Hi list,
A question occurred to me just now (it is Friday afternoon after all)
- the Lua manual https://www.lua.org/manual/5.4/manual.html#pdf-next
states that
"The behavior of next is undefined if, during the traversal, you
assign any value to a non-existent field in the table"
Which I understand the general meaning of, but the manual's wording
got me wondering. Normally "undefined behavior" in the context of a
programming language means "your code may do absolutely anything
including crash or evolve into Skynet".
With Lua however I've always
understood that (unless you use the debug library, or other native
calls into 3rd party libraries) it should never be possible to write
Lua source which compiles but crashes the interpreter, or otherwise
trashes the native interpreter state. I'm ignoring invalid bytecode
here, as that's understood to offer no guarantees. And of course
there have been a handful of bugs over the years that let you do
that, but those were clearly bugs and got fixed.
As far as I can see, this is the only place in the manual that uses
the phrase "undefined behaviour", so there isn't anything else to
compare it to.
So, what exactly are the guarantees on what `next` might do if you
violate the assignment constraint? "Not correctly let you traverse
the entire table" is a given, but what else? Get stuck in a loop
hanging the interpreter? Throw an "invalid key to next" error?
Actually crash? My guess would be "not correctly traverse", based on
my rough understanding of how table traversal works, but I'd be
interested to know what the intended behavior is, and of course if
anyone happens to know how any particular Lua version actually
behaves.
