Malformed YAML can produce confusing errors

[jkachmar]

When a user supplies curator with malformed YAML, it returns the parse errors from the yaml package verbatim, with little to no additional context.

For example, given this malformed input to build-constraints.yaml:

# Constraints for brand new builds
packages:
  "Alice Roberts" <alice@roberts.com> @aroberts":

...curator will produce the following error (or similar):

curator: InvalidYaml (Just (YamlParseException {yamlProblem = "did not find expected key", yamlContext = "while parsing a block mapping", yamlProblemMark = YamlMark {yamlIndex = 388, yamlLine = 11, yamlColumn = 18}}))

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It's entirely reasonable that this issue is outside of the scope of curator, since it would rely on yaml returning more structured information for us to turn into a more meaningful response.

This was just reported in commercialhaskell/stackage#5520, and I figured this would be a better place to open a discussion.

[bergmark]

Ideally the error message would be improved in yaml. We could improve the error message ad hoc in curator, but it could be fragile as we need to match on strings which may not be considered stable in yaml's api (it's not in aeson's).

But we could do some small improvements here by printing YamlParseException's in a nicer way.

[juhp]

Perhaps this can be closed with Adam's newly introduced workflow