I’ve been out of the game for a number of years but I used something called ghc-mod [1] with its associated vim plugin [2] to get this functionality. Moving forward, it seems that all of the effort has moved over to Haskell IDE Engine [3]. It looks like this exact feature hasn’t been brought over yet but it is planned. In the mean time you could still use ghc-mod though.

[1] https://github.com/DanielG/ghc-mod

[2] https://github.com/eagletmt/ghcmod-vim

[3] https://github.com/haskell/haskell-ide-engine

Thanks for the tip! Haskell IDE engine is exactly what I've been using til now and would've been very pleased to find out it had such a convenient feature.

Ghcide is on my list to try next as it's gotten a lot of attention lately.

Yes, it already has a code command to automatically add top level type annotations. The caveat I that it can only work if you add -Wall to your compilations flags.

It seems that "ghcide" can do this: https://twitter.com/chris__martin/status/1218331415869190144 https://github.com/digital-asset/ghcide

Template Haskell not working on ghcide has sadly been holding me back with it. I've heard great things about it otherwise

You can use HIE. It supports template Haskell

Recent ghcide can do it now as well as https://github.com/mpickering/apply-refact for hlint