Difficult errors when no appropriate GHC is found

[hasufell]

If HLS is managed by the extension, but there is no ghc in PATH and no ghc-<project-version> either, then the error is:

---

If HLS is managed by the extension and only a project-incompatible ghc is in PATH, hie-bios seems to detect a wrong project GHC version (with-compiler: ghc-8.10.7 is set in this project):

https://user-images.githubusercontent.com/1241845/158057243-dffe75f1-9351-4d8e-94ce-14df428126dc.png

Apparently this is fixed in master hie-bios, see new error below

@fendor

[hasufell]

haskell/hie-bios#340

[hasufell]

Alright, latest hie-bios doesn't have the second error, but it's still not very informative:

[hasufell]

There appears to be some code:

vscode-haskell/src/hlsBinaries.ts

Lines 45 to 80 in 70f6883

	class MissingToolError extends Error {
	public readonly tool: string;
	constructor(tool: string) {
	let prettyTool: string;
	switch (tool) {
	case 'stack':
	prettyTool = 'Stack';
	break;
	case 'cabal':
	prettyTool = 'Cabal';
	break;
	case 'ghc':
	prettyTool = 'GHC';
	break;
	default:
	prettyTool = tool;
	break;
	}
	super(`Project requires ${prettyTool} but it isn't installed`);
	this.tool = prettyTool;
	}
	public installLink(): Uri | null {
	switch (this.tool) {
	case 'Stack':
	return Uri.parse('https://docs.haskellstack.org/en/stable/install_and_upgrade/');
	case 'Cabal':
	case 'GHC':
	return process.platform === 'win32'
	? Uri.parse('https://www.haskell.org/platform/index.html#windows')
	: Uri.parse('https://www.haskell.org/ghcup/');
	default:
	return null;
	}
	}
	}

But we deleted some of the try-catch:

vscode-haskell/src/hlsBinaries.ts

Lines 363 to 378 in 70f6883

	} catch (error) {
	if (error instanceof MissingToolError) {
	const link = error.installLink();
	if (link) {
	if (await window.showErrorMessage(error.message, `Install ${error.tool}`)) {
	env.openExternal(link);
	}
	} else {
	await window.showErrorMessage(error.message);
	}
	} else if (error instanceof NoBinariesError) {
	window.showInformationMessage(error.message);
	} else if (error instanceof Error) {
	// We couldn't figure out the right ghc version to download
	window.showErrorMessage(`Couldn't figure out what GHC version the project is using: ${error.message}`);
	}

I'm not entirely sure what this object hierarchy does.

[hasufell]

Fixed in #543