Don't write out the cabal file if it matches the old one

[Gabriella439]

The motivation behind this change is avoid the --force option from triggering an unnecessary reload of ghcid if nothing changed.

The context is that at work we use hpack --force to correct manual edits that users might make to the .cabal file, and we run hpack --force whenever users enter our Nix shell. If a user uses direnv on top of that then hpack --force runs every time they cd to the project directory.

However, suppose that they load ghcid in one window and then cd to the project directory in another window. This causes hpack --force to run and even if nothing changed the mwb.cabal file is touched which causes ghcid to reload (which is expensive). This change fixes that.

[Gabriella439]

Note that I could also structure this to check if the new/old cabal files are textually identical instead of the abstract syntax being identical. However, that would be a slightly more invasive change so I wanted to propose this first to solicit feedback.

[sol]

we run hpack --force whenever users enter our Nix shell.

What is the reason that you use --force in the first place? If you can remove the --force option then I think things would already work for your use case.

Do you have a modification hash in the generated .cabal file? If yes, then run hpack --no-hash --force once to get rid of it. After that hpack should work just fine, even with manual modifications and without --force.

Or is it, that you use different versions of hpack across the team?

Note that I could also structure this to check if the new/old cabal files are textually identical instead of the abstract syntax being identical.

Yes, if we do anything here, then we would want textual comparison. But please hold back for a second. This code is already more complicated than it should be. We might want to remove support for modification hashes (and possibly even the hpack min version check) first, before doing anything here.

[sol]

@Gabriella439 please take a look at #557. Does this work for you?

[Gabriella439]

Yeah, #557 looks good to me. To answer your prior question, we would use --force so that if the user made any manual modification to the .cabal file then we'd correct and override that modification.

[sol]

• Release 0.35.4 #560