Nested comment systems tend to use one of two patterns - both of which assume you're using a relational database. Arc forums doesn't, which IMO greatly reduces flexibility in exchange for Lisp purity.
The most common pattern is the "adjacency list" model[0]. Every entry has an ID and parent ID, at least. You can also include an "origin" ID for the root of the tree. For this, you need to build an entire thread with recursive queries (for each ID, get children of ID, etc.) HN and other Arc based forum use this.
Slightly more involved is the "nested set" model[1] which actually maintains a balanced tree. It's easier to query an entire tree than with the adjacency list, but insertions and deletions are more costly.
From what I can tell from the introduction[0] and initial HN thread[1] Bel is an academic exercise in developing a programming language by focusing on axioms rather than implementation. Also:
I think the point of a high-level language is to make your programs shorter. All other things (e.g. libraries) being equal, language A is better than language B if programs are shorter in A. (As measured by the size of the parse tree, obviously, not lines or characters.) The goal of Bel is to be a good language. This can be measured in the length of programs written in it.
There is a database wrapper (database.arc) in /lib that should support postgres.
It isn't well documented and only tested on a SQL connection but it's just a set of wrappers around Racket's DB drivers, so you could try including that into your app.
Unfortunately I seem to have completely lost my test project but the connection should work as follows:
I don't think so, since OP mentioned Arc 3.2 without any Nginx. I just tried it and I'm able to reproduce this issue with just the official branch in anarki (which is identical to Arc 3.2) without any Nginx.
Arc 3.2 doesn't get very frequent updates, but we should probably fix this in the stable branch. It shows the same error, and also has some errors in the log. Unfortunately I'm not sure when I can get to this. I've been having some RSI issues and time at the computer is limited at the moment.