At the moment, in the Lua support extension we have been developing, wikitext is output to the wiki via the return value of a function. For example in wikitext you would have:
{{#invoke:MyModule|myFunction}}
Then in [[Module:MyModule]]:
local p = {} function p.myFunction() return 'Hello, world!' end return p
This is all nice and elegant and will work. There is an alternative convention commonly used in scripting languages (and programming in general for that matter), using a print function:
local p = {} function p.myFunction() print('Hello, world!') end return p
I would have been happy to leave it as Victor Vasiliev made it, i.e. using return values, but I happened across a performance edge case in Lua which made me think about it. Specifically, this:
function foo(n) s = '' for i = 1, n do s = s .. toString(i) end return s end
has O(n^2) running time. For 100,000 iterations it takes 5 seconds on my laptop. Apparently this is because strings are immutable, so the accumulator needs to be copied for each concatenation. It's very similar to the situation in Java, where a StringBuffer needs to be used in such an algorithm.
It's easy enough to work around, but the problem is obscure enough that I think probably most of our users will not realise they need to work around it until it becomes severe.
It would be possible to provide a print() function which does not suffer from this problem, i.e.
function foo(n) for i = 1, n do print(i) end end
could run in O(n log(n)) time. Intuitively, I would expect that providing such a print function would encourage a programming style which would avoid at least some instances of repetitive concatenation.
The performance issue is probably no big deal, since most templates are probably not going to be concatenating hundreds of thousands of strings, and 5 seconds is still quicker than the time it takes most of our featured articles to render at the moment. But like I say, it got me thinking about it.
Does anyone have any thoughts on return versus print generally? Are there other reasons we would choose one over the other?
-- Tim Starling