Re: POC: converting Lists into arrays

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: POC: converting Lists into arrays
Date: 2019-08-01 01:15:11
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On 2019-07-31 19:40:09 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > Unfortunately foreach(ListCell *lc, ...) doesn't work with the current
> > definition. Which I think isn't great, because the large scopes for loop
> > iteration variables imo makes the code harder to reason about.
> Yeah, I tried to make that possible when I redid those macros, but
> couldn't find a way :-(. Even granting that we're willing to have
> a different macro for this use-case, it doesn't seem easy, because
> you can only put one <declaration> into the first element of a
> for (;;).

I remember hitting that at one point and annoyed/confused as there that
restriction came from. Probably some grammar difficulties. But still,

> 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)


That's quite similar to your:

> One idea is that we could do something like
> foreach_variant(identifier, list_value)
> {
> type *v = (type *) lfirst_variant(identifier);
> ...
> }
> where the "identifier" isn't actually a variable name but just something
> we use to construct the ForEachState variable's name. (The only reason
> we need it is to avoid confusion in cases with nested foreach's.) The
> lfirst_variant macro would fetch the correct value just by looking
> at the ForEachState, so there's no separate ListCell* variable at all.

but would still allow to avoid the variable.


Andres Freund

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