threepenny-gui VS nixos-manager

Maybe? https://github.com/HeinrichApfelmus/threepenny-gui

Not if you use my Threepenny-gui library. 😉

If you really want FRP, use Obelisk (based on Reflex-Dom. I will call just say "Obelisk" to refer to Obelisk's dependencies Reflex-Dom, Reflex,... too). I like the author of threepenny-gui and the library has the great advantages of not needing GHCJS (Obelisk does not need GHCJS either theoretically but I do not know how easy it currently is to make it work without) nor Nix. But the library was originally built as a non-FRP library and it has been lacking essential FRP combinators for a long time.

The threepenny-gui library is probably best if you just want a simple way to make GUIs. However, it doesn't really do desktop GUIs as such: it displays your GUI as a webpage running on localhost. However, it does work pretty well with Electron, which gives you a desktop application.

For this purpose, I normally use threepenny-gui, which can be used with Electron. (As it happens, that combination is what I’m using for my current Haskell program.)

the version of GTK supported by gi-gtk is far from current, not to mention that gi-gtk itself, according to this discussion, isn't being as actively developed as it used to be.

You may want to check out this link. I have not checked it myself yet but it looks promising, although it looks like it has not been updated for some time.