Setup ghcide for rules_haskell repository

[aherrmann]

Depends on #1350.

This configures hie-bios for ghcide for rules_haskell.

• Adds ghcide as built by stack_snapshot and haskell_cabal_binary to the repository. stack_snapshot build executable components #1304 will make this easier in the future.
• Adds a haskell_repl target to determine GHC flags required for the REPL session.
• Adds a .hie-bios script to the repository that is picked up by hie-bios's bios cradle. When executed that should write all GHCi flags to a file specified by the HIE_BIOS_OUTPUT environment variable. See hie-bios README. Once multi-component support is implemented in ghcide this should become more flexible. See Multi Component haskell/ghcide#522 (comment)
• Adds a simple test case that builds ghcide and checks that ghcide --version works. This is run on nixpkgs and bindist configurations.
• Adds a more involved test case to RunTests that invokes ghcide on a target and checks if it can successfully determine the required GHC flags. This is only run on nixpkgs configurations, since run-tests is only executed there.
• Adds a section to the use-case documentation explaining how to setup ghcide with rules_haskell. The setup is still pretty involved, stack_snapshot build executable components #1304 will simplify it a fair bit. We may want to hold off wider announcement, e.g. a tweet, until this is merged.

---

In order to make use of ghcide on rules_haskell you need to configure your editor to use it. See setup instructions. For example, the following works for neovim with coc:
.vim/coc-settings.json

{
    "languageserver": {
        "haskell": {
            "command": "nix-shell",
            "args": [
                "--run",
                "./.ghcide --lsp"
            ],
            "rootPatterns": [
                ".hie-bios"
            ],
            "filetypes": [
                "hs",
                "lhs",
                "haskell"
            ]
        }
    }
}

[smelc]

Cool! I could successfully reproduce the setup as follows:

> nix-shell --pure
pure> bazel test //... # To make sure everything is right
pure> bazel build @ghcide//:ghcide # Took approximately 10 minutes

Then in another shell, because I didn't want to pull neovim's coc dependencies from nix; and used my system install instead:

Video is a bit laggy because of the recording, but it's really smooth otherwise. In case things aren't working as expected:

• In the pure shell, check that executing bazel-bin/external/ghcide/_install/bin/ghcide works (It should stop at Step 1/6: Finding files to test in ...)
• Within neovim, check that content displayed at :CocConfig is the one posted by @aherrmann above. If you're using neovim in other contexts, it may be configured differently.

[mboes]

Once this merged, don't forget to schedule a tweet.

[thufschmitt]

That's awesome!

My only pain point is that setting this up is a bit involved. I'd really like this to be easier (as close to zero-config as can be). Though that's not a blocker for this PR and can be solved incrementally later

[thufschmitt]

After a bit more testing it looks like hover only display the haddock documentation for the values that come from libraries bundled with GHC (so not from cabal_library nor haskell_library deps). Is that expected?

[aherrmann]

This is now ready for review.

I've updated the PR description to reflect the latest state.

[thufschmitt]

I'm not sure where to put this, but I ran into some issues with targets building with -Werror because ghcide forcibly sets some warnings (like missing-signatures), so the combination of both might lead to spurious errors. The common way out of this is that most hie bioses seem to filter out -Werror or add -Wwarn, and that's what I also did by added an echo -Wwarn at the end of .hie_bios

[aherrmann]

That's a good point, another way to deal with this is to add -Wwarn to the toolchain's GHCi flags, e.g. https://github.com/digital-asset/daml/blob/a47c734401a70e5b9dd94cd9717a2192bc28c703/WORKSPACE#L382.

[aherrmann]

Nvm, I see that the hie-bios output group doesn't include those. Though maybe it should.

[aherrmann]

@smelc I made the docs a bit more explicit regarding pointing the editor to the correct instance of ghcide.

[smelc]

👍

[thufschmitt]

I stumbled across a strange error with this on #1342

[thufschmitt]

Just one minor comment but can be solved as a follow-up if needs be

[thufschmitt]

This is out-of-sync with the .hie-bios script in this repo.

I guess it's on purpose (to show a simple-but-working script in the docs), but maybe we could export the full-featured script as a bazel target and let the client-side .hie-bios script just be something like

#!/bin/bash

bazel run @rules_haskell//:hie-bios //:hie-bios

[aherrmann]

Indeed that was intentional to keep the documentation simple.

but maybe we could export the full-featured script as a bazel target

That's a very nice idea, thanks!

[dpulls]

🎉 All dependencies have been resolved !