Dark lang is a new language for writing “deployless” backends (no server management, no build pipelines, etc.). The language looks like Elm mixed with a little bit of Rust. You write your HTTP handlers, DB tables, CRONs and others in their browser editor / IDE (it shows you live values, it’s not possible to write wrong syntax, etc.)
I’ve migrated my RealWorld example to Quickstart with Webpack and written Dark backend for it. The result is MartinKavik/seed-realworld-darklang.
I recommend to write your Seed app first and then build your Dark backend. This way, you can use Trace Driven Development (i.e. you fire a HTTP request from Seed and then create automatically a new HTTP handler in Dark and you can follow live values for implementation). Also strict Rust/Serde helps you with writing Dark HTTP handlers because it shows you problems with endpoint responses.