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If you choose to use Common Lisp as a functional language, CLOG is available: https://github.com/rabbibotton/clog

If you count Rust as a "functional language", my personal favorite is iced, which is bloody brilliant and blazing fast https://github.com/iced-rs/iced

It's desktop-first but compiles to WASM

It's honestly "just Rust", so I guess if you're looking for a DSL-type declarative, it doesn't quite fit the bill... but there are design reasons for why it's "just rust" and honestly I much prefer it. It still feels declarative enough

slint is more declarative, supports Rust and Python

I prefer less declarative

elixir's liveview is perhaps the best known and arguably inspired a lot of the ones in other languages

yes but doesn't that expect view code that looks a lot like HTML and is usually styled with Tailwind?

the view code compiles to elixir code, it just uses html as a familiar syntax. it's written inline as functions in the source code though so I tend to think of it as closer to a "pure elixir" solution.