From the outside Lua is close to the perfect language for scripting. It is small and understandable, without sacrificing power. However, there are a few things about it which have stumped me at first.

Unicode ‘support’

Here’s an easy one:

s = "‡"
print(s:len()) -- returns "3"

Pretty-printing non-ASCII documents using string.format is basically impossible without resorting to additional libraries. And unfortunately it seems that the unicode library in LuaTeX doesn’t ‘fix’ the formatting problem:

s = "‡"
-- "1", as expected 

print(unicode.utf8.format("string: [%-4s]",s))
-- "[‡ ]", only 2 chars, not 4; same as string.format

Oh well. This can be addressed by manually calculating the number of spaces to insert:

L = 4
s = "‡"
Nspaces = L - unicode.utf8.len(s)
print(string.format("string: [%s]",s .. string.rep(" ",Nspaces)))

Table variables are pointers

I think I understand this, but it sure took me a while to figure out the first time around:

pp = {x=2}
print(pp.x) -- "2"
local ll = pp
print(pp.x) -- "0" (!)

Coming from a Matlab background, this behaviour was unexpected to say the least. The canonical way to copy a table appears to be:

local ll = {}
for k,v in pairs(pp) do 
  ll[k] = v