Introduce 'haskell.toolchain' setting

[hasufell]

Wrt #561

---

Example config in current workspace:

{
    "haskell.toolchain": {
        "hls": "1.6.1.1",
        "cabal": "recommended",
        "stack": null
    }    
}

This means:

1. install the ghc version corresponding to the project (default, because it's omitted)
2. install hls 1.6.1.1
3. install the recommended cabal version from ghcup
4. don't install any stack version

Another config:

{
    "haskell.toolchain": {
        "ghc": "9.2.2",
        "hls": "latest"
        "cabal": "recommended",
    }    
}

1. install ghc 9.2.2 regardless of what the project requires
2. always install latest HLS, even if it doesn't support the given GHC version
3. install recommended cabal
4. install latest stack (default, because it's omitted)

[hasufell]

This is a bugfix regardless of this PR.

[wz1000]

Huh, I wasn't aware undefined and null were different values in JS.

[hasufell]

Yeah, I'm abusing this fact to force different codepath. Great engineering.

[wz1000]

Look sensible to me from a quick glance. I will trust you on the details.

Thanks for implementing this and all the other work too!

[wz1000]

Is there a link in the GHCup documentation we could point to?

[hasufell]

This is rather vscode specific and described in the readme.

[fendor]

Awesome! My only complaint is regarding usability, we need to add more explanations why certain messages happen and what users can do to understand the pop-up and why it happens.
Example:
For me, hls-wrapper failed to load, but nothing helped me to understand what I could do to reproduce it or figuring out what is going wrong. Just this:

Found "/home/hugin/Documents/haskell/fpex-eval/hie.yaml" for "/home/hugin/Documents/haskell/fpex-eval/a"
Run entered for haskell-language-server-wrapper(haskell-language-server-wrapper) Version 1.6.1.0, Git revision f4022c5bb8530cd306c53b941878244bf27a5d41 (dirty) x86_64 ghc-8.10.7
Current directory: /home/hugin/Documents/haskell/fpex-eval
Operating system: linux
Arguments: ["--lsp","--debug"]
Cradle directory: /home/hugin/Documents/haskell/fpex-eval
Cradle type: Cabal

Tool versions found on the $PATH
cabal:		3.6.2.0
stack:		2.7.5
ghc:		8.8.4


Consulting the cradle to get project GHC version...
Failed to get project GHC version:CradleError {cradleErrorDependencies = [], cradleErrorExitCode = ExitFailure 1, cradleErrorStderr = ["Error when calling cabal --builddir=/home/hugin/.cache/hie-bios/dist-fpex-eval-22f998bc1b8ed0a562686ad76442afff v2-exec --with-compiler /home/hugin/.cache/hie-bios/wrapper-13a09b18ea883dd377d59db5e821a86b ghc -v0 -- --numeric-version","","cabal: Could not resolve dependencies:\n[__0] trying: fpex-0.0.0 (user goal)\n[__1] next goal: base (dependency of fpex)\n[__1] rejecting: base-4.13.0.0/installed-4.13.0.0 (conflict: fpex =>\nbase>=4.14.0.0 && <4.17.0.0)\n[__1] rejecting: base-4.16.1.0, base-4.16.0.0, base-4.15.1.0, base-4.15.0.0,\nbase-4.14.3.0, base-4.14.2.0, base-4.14.1.0, base-4.14.0.0, base-4.13.0.0,\nbase-4.12.0.0, base-4.11.1.0, base-4.11.0.0, base-4.10.1.0, base-4.10.0.0,\nbase-4.9.1.0, base-4.9.0.0, base-4.8.2.0, base-4.8.1.0, base-4.8.0.0,\nbase-4.7.0.2, base-4.7.0.1, base-4.7.0.0, base-4.6.0.1, base-4.6.0.0,\nbase-4.5.1.0, base-4.5.0.0, base-4.4.1.0, base-4.4.0.0, base-4.3.1.0,\nbase-4.3.0.0, base-4.2.0.2, base-4.2.0.1, base-4.2.0.0, base-4.1.0.0,\nbase-4.0.0.0, base-3.0.3.2, base-3.0.3.1 (constraint from non-upgradeable\npackage requires installed instance)\n[__1] fail (backjumping, conflict set: base, fpex)\nAfter searching the rest of the dependency tree exhaustively, these were the\ngoals I've had most trouble fulfilling: base, fpex\n\n"]}

while the error message itself is coming from HLS (and is objectively bad), we ought to print the exact command a user can run to reproduce this error. For example:

PATH="pathstuff" hls-wrapper --project-ghc-version

or something similar.

[hasufell]

while the error message itself is coming from HLS (and is objectively bad), we ought to print the exact command a user can run to reproduce this error. For example:

Doesn't seem like a regression.

[fendor]

It's not, but it is something that I think needs improvement.
Previously, it was enough to run hls-wrapper to reproduce the issue, with the managed tools, that's no longer valid as cabal/stack/ghc might not be on the system PATH. Hence, I would like to have a very simple way for the user to figure out what failed in the cli.

[hasufell]

It's not, but it is something that I think needs improvement.

I don't think this needs to block the release. Otherwise it will cause another 2 weeks of roundtrip time and then someone discovers yet something else they don't like.

[fendor]

No, we can start with the pre-release today and ask on discourse / reddit / discord for volunteers and feedback