Re: POC: converting Lists into arrays

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2019-07-31 19:40:09 -0400, Tom Lane wrote:
>> That makes the other idea (of a foreach-ish macro declaring the
>> listcell value variable) problematic, too :-(.

> Hm. One way partially around that would be using an anonymous struct
> inside the for(). Something like
> #define foreach_node(membertype, name, lst) \
> for (struct {membertype *node; ListCell *lc; const List *l; int i;} name = {...}; \
> ...)
> which then would allow code like

> foreach_node(OpExpr, cur, list)
> {
> do_something_with_node(cur.node);
> foreach_delete_current(cur);
> }

I'm hesitant to change the look of our loops quite that much, mainly
because it'll be a pain for back-patching. If you write some code
for HEAD like this, and then have to back-patch it, you'll need to
insert/change significantly more code than if it's just a matter
of whether there's a ListCell variable or not.

I experimented with the "aforeach" idea I suggested upthread,
to the extent of writing the macros and then converting
parse_clause.c (a file chosen more or less at random) to use
aforeach instead of foreach. I was somewhat surprised to find
that every single foreach() did convert pleasantly. (There are
several forboth's that I didn't try to do anything with, though.)

If we do go in this direction, I wouldn't suggest trying to
actually do wholesale conversion of existing code like this;
that seems more likely to create back-patching land mines than
do anything helpful. I am slightly tempted to try to convert
everyplace using foreach_delete_current, though, since those
loops are different from v12 already.

Thoughts?

regards, tom lane