Detect when user changed .cabal file instead of package.yaml

[mgsloan]

Use of package.yaml can be confusing when the user doesn't expect it - see #3380 . I think this case should somehow be detected and a noisy warning emitted.

One way to do this might be to output a copy of the *.cabal on hpack generation. Then, when later re-generating, check if the cabal file changed since the last hpack generation. If so, then the user modified it and a noisy warning should be displayed.

[borsboom]

A nice way would be to write a checksum into the .cabal file itself (of everything after the header comment), and then compare it before overwriting. That way don't need to keep track of two files (especially if the .cabal file is checked into git in addition to the package.yaml).

[snoyberg]

Here's the logic I'd recommend:

1. If package.yaml is newer than foo.cabal, ignore foo.cabal and overwrite it
2. If foo.cabal is newer, check if the first line is of the form -- AUTOGEN CHECKSUM: sha256
3. If the checksum does not exist:
   1. Generate a new copy of foo.cabal in memory
   2. If the new copy matches the existing one perfectly, write a new foo.cabal with the added CHECKSUM line
   3. If it does not match, error out with an explanation to the user that they need to compare the two files and then delete foo.cabal
4. If the checksum exists and matches, all is well
5. If the checksum exists and does not match, error out with an explanation to the user

Does this seem reasonable to everyone?

[mgsloan]

@snoyberg Makes sense! I think that this should be implemented in hpack itself.

Marking this as P0 so that commercialhaskell/stack-templates#112 can be done sooner. If this ends up blocking release for too long, we can go ahead and ignore it. It might take a little while to add it to hpack and do a release of hpack. I will hopefully get around to it in the next few days, I have it on my TODO list.

[snoyberg]

I hadn't thought about this being in hpack instead. That makes sense, but also makes it pretty difficult to list as a P0 issue, since we have no control over the hpack release schedule.

In any event: CCing @sol. This issue is about the fact that a number of users have been tripped up by editing the generated cabal files instead of package.yaml files. We've put together a proposed solution above. How do you feel about including something like this in hpack?

[sol]

@snoyberg sounds sensible. I'm looking into it.

[sol]

As you guys want this as soon as possible, I implemented this based on the 0.19.3 release, so that we can do a minor release that just adds this feature.

I did some refactoring first, the actual change in behavior is in
sol/hpack@fc51e3e.

[snoyberg]

That's much appreciated, thanks @sol! I'll try to look at this today and provide some feedback.