Fix #3095: disabling default flags in the cradle is not propagated to hlint properly

[lf-]

There is a lot of detail of the exact issue in #3095.

I assume that all the flags that are not enabled are disabled (the default flags have already been applied at this point; so this is probably actually true).

It seems to fix the issue but it is kind of a hack.

[pepeiborra]

So I think the problem is that hlint enables lots of extensions by default in order to maximise its chances of success when used from the command line.

But in our setting, where we know with full precision the set of extensions that need to be enabled for parsing, this is completely unnecessary.

Have you considered telling hlint not to enable any extensions by default? There should be some config for that.

[pepeiborra]

Can you extend the comment to clarify why we need to assume? Maybe reference the GHC MR with the fix?

[lf-]

done :)

[lf-]

It seems that we would have to hack up hlint upstream to achieve that. Currently there is no way of passing "exactly these flags" into parseModuleEx, and there does not seem to me to be any obvious deeper entry point into hlint.

The bug on the hlint side is that lang_set sets the default flags for the language, absent any explicit enable/disables. So the only way to currently get hlint to get the right flags is to exhaustively force all of them to the known values, as we don't have a disables list. (besides using the Show OnOff instance which is kind of bad but might actually be cleaner nope that doesn't work because there's no Read instance and the code gets quite ugly quite fast)

We aren't hitting the "many default extensions" trouble, because we wholesale replace the enabledExtensions.

https://github.com/ndmitchell/hlint/blob/c0e77b59adb75d2b043a6f41f9a204cd7d4b42ef/src/GHC/All.hs#L173-L183

[pepeiborra]

There is a build error with ghc 8.10.7:

https://github.com/haskell/haskell-language-server/runs/7814129905?check_suite_focus=true#step:5:373

[lf-]

Yeah, it's odd. I'll look at it on Monday.