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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
In-reply-to: <20190731225756.nozn3j6lgwhm6qvq@alap3.anarazel.de>
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 <20190731225756.nozn3j6lgwhm6qvq@alap3.anarazel.de>
Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Wed, 31 Jul 2019 15:57:56 -0700"
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Date: Wed, 31 Jul 2019 19:40:09 -0400
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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