setup-dev-env.md

Set Up a Haskell Development Environment in Three Steps

a Haskell Development Environment consist in a compiler (ghc), a language server (hls), a building tool (cabal or stack), and an editor compatible with the language server protocol.

The best way to have a coherent installation of these components is with the ghcup tool.

• Step 1: Install GHCup. You'll be prompted to install some tools. Say yes to hls. Do not install stack yet (see below).
• Step 2: Install vscode
• Step 3: Open vscode and install the haskell extension either using the extension panel or pressing CTRL+P and ext install haskell.haskell.

Integration with other editors

If you want an editor different from vscode, check the documentation here

Integration with stack.

to integrate stack with haskell-language-server, you should configure it properly.

The key point is to prevent stack to install ghc on its own. Follow these steps:

• First, You still need to follow the three steps at the top of this guide. You can't get a proper working environment solely with stack.

• Skip this point if you installed stack independently from ghcup. In a terminal run ghcup install stack

• Configure stack to use system's ghc. Configuration can be either global (recomended) or local

  • Global (recomended)

    # Run on a terminal if you want to set a global configuration
    stack config set install-ghc --global false
    stack config set system-ghc --global true 

  • Local:

    # On stack.yaml add this line if you want to set a local configuration
    system-ghc: true 

• You must use a stack snapshot compatible with the ghc version installed by ghcup. Visit stackage to get the ghc version on each sanpshot. For example lts-18.28 uses ghc-8.10.7. You can check version compatibility with:

  # This prints the ghc's version installed by ghcup
  > ghcup list -c installed -t ghc
  ✔✔ ghc  8.10.7  recommended,base-4.14.3.0 hls-powered
  
  # Your resolver should point to 18.28      ---------------------------------------------------
  > cat stack.yaml    #                                                                    vvvvv    
  resolver:
    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/18/28.yaml
  

What if I need to use a different snapshot?

In case you need to use a different snapshot you have to install it's ghc version with ghcup. For the sake of example, let say you are using lts-16.31, which uses ghc-8.8.4. Then you need to run ghcup install ghc 8.8.4 on a terminal. Also, depending on which snapshot you are using, you might loose integration with hls. Check hls documentation for more info.

Saying no to GHCup HLS (yes to everything else), and then letting the Haskell extension install HLS seems to work fine. Either GHCup or the extension, I don't know which one, creates a stack hook to prevent it from installing its own GHC. There's no further need to mess with stack anyway.
https://www.haskell.org/ghcup/guide/#strategy-1-stack-hooks-new-recommended

Debugging and testing (HSpec) still don't work from VSCode though. The debug adapter extension is $shit.