Re: Lua plans to add meta-programming support?

[steve donovan <steve.j.donovan@...>]

On Sat, Mar 10, 2012 at 8:08 AM, Xavier Wang <weasley.wx@gmail.com> wrote:
> Now we are discussing some semantics proposals in Lua. I believe that
> the main solution is makes Lua easy to meta-programming. i.e., add
> some hooks to llex, makes Lua can callbacks on lexing).

We did have the token-filter patch by lhf, which did pretty much what
you suggested. I based the first version of LuaMacro on top of it, but
requiring a patched Lua is irritating, so the current version uses
LPeg to do the tokenizing [1]  (The tricky part is preserving original
line information so that error messages and error tracebacks make
sense)

These are lexically-scoped macros, so they don't contaminate the whole
global namespace.  For instance, consider replacing mytable[#+1] with
mytable[#mytable+1] which was suggested recently.  That is, you can
write code like this:

 require_ 'rawhash'

 Tab mytable, another

  t = {1,3}

  -- Here # is short for #mytable
 mytable[#+1] = 1
 mytable[#+1] = 2

 -- without indexing, behaves just like a table reference
 assert(type(mytable)=='table')

 -- it is still possible to use #t explicitly
 assert(mytable [#]==mytable[#t])

 assert(mytable[#-1] == mytable[1])

Here 'Tab' is a macro which generates locally-scoped macros, and the
implementation is here [2]

Also works interactively, where you have access to other things like
lambda short-forms:

C:\Users\steve\lua\LuaMacro\tests> luam -lrawhash -i
Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
Lua Macro 2.3.0 Copyright (C) 2007-2011 Steve Donovan
> Tab mytable
> mytable = {10,20,30}
> = mytable[#]
30
> f = \x(2*x)
> = f(2)
4

There are limitations - this is smart substitution but it is still on
the lexical level. I cannot implement the most commonly suggested |x|
2*x syntax for short lambdas because it's hard to know where the
expression ends without parsing it.  Some of the freedom of
token-filtering is also lost, so I can't show you a filter that
replaces newlines in tables with commas (I'm thinking of an elegant
yet efficient way of doing this)

steve d.

[1] https://github.com/stevedonovan/LuaMacro
[2] https://github.com/stevedonovan/LuaMacro/blob/master/tests/rawhash.lua