The way the language provider and engine communicate is via gRPC, so you'll need some way to crack that nut (https://github.com/grpc/grpc-haskell looks like it could have been a thing but the

README

says it is experimental and it looks like there have been no commits in 9 months). Another option would be to have some external bridge that translates gRPC into some wire format that makes more sense there. When I was starting the dotnet language provider my plan of attack was first to just get the language plugin launching and have it print hello world. Pulumi will display things written to stdout as info messages during a deployment. The next thing was to start using the gRPC interface to send logging messages from the language host back to the engine. Once you have that, you can actually start on the "hard work".