Re: POC: converting Lists into arrays

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 (;;).

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