lua语言基础（4）闭包closuer

1. 回调一般发生在digitbutton函数执行完之后，那个时候局部变量digit已经超出了作用范围，但closuer仍可以访问。

function digitButton (digit)

return Button{ label = digit,

action = function ()

add_to_display(digit)

end

}

end

2. Hook原来的函数

do

local oldSin = math.sin

local k = math.pi/180

math.sin = function (x)

return oldSin(x*k)

end

end

do

local oldOpen = io.open

io.open = function (filename, mode)

if access_OK(filename, mode) then

return oldOpen(filename, mode)

else

return nil, "access denied"

end

  end

end

3. lua的尾调用可以防止堆栈溢出

function foo (n)

if n > 0 then return foo(n - 1) end

end

不是：

function f (x)

g(x)

return

end

return g(x) + 1 -- must do the addition

return x or g(x)-- must adjust to 1 result

return (g(x))   -- must adjust to 1 result

4 . 一个例子：实现状态机

function room1 ()

local move = io.read()

if move == "south" then return room3()

elseif move == "east" then return room2()

else print("invalid move")

 return room1()   -- stay in the same room

  end

end

function room2 ()

local move = io.read()

  if move == "south" then return room4()

  elseif move == "west" then return room1()

else print("invalid move")

 return room2()

  end

end

function room3 ()

local move = io.read()

  if move == "north" then return room1()

  elseif move == "east" then return room4()

else print("invalid move")

 return room3()

  end

end

function room4 ()

  print("congratulations!")

end