Lua Compatibility
Phew! Yesterday afternoon, I decided to upgrade my laptop’s OpenIndiana from 151a9 to “Hipster”. I did it in a bit convoluted way, and hopefully I’ll write about that some other day. In the end, I ended up with a fresh install of the OS with X11 and Gnome. If you’ve ever seen my monitors, you know that I do not use Gnome — I use Notion. So, of course I had it install it. Sadly, OpenIndiana doesn’t ship it so it was up to me to compile it. After the usual fight to get a piece of software to compile on Illumos (a number of the Solaris-isms are still visible), I got it installed. A quick gdm login later, Notion threw me into a minimal environment because something was exploding.
After far too many hours of fighting it, searching online, and trying random things, I concluded that it was not Notion’s fault. Rather, it was something on the system. Eventually, I figured it out. Lua 5.2 (which is standard on Hipster) is not compatible with Lua 5.1 (which is standard on 151a9)! Specifically, a number of functions have been removed and the behavior of other functions changed. Not being a Lua expert (I just deal with it whevever I need to change my window manager’s configuration), it took longer than it should but eventually I managed to get Notion working like it should be.
So, what sort of incompatibilies did I have to work around?
loadstring
loadstring got renamed to load. This is an easy to fix thing, but still a headache especially if you want to support multiple versions of Lua.
table.maxn
table.maxn got removed. This function returned the largest positive integer key in an associative array (aka. a table) or 0 if there aren’t any. (Lua indexes arrays starting at 1.) The developers decided that it’s so simple that those that want it can write it themselves. Here’s my version:
local function table_maxn(t)
local mn = 0
for k, v in pairs(t) do
if mn < k then
mn = k
end
end
return mn
end
table.insert
table.insert now checks bounds. There doesn’t appear to be any specific way to get old behavior. In my case, I was lucky. The index/positition for the insertion was one higher than table_maxn returned. So, I could replace:
table.insert(ret, pos, newscreen)
with:
ret[pos] = newscreen
Final Thougths
I can understand wanting to deprecate old crufty interfaces, but I’m not sure that the Lua developers did it right. I really think they should have marked those interfaces as obsolete, make any use spit out a warning, and then in a couple of years remove it. I think that not doing this, will hurt Lua 5.2’s adoption.
Yes, I believe there is some sort of a compile time option for Lua to get legacy interfaces, but not everyone wants to recompile Lua because the system installed version wasn’t compiled quite the way that would make things Just Work™.
RAID6