Cache in Github Actions doesn't work

[BinderDavid]

The cache in the github actions for both the deploy actions and the actions which are run on every PR doesn't work correctly. As a consequence, hakyll is rebuilt from scratch every time which can take half an hour.

I think I know what the problem is: The key for the cache consists of the cabal file. I.e. we have the following line:

key: cabal-cache-0-${{ hashFiles('message-index/message-index.cabal') }}

This means that the last time the cache was updated was the last time we changed the cabal file.
Since we invoke cabal update directly afterwards, we invalidate the cached content and build from scratch.

Proposed solution: Use a cabal.freeze file.

[BinderDavid]

Related work:

• cabal freeze file is not so suitable as cache key jonathanknowles/haskell-example#22
• https://lambdafoo.com/posts/2021-08-27-hakyll-blog.html
• Recommended way to use cabal freeze for an executable? haskell/cabal#8047