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