Purple Yolk
Purple Yolk is a Haskell IDE for Visual Studio Code.
Purple Yolk works best for Haskell projects that can be loaded into GHCi. Behind the scenes it launches GHCi for you, reloads it when you make changes, and displays GHCi's output in VSCode.
Purple Yolk doesn't care which build tool you use. The default configuration uses Stack, but you can just as easily use Cabal, Nix, or anything else. You can even use different build tools in different workspaces thanks to VSCode's built-in workspace configurations.
Similar extensions
Development
$ npm install
$ rm -f purple-yolk-*.vsix
$ npm run vsce package
$ code --install-extension purple-yolk-*.vsix