[Dev-luatex] new/missing feature in luamplib

Hans Hagen pragma at wxs.nl
Tue Aug 7 10:12:44 CEST 2012

On 7-8-2012 09:29, Keith J. Schultz wrote:
> Hi Manuel, Everybody,
> I am just getting into LuaLaTeX.
> I needed to do some work with MetaPost.
> While working I came across a kudo. That is once inside the mblibcode
> environment one can not call Lua. THIS I consider a missing feature and
> a VERY important one.

Some (simple) support for lua calls is on the (long term) agenda but not 
before the current mplib extensions project is done (i.e. the decimal 
calculation plugins).

Calling lua in itself is not so much an issue; after all one can just do 
a \directlua inside a blob of mp passed to mplib (I do that all the 
time). For an lua 'primitive' in mplib (which then probably would just 
be called just 'lua') we first need to figure out:

1 - how to access mp internals (and that relates to the mentioned 
decimal extensions, as we might need to get and set)
2 - if and how we can share the state with the (main) tex lua instance
3 - how we want to print back to mp (like tex.[s]print)

It's one of those feature we have discussed (maybe too) long ago, but 
because it has to be more than just (3) it never ended up first on the 
todo list (first finishing the main lib).

The problem with these things is that it has to be mostly 'right from 
the start' in order not to conflict too much with long term usage).

Anyhow, if you play with such things, best have something

   lua "mp.print(123)"

Given mp's somewhat different parsing and expansion compared to tex 
there are also some conceptual issues involved. It also relates to input 
buffers (and file reading) and io is somewhat messy in mp and on the 
agenda for cleaning up.

(Luigi Scarso has done this with metafont, so that might be a starting 
point for experimenting.)


                                           Hans Hagen | PRAGMA ADE
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