> (Or in other words, let's see \while ... \endloop in the minimal proposal > as well, or at least a sketch of how to get there.)
It seems to me that we could implement \if ... \else ...\endif by having some kind of stack that indicates which constructs we're inside of and whether we're currently executing commands or discarding them. I think we want to avoid waiting until we see \endif to start running commands; it's better to execute or skip/discard each command as we see it.
+1
To implement \while, we'd also need to remember previous commands so that when we reach the end of the loop, we can rewind and put all of those commands back on the stack to be executed again, or perhaps to be skipped if the \while condition turns out now to be false.
This might be what you meant when you said "those commands back on the stack", but I think we'd have to remember not a list of commands, but the raw string of bytes from the start of the \while to the \endwhile (or equivalent), because any psql vars within that block could themselves be a non-parameter part of a command:
-- this is how I fake an 'exit 0' now:
\set work_needs_to_be_done t select
case
when :'work_needs_to_be_done'::boolean then ''
else '\q'
end as cmd
\gset
:cmd
-- ridiculous example to illustrate complications in remembering past commands
select
case
when random() < 0.5 then '\ir my_script.sql' when random() < 0.7 'select * from a_table; select count(*) from another_table;'
else 'select null as foo;' end as cmd \gset
:cmd
And even then, things get complicated, because an \ir include which makes it this iteration might not make it the next, and the \endwhile might have been inside that include, or vice-versa, an included file starts a \while it doesn't finish.
So maybe what we store is a stack of buffers that are currently open (STDIN being captured as a buffer only when a \while starts, everything else being files), and additionally have a stack of positions where a \while started (buffer_id, position in buffer).
Additionally, we could assert that all \while-\endwhile pairs must happen in the same MainLoop (aka file), and mismatches are an error.
I'm happy to keep sketching out what control structures might look like and how to implement them. But I'm also happy to leave while/for loops out for now.