stack_snapshot ignores the cabal flags defined in the snapshot

[thufschmitt]

Describe the bug

Stack snapshots can define custom cabal flags for building certain packages. The stack_snapshot rule ignores them.

To Reproduce

From the rules_haskell repository

1. Add cassava to the packages argument of stack_snapshot in the WORKSPACE
2. Build it with bazel build @hackage//:cassava

The build will fail with

Setup.hs: Encountered missing dependencies:
bytestring >=0.9.2 && <0.10.4, text-short ==0.1.*

Now add flags = { "cassava": ["-bytestring--LT-0_10_4"], }, to the arguments of stack_snapshot and build it again. The build will succeed

Expected behavior

Cassava should be built with the correct flag out of the box because it is specified in the snapshot file

Environment

• OS name + version: Linux 4.19.75 Initial haskell_binary implementation. #1-NixOS
• Bazel version: 1.1.0 (from the nix-shell)
• Version of the rules: 33743d2 (latest master)

/cc @mboes

[mboes]

@aherrmann I'm at a loss as to how to resolve this. Stack doesn't expose enough metadata. We'd need
to add new subcommands to stack ls to expose this information, or augment the new JSON output. Any other ideas?

[aherrmann]

Yes, it seems this requires more output from stack ls dependencies --json. This makes me wonder, if this becomes available, should stack_snapshot still accept Cabal flags, or should users specify those in custom stack snapshot files?

[mboes]

Using a custom snapshot file may well be cleaner. Best to keep as much Stack configuration in Stack specific files, rather than have that split between Stack specific files and the WORKSPACE file.

[aherrmann]

Upstream feature request about exposing the needed metadata: commercialhaskell/stack#5372