HLint in the GHC CI, an eight-months retrospective

[Hécate]

Hello fellow devs,

this email is an activity report on the integration of the HLint[0] tool 
in the Continuous Integration (CI) pipelines.

On Jul. 5, 2020 I opened a discussion ticket[1] on the topic of code 
linting in the several components of the GHC code-base. It has served as 
a reference anchor for the Merge Requests (MR) that stemmed from it, and 
allowed us to refine our expectations and processes. If you are not 
acquainted with its content, I invite you to read the whole conversation.

Subsequently, several Hadrian lint rules have been integrated in the 
following months, in order to run HLint on targeted components of the 
GHC repository (the base library, the compiler code-base, etc).
Being satisfied with the state of the rules we applied to the code-base, 
such as removing extraneous pragmata and keywords, it was decided to 
integrate the base library linting rule in the CI. This was five months 
ago, in September[2], and I am happy to report that developer friction 
has been so far minimal.
In parallel to this work on the base library, I took care of cleaning-up 
the compiler, and harmonised the various micro coding styles that have 
emerged quite organically during the decades of development that are 
behind us (I never realised how many variations of the same ten lines of 
pragmata could coexist in the same folders).
Upon feedback from stakeholders of this sub-code base, the rules file 
was altered to better suit their development needs, such as not removing 
extraneous `do` keywords, as they are useful to introduce a block in 
which debug statements can be easily inserted.

Since today, the linting of the compiler code-base has been integrated 
in our CI pipelines, without further burdening our CI times.
Things seem to run smoothly, and I welcome comments and requests of any 
kind related to this area of our code quality process.

Regarding our future plans, there has been a discussion about 
integrating such a linting mechanism for our C code-base, in the RTS. 
Nothing is formally established yet, so I would be grateful if people 
who have experience and wisdom about it can chime in to contribute to 
the discussion: https://gitlab.haskell.org/ghc/ghc/-/issues/19437.

And I would like to say that I am overall very thankful for the 
involvement of the people who have been giving us feedback and have been 
reviewing the resulting MRs.

Have a very nice day,
Hécate

---
[0]: https://github.com/ndmitchell/hlint
[1]: https://gitlab.haskell.org/ghc/ghc/-/issues/18424
[2]: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4147

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