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: <30240.1564616409@sss.pgh.pa.us>
References: <11587.1550975080@sss.pgh.pa.us>
 <20190731225756.nozn3j6lgwhm6qvq@alap3.anarazel.de>
 <30240.1564616409@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Wed, 31 Jul 2019 19:40:09 -0400"
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Date: Wed, 31 Jul 2019 20:16:06 -0400
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Precedence: bulk

I wrote:
> 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.)

On second thought, there seems no strong reason why you should need
to fetch the current value of a foreach-ish loop that's not the most
closely nested one.  So forget the dummy identifier, and consider
this straw-man proposal:

#define aforeach(list_value) ...

(I'm thinking "anonymous foreach", but bikeshedding welcome.)  This
is just like the current version of foreach(), except it uses a
fixed name for the ForEachState variable and doesn't attempt to
assign to a "cell" variable.

#define aforeach_current() ...

Retrieves the current value of the most-closely-nested aforeach
loop, based on knowing the fixed name of aforeach's loop variable.
This replaces "lfirst(lc)", and we'd also need aforeach_current_int()
and so on for the other variants of lfirst().

So usage would look like, say,

	aforeach(my_list)
	{
		type *my_value = (type *) aforeach_current();
		...
	}

We'd also want aforeach_delete_current() and aforeach_current_index(),
to provide functionality equivalent to foreach_delete_current() and
foreach_current_index().

These names are a bit long, and maybe we should try to make them
shorter, but more shortness might also mean less clarity.

BTW, I think we could make equivalent macros in the old regime,
which would be a good thing because then it would be possible to
back-patch code using this notation.

Thoughts?

			regards, tom lane