[luatex] Problem with mplib format in Lua script

[Nicola]

Hi,
I've got stuck with the following problem: running a Lua script that 
makes use of mplib (with the command 'luatex myscript.lua'). No TeX 
involved. First I've tried:

local mplib = require('mplib') 
local mp = mplib.new ({ ini_version = false })
if mp then
   print "MPLib instantiated!"
else
   print "MPLib not loaded :("
end

but mplib was not instantiated. Since I've installed mpost 1.208 and 
mplib uses 1.207, I thought there was a format mismatch. So, I've tried 
to generate a format file through mplib:

local mplib = require('mplib') 
local mp = mplib.new ({ ini_version = true, mem_name = 'plain' })
if mp then
   mp:execute('dump;')
   mp:finish();
end

This has generated plain.mem in the current directory (but it's only 
~13kb). But then:

local mplib = require('mplib') 
local mp = mplib.new ({ ini_version = false, mem_name = 'plain' })
if mp then 
   local l = mp:execute([[
         addto currentpicture doublepath (0,0)--(20,30);
         shipout currentpicture;
   ]]) 
   if l and l.status == 2 then
      print(l.log)
   end
   mp:finish(); 
end

has resulted in the following output, as if the plain macros were not 
loaded at all:

This is MetaPost, Version 1.207 (mem=plain 2009.12.20)  20 DEC 2009 11:51
>> (0
! Improper `addto'.
<to be read again> 
                   ,
<*>          addto currentpicture doublepath (0,
                                                0)--(20,30);
This expression should have specified a known path.
So I'll not change anything just now.

! Variable currentpicture is the wrong type (numeric).
<to be read again> 
                   ,
<*>          addto currentpicture doublepath (0,
                                                0)--(20,30);
I was looking for a "known" picture variable.
So I'll not change anything just now.

! Extra tokens will be flushed.
<to be read again> 
                   ,
<*>          addto currentpicture doublepath (0,
                                                0)--(20,30);
I've just read as much of that statement as I could fathom,
so a semicolon should have been next. It's very puzzling...
but I'll try to get myself back together, by ignoring
everything up to the next `;'. Please insert a semicolon
now in front of anything that you don't want me to delete.
(See Chapter 27 of The METAFONTbook for an example.)


>> currentpicture
! Not a known picture.
<to be read again> 
                   ;
<*>          shipout currentpicture;
                                    
I can only output known pictures.

I'm using TeX Live 2009 on Intel Mac OS X, and luatex  
beta-0.46.0-2009112820. Probably I am not dealing with file formats 
correctly. Any hint?

Nicola