Luaチュートリアル(10)。グローバル変数と非グローバル環境

-- This table is used to store the names of all the global variables that have been declared
local declaredNames = {}
local mt = {
    __newindex = function(table,name,value)
        -- First check if the new name has already been declared, and if it exists, this is set directly via the rawset function.
        if not declaredNames[name] then
            -- Check again if this operation was done in the main program or C code, and if so, continue to set it, otherwise report an error.
            local w = debug.getinfo(2,"S").what
            if w ~= "main" and w ~= "C" then
                error("attempt to write to undeclared variable " . name)
            end
            -- Update the declaredNames table before you actually set it, so you don't have to check it the next time you set it.
            declaredNames[name] = true
        end
        print("Setting " . name ... " to " ... value)
        rawset(table,name,value)
    end,
    __index = function(_,name)
        if not declaredNames[name] then
            error("attempt to read undeclared variable " . name)
        else
            return rawget(_,name)
        end
    end
}   
setmetatable(_G,mt)

a = 11
local kk = aa

-- The output is.
--[[
Setting a to 11
lua: d:/test.lua:21: attempt to read undeclared variable aa
stack traceback:
        [C]: in function 'error'
        d:/test.lua:21: in function <d:/test.lua:19>
        d:/test.lua:30: in main chunk
        [C]: ?
--]]

a = 1
setfenv(1,{})
print(a)

-- The output is.
--[[
lua: d:/test.lua:3: attempt to call global 'print' (a nil value)
stack traceback:
        d:/test.lua:3: in main chunk
        [C]: ?
--]]

a = 1
local newgt = {} -- New environment table
setmetatable(newgt,{__index = _G})
setfenv(1,newgt)
print(a) -- output 1

a = 10
print(a) -- output 10
print(_G.a) -- output 1
_G.a = 20
print(a) -- output 10

function factory()
    return function() return a end
end
a = 3
f1 = factory()
f2 = factory()
print(f1()) - output 3
print(f2()) --output 3

setfenv(f1,{a = 10})
print(f1()) -- output 10
print(f2()) -- output 3