Re: destructors

[Rici Lake <lua@...>]

On 23-Sep-06, at 1:59 AM, Glenn Maynard wrote:
> I think Lua's GC scheme isn't capable of actually implementing dtors
> efficiently, but how do people deal with this?  I can't just force a
> complete GC cycle; aside from performance, that's just the "close"
> problem in another form.

Garbage collectors aren't really designed to handle this problem.

Here's something I've been playing with. I'm not crazy about the extra 
argument to the protected function; I could have gotten around that by 
stashing the finalizers in the environment (since I can reset the 
finalizers array on the way out) but that's not guaranteed to work if 
people mess around with function environments.

----

local function collect(finalizers, ind, ...)
  for i = #finalizers, 1, -1 do pcall(finalizers[i]) end
  return ind, ...
end

local function addf(self, func, targ)
  self[#self+1] = function() func(targ) end
end

function protect(f, ...)
  local finalizers = {add = addf}
  return collect(finalizers, pcall(f, finalizers, ...))
end


function close(x) print("Closing: ", x) end

function doSomeWorkBadly(finally, a, b)
  local c = "some precious resource"
  finally:add(close, c)
  print("a+3 is ", a+3)
  local d = "another precious resource"
  finally:add(close, d)
  print("b+3 is ", b+3)
  finally:add(print, "Almost done")
  return a+b
end