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 same x
.