dependent-sum VS monad-bayes

What is the dependent-sum library for? I must be missing something subtle, because it looks like an over-complicated sum type?

source-repository-package type: git location: https://github.com/tweag/monad-bayes.git

I'm extending https://github.com/tweag/monad-bayes, which had it already, not sure where from. To be fair, it only had it in "dev" mode. Here's the flags in the cabal file:

So this summer I'll be working as part of a Tweag fellowship (https://www.tweag.io/blog/2020-02-14-os-fellowship/) on extending the Bayesian inference and probabilistic programming library monad-bayes (https://github.com/tweag/monad-bayes). It's a super nice library, and my goal is to make it much more user friendly (see these draft docs for example: https://monad-bayes.netlify.app/).

These questions are pretty basic, but I'm having a hard time making sure I'm doing things right. I have a .cabal file which I want to change in order to enable e.g. threading and optimization (https://github.com/tweag/monad-bayes/blob/master/monad-bayes.cabal). Looking at it, I don't see -O2 or -threading anywhere, so am I right in thinking those things aren't happening, or am I looking in the wrong place?

Thanks so much! Streamly seems like a good idea. I'll almost certainly follow up with more detailed questions after trying some things, but just for context for now, here's the code I'm trying to modify: https://github.com/tweag/monad-bayes/blob/master/src/Control/Monad/Bayes/Population.hs .