Re: Gotchas in math metamethods etc

[Dirk Laurie <dpl@...>]

On Sun, Mar 13, 2011 at 07:15:20AM +0200, HyperHacker wrote:
> Something I stubbed my toe on the other day:
> > obj={val=2}
> > meta={}
> > function meta:__mul(num) return self.val * num end
> > setmetatable(obj, meta)
> > =obj*4
> 8
> > =4*obj
> stdin:1: attempt to index local 'self' (a number value)
> stack traceback:
> 	stdin:1: in function <stdin:1>
> 	stdin:1: in main chunk
> 	[C]: ?
> While this makes perfect sense, it seems like something that's likely
> to give a hard time to newbies who don't realize why the second
> statement fails ('self' is actually 4). I was just thinking this
> should be noted in the manual to avoid that potential confusion. 

This particular mistake has little to do with Lua.

I suppose that you are looking for a vector space structure, otherwise 
you would have coded the possibility of multiplying two objects.  Now 
in a vector space, you are supposed always to write scalars on one
pre-chosen side of vectors when you multiply.  Left is usual, but right
is equally possible and in fact preferred by some authors.  Whatever,
you would expect one of scalar*vector and vector*scalar to be valid
and the other invalid.

Which is exactly what you found.

Dirk