If ghc --version fails for an installed GHC, recover more intelligently

[Blaisorblade]

In #2415 (comment) @jhance reported that stack has

no way to recover from the --version failing and treating that as "ghc is not installed"'

while this is probably a seldom occurrence, it could certainly happen in strange scenarios. I'm not sure we should detect all of them (what if ghc --version works but half of the libraries were deleted?) but at the very least a setup option for "setup GHC again" sounds useful. Also, if ghc --version fails, error messages could mention this option, or even do it automatically.

Could we at least accept a PR for this?

[Blaisorblade]

I think that a broken --version is definitely something that could be reasonably handled, though, or at least give the user instructions on how to purge the broken ghc manually.

At least the non-automated part is uncontroversial then 😄

And IIUC the automation could be a regression for those who install special GHCs with --ghc-bindist.

Thinking more, "remove an installed GHC" is probably even more useful. I do it by hand hesitantly often, by also remove the .installed part, even just to save space.

In sum, it'd be nice to have options to:

• force setup again. I do see stack setup --reinstall here though, so that doesn't need to be added! Or did it fail somehow for you?
• mention "force setup again" in the error when --version fails.
• separately, allowing to remove downloaded GHCs would be a nice (but separate) addition.

[mgsloan]

mention "force setup again" in the error when --version fails.

Makes sense! In the future we might actually skip the version check in many cases: #2433

separately, allowing to remove downloaded GHCs would be a nice (but separate) addition.

Covered by #133

[snoyberg]

This was addressed in #4714, closing.