Table Funcs

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The flip side of tables which look like functions is functions which look like tables.

  local meta = {__call = function(t, ...)
                           return t.__function(t, unpack(arg))

  function Tablefunc(fn)
    return setmetatable({__function = fn}, meta)

The result of Tablefunc is a table with call semantics, but the function has an implicit first argument which is its own table, in which it can store persistent state.

I know what you're thinking. So what? This is just a standard object call without the key. And what if I want private persistent state? OK, take a look at TableFuncsTwo

Meanwhile, here is a quick and silly example:

repeater = Tablefunc(
  function(self, n, str)
    -- I've got persistent state variables
    self.times_called = (self.times_called or 0) + 1
    -- I can get at the base function and even change it
    -- so it will do something different next time
    if self.times_called >= 99 then
      self.__function =
        function() return "Sorry, I'm tired of repeating myself" end
    -- I can use the table for configuration, and
    -- the functable itself is self, so I can recurse
    if n == 0 then return ""
      elseif n == 1 then return str
      else return str .. (self.delim or ", ") .. self(n - 1, str)

The obligatory sample run:

$ lua
Lua 5.0 (beta)  Copyright (C) 1994-2002 Tecgraf, PUC-Rio
> -- I can get at the state variables from here, too
> repeater.delim = "; "
> print(repeater(7, "hello"))
hello; hello; hello; hello; hello; hello; hello
> print(repeater.times_called)
> _ = repeater(90, "hello")
> print(repeater.times_called)
> print(repeater(7, "hello"))
hello; hello; Sorry, I'm tired of repeating myself


Another implementation:
function wrap(fn)
  local t = {}
  return function(...) return fn(t, ...) end, t
or this:

local states = setmetatable({}, {__mode = "kv"})
function addstate(fn)
  local t = {}
  local fn2 = function(...) return fn(t, ...) end
  states[fn2] = t
  return fn2
function getstate(fn)
  return states[fn]

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Last edited May 28, 2007 4:15 pm GMT (diff)