stack-to-nix: Cache entry prevents generation of missing file

[rvl]

There is a usability problem/bug with the stack-to-nix cache file.

Firstly, users don't understand what it is, and assume that this file is the real product of stack-to-nix (e.g. cardano-foundation/cardano-wallet@17485e1).

Secondly, after running stack-to-nix and then git commit -a -m "regenerate stuff", there may have been new .stack.nix/*.nix files that are not checked in to git and then lost.

Thirdly, after the .nix file is lost, but .stack-to-nix.cache is updated with an entry, the necessary file will never again be generated, and stack-to-nix will not work properly until the cache file is removed.

There need to be additional checks that cache entries are valid before using them.

[rvl]

cc @jbgi

[chreekat]

Ah shoot, just ran into this. Considering there's a good handful of bugs around it, I wonder if having .stack-to-nix.cache is worth the effort?

[angerman]

@chreekat that is a valid question. Maybe we should put it behind a flag for now and disable it by default? Are you keen on a challenge?

[chreekat]

Haha, maybe eventually. :) Unfortunately I'm already full up on hobby Haskelling. I was hoping to use haskell.nix for a work project, but seeing as we already have a (mostly) working Nix expression, I think I'll have to come back after the beta release. Looking forward to it!

[stale]

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