Error message for missing package list is unclear when looking for build-tool-depends

[roberth]

Describe the bug

When the package list is missing, build-tool-depends causes a cryptic error message.

cc @chuahou

To Reproduce
Steps to reproduce the behavior:

See https://stackoverflow.com/a/65287190/428586

$ rm -rf ~/.cabal

$ cabal v2-build
[... omitted ...]
[__0] trying: aoc-0.1.0.0 (user goal)
[__1] unknown package: aoc:happy:exe.happy (dependency of aoc)
[__1] fail (backjumping, conflict set: aoc, aoc:happy:exe.happy)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: aoc, aoc:happy:exe.happy

$ cabal update
Downloading the latest package list from hackage.haskell.org

$ cabal v2-build
Resolving dependencies...
Build profile: -w ghc-8.10.2 -O1
In order, the following will be built (use -v for more details):
 - alex-3.2.6 (exe:alex) (requires download & build)
 - happy-1.20.0 (exe:happy) (requires download & build)
 - aoc-0.1.0.0 (lib) (configuration changed)
 - aoc-0.1.0.0 (exe:aoc) (dependency rebuilt)
Downloading  happy-1.20.0
[... omitted ...]
Configuring library for aoc-0.1.0.0..
Preprocessing library for aoc-0.1.0.0..
Building library for aoc-0.1.0.0..
[... omitted ...]

Please use version-prefixed commands (e.g. v2-build or v1-build) to avoid ambiguity.

Expected behavior

A clear error message indicating that the package list wasn't present on the system.
Perhaps it could be downloaded automatically.

System information

• Operating system
• cabal, ghc versions

Additional context
Add any other context about the problem here.

[phadej]

If you read the output properly:

% cabal build --dry
Warning: The package list for 'hackage.haskell.org' does not exist. Run 'cabal
update' to download it.RemoteRepo {remoteRepoName = RepoName
"hackage.haskell.org", remoteRepoURI = http://hackage.haskell.org/,
remoteRepoSecure = Just True, remoteRepoRootKeys =
["fe331502606802feac15e514d9b9ea83fee8b6ffef71335479a2e68d84adc6b0","1ea9ba32c526d1cc91ab5e5bd364ec5e9e8cb67179a471872f6e26f0ae773d42","2c6c3627bd6c982990239487f1abd02e08a02e6cf16edb105a8012d444d870c3","0a5c7ea47cd1b15f01f5f51a33adda7e655bc0f0b0615baa8e271f4c3351e21d","51f0161b906011b52c6613376b1ae937670da69322113a246a09f807c62f6921"],
remoteRepoKeyThreshold = 3, remoteRepoShouldTryHttps = True}
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: splitmix-tools-0 (user goal)
[__1] unknown package: vector (dependency of splitmix-tools)
[__1] fail (backjumping, conflict set: splitmix-tools, vector)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: splitmix-tools, vector

it literally starts with

Warning: The package list for 'hackage.haskell.org' does not exist. Run 'cabal
update' to download it.

This is no bug.

Whether to download automatically, I have no opinion. In cabal-install-3.4 there is a way to disable use of configured package repositories (previously the only way was to alter the actual config), so enabling download when repository doesn't exist at all should be fine.

[roberth]

I agree that it's not a bug. It's "just" bad user experience.

Maybe I got numbed by (other) verbose build logs or may I got demotivated by the weird "show RemoteRepo" logging.
Either way, it seems like a bad idea to require extra steps for something that 99% of users need to do anyway.

Possible actions

• remove "show RemoteRepo" logging. Users may skip past such logs. (this might have been a patched cabal? idk)
• add some logic to the final message specifically for this case (no package list)
• download the package list automatically. Not sure if that's entirely great, because new users will have to learn the relevance of cabal update at some point.

[chuahou]

Thank you @roberth for bringing this here from Stack Overflow.

I personally feel this isn't a cabal problem, but rather a nix problem, though. I doubt anyone using cabal by itself without nix would try to pull dependencies without cabal update, and the package list warning is indeed front and centre at the top as @phadej mentioned.

The only reason I ignored that is because I would have liked to use nix to provide those dependencies to cabal, rather than pull from Hackage—this had been working for build-depends, and nix-build manages to do this properly even for build-tool-depends, but only nix-shell seems to have this problem.

Hence, it seems to me that it's likely to be an issue with nix-shell's provision of dependencies to cabal.

---

Edit: @phadej's errors appearing for build-depends as well is because I hadn't had shell.nix set up correctly in the reproduce environment previously. It has now been rectified.

[ScottFreeCode]

About the nix-shell version of this problem, where the error occurs even if the tools are provided, I have a straightforward reproduction here: https://gist.github.com/ScottFreeCode/c1adb5681c5b61c61373615f63a4a7d2

And I've collected GitHub issues relating to this problem here: https://gist.github.com/ScottFreeCode/ef9f254e2dd91544bba4a068852fc81f