From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: POC: converting Lists into arrays
Date: Wed, 31 Jul 2019 19:40:09 -0400
Message-ID: <30240.1564616409@sss.pgh.pa.us> (raw)
In-Reply-To: <20190731225756.nozn3j6lgwhm6qvq@alap3.anarazel.de>
References: <11587.1550975080@sss.pgh.pa.us>
<20190731225756.nozn3j6lgwhm6qvq@alap3.anarazel.de>
Andres Freund <andres@anarazel.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 (;;).
That makes the other idea (of a foreach-ish macro declaring the
listcell value variable) problematic, too :-(.
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.
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, andres@anarazel.de, pgsql-hackers@lists.postgresql.org
Subject: Re: POC: converting Lists into arrays
In-Reply-To: <30240.1564616409@sss.pgh.pa.us>
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