Looks like tmux + vim, using the Solarized Dark theme.

I gathered the vim part. Is he pressing a separate key to run the stuff in the lower-right window or is there a vim hook to do that automatically?

For that I think he's using Guard (https://github.com/guard/guard/)

I'll make a video on my setup, but:

Left: vim with Vim Haskell Mode[1], a pale imitation of emacs haskell minor mode that nevertheless gets the job done. ;)

Top right: GHCi

Bottom right: Guard, as mentioned, using guard-shell[2] to compile the file and run main using runghci.

[1] https://github.com/lukerandall/haskellmode-vim [2] https://github.com/guard/guard-shell

Maybe that info should be on the page itself somewhere. I wouldn't know to look at the github to find your guard script: https://github.com/haskelllive/haskelllive/blob/master/Guard...

Good call. I'll add that to the About. I'm going to do a quick supplemental vide on the setup as well between now and next episode.