Re: local scope once again

[Rici Lake <lua@...>]

On 12-Dec-06, at 10:42 AM, Leo Razoumov wrote:

> Hi Everyone,
> I am puzzled by the behaviour of the following code:
>
> function foo()
>  local x= 1
>  local x= 20
>  local x= 300

<snip>

> I am surprised to see that there are three different local 'x'
> variables. All of them belong to the same "block" which is foo()
> function body and, thus, should have the same scope. Why does not Lua
> collapses them into just one local x variable??

Because they are three different variables which happen
to have the same name. (Some might consider that bad style.)

You don't need the debug library to see this:

function foo()
  local x = 1
  local function f1(inc)
    local old = x
    if inc then x = x + inc end
    return old
  end
  local x = 20
  local function f2(inc)
    local old = x
    if inc then x = x + inc end
    return old
  end
  local x = 300
  local function f3(inc)
    local old = x
    if inc then x = x + inc end
    return old
  end
  return f1, f2, f3
end

> x1, x2, x3 = foo()
> =x1(1), x2(1), x3(1)
1       20      300
> =x1(-2), x2(-2), x3(-2)
2       21      301
> =x1(), x2(), x3()
0       19      299


A slightly more interesting example:

function counters(...)
  local funcs = {}
  for i, init in ipairs{...} do
    funcs[i] = function(inc)
      local old = init
      if inc then init = init + inc end
      return old
    end
  end
  return unpack(funcs)
end

x1, x2, x3 = counters(1, 20, 300)
> =x1(1), x2(1), x3(1)
1       20      300
> =x1(-2), x2(-2), x3(-2)
2       21      301
> =x1(), x2(), x3()
0       19      299
>