Re: Custom extensions to Lua

[Rici Lake <lua@...>]

On 10-Aug-05, at 2:57 PM, Rici Lake wrote:

> In fact, you could do it without patching the Lua core at all, using 
> only a preprocessor ( :) ), by representing a box as a table with one 
> distinguished key. The transformation would turn
>
>   a <- *a + 7
>
> into
>
>   a.value = a.value + 7
>
> or some variation on that theme. Mind you, you couldn't call the 
> preprocessor input Lua either, but you could certainly call it a 
> "language which can be compiled into Lua".

Just in case it amuses someone other than me:

do
  local function donothing() end
  local function show(_, k, v)
    if type(v) == "string" then
      v = string.format("%q", v)
    else
      v = tostring(v)
    end
    print(string.format("Global `%s' is being changed to %s",
                        tostring(k), v))
  end

  function ref(tab, key, preset, postset)
    preset, postset = preset or donothing, postset or donothing
    return setmetatable({}, {
      __index = function() return tab[key] end,
      __newindex = function(_, _, new)
                     preset(tab, key, new)
                     tab[key] = new
                     postset(tab, key, new)
                   end
    })
  end

  -- reference global by name, assuming we have the
  -- same environment table as our caller
  function gref(name, preset, postset)
    return ref(getfenv(), name, preset, postset)
  end

  function traceref(name)
    return gref(name, show)
  end
end

> a = "Hello"
> b = gref "a"
> c = traceref "a"
> b.value = b.value .. ", world"
> print(a)
Hello, world
> c.value = string.gsub(c.value, ", world", "")
Global `a' is being changed to "Hello"
> print(a)
Hello