Fix installation instructions

[spacekitteh]

No description provided.

[korayal]

Thank you for this, but I think we shouldn't be selecting a specific binary (in this case haskell-language-server) as I stated in my comment here. I think this branch ( multi-version ) is still WIP until we find a way to expose both binaries.

On the other hand, I'm not using NixOS so it looks like packages.haskell-language-server.components.exes doesn't work ( it does work with nix-env -iA), so I guess the correct equivalent would be something like:

environment.systemPackages = [
  (import (builtins.fetchTarball "https://github.com/korayal/hls-nix/tarball/master") {}).hls-ghc865.haskell-language-server
  (import (builtins.fetchTarball "https://github.com/korayal/hls-nix/tarball/master") {}).hls-ghc865.haskell-language-server-wrapper
];