines new local variables. Consider the following example:

a = {}     local x = 20     for i=1,10 do       local y = 0       a[i] = function () y=y+1; return x+y end     end

The loop creates ten closures (that is, ten instances of the anonymous function). Each of these closures uses a different y variable, while all of them share the