GitHub CI: Deduplicate workflow configuration

[Anton-Latukha]

After adding #2419, workflow configs, platform-specific things & matrix'es became shotgun-surgery enough to think about doing the https://docs.github.com/en/actions/learn-github-actions/reusing-workflows#overview work to configure configuration in workflows in one place as much as possible.

[Anton-Latukha]

At the same time it would be useful to look into: does those $CABAL*-platform depended env variables even need to be set, or the:

      - name: "Haskell env setup"
        id: HaskEnvSetup
        uses: haskell/actions/setup@v1
        with:
          ghc-version: ${{ matrix.ghc }}
          
### ...
      - name: "Configuring GitHub cache"
        uses: actions/cache@v2
        with:
          path: |
            ${{ steps.HaskEnvSetup.outputs.cabal-store }}

is enough (seems enough).

[jneira]

Thanks for opening the issue, reusing workflows looks promising

[Anton-Latukha]

Bad news: seems like OS/GHC matrix seems to be needed to be restated for every reusable workflow.

Good news: It would save in code complexity & maintenance anyway. I already today churned to keep in sync the cache-deps & test code & decided to concentrate only on the first one. And deduplicating that code would make CI work & setup more robust.

[Anton-Latukha]

The first thing to do with reusable workflows - is to start with having a workflow for cabal.project processing. The file basically can be passed even as a value. But it is useful to do it through artifact passing.

[Anton-Latukha]

I looked into what GitHub has & it is ...

I basically wanted/imagined simple things:

• as loading & sharing the steps or jobs (for example loading them from json).
• macros are impossible.

Also, GitHub CI allows only 1 level of inheritance (does not allow to inherit workflow that inherited anything).

Caching is designed by GitHub so - that its setup is not sharable, every workflow that uses caching - needs the action setup.

So ... deduplication is limited. The best possible I see is still sync files into seamless form & then look at how to do #2422 (comment)

I hope in the future GitHub would expand reuse abilities.

[jneira]

What about custom actions? could we extract some reusable bits and make it actions (like our use of skip actions)?

[Anton-Latukha]

Custom actions are also partial solutions & not flawless.

There are JS, sh & docker actions, each has its pros & cons. I know that now GitHub allows caching inside actions, but do not how system works or its limitations.

[Anton-Latukha]

I've looked into DHall & its alternatives.

& DHall syntax is not pretty, & it is { } based. But seems there are no real alternatives with such abilities & it should do the job.

I am not holding my breath to be surprised or anything, but we would see.

I for this purpose tried to unify a subset of yml as much as possible.

[Anton-Latukha]

There are also automations for CI to keep generated files in accordance with dhall changes.