YAML: Node Anchor followed by a blank line: "Nested mappings are not allowed in compact mappings"

[jdufresne]

Prettier 2.2.1
Playground link

Input:

key1: &default

  subkey1: value1

key2:
  <<: *default

Output:

test.yml[error] test.yml: SyntaxError: Nested mappings are not allowed in compact mappings (1:7)
[error] > 1 | key1: &default
[error]     |       ^^^^^^^^
[error] > 2 | 
[error]     | ^
[error] > 3 |   subkey1: value1
[error]     | ^
[error] > 4 | 
[error]     | ^
[error]   5 | key2:
[error]   6 |   <<: *default
[error]   7 | 

Expected behavior:

No error. The file should be formatted.

Both Python and Ruby YAML libraries load this file with the expected results.

In my production code, the blank line exists there to separate a comment.

key1: &default

  # This key ...
  subkey1: value1

key2:
  <<: *default

Which actually results in a different error:

test.yml[error] test.yml: SyntaxError: All collection items must start at the same column (1:1)
[error] > 1 | key1: &default
[error]     | ^^^^^^^^^^^^^^
[error] > 2 | 
[error]     | ^
[error] > 3 |   # This key ...
[error]     | ^
[error] > 4 |   subkey1: value1
[error]     | ^
[error] > 5 | 
[error]     | ^
[error] > 6 | key2:
[error]     | ^
[error] > 7 |   <<: *default
[error]     | ^
[error] > 8 | 
[error]     | ^

[thorn0]

To confirm that this is valid YAML and the parsing errors are incorrect, please use the spec, not 3rd party implementations. I'm really sure about the robustness of Prettier's underlying parser: yaml-unist-parser, which in turn uses eemeli/yaml, but if you can prove this is indeed an error on their part, we'll open an issue in the corresponding repo.

[jdufresne]

Yes, this is valid YAML. There is nothing in the spec suggesting blank lines between keys are invalid, even when a node anchor is involved.

I only presented other tools as examples of compliant tools, not as an authority. Sorry for the confusion.

[thorn0]

Okay, thanks for the report. I'll try to find time to look into it. Meanwhile, to make things happen faster, you can open an issue in https://github.com/eemeli/yaml yourself.

[jdufresne]

👍 I'll go ahead with that. Thanks for the response.

[thorn0]

Great. Thanks. Let's keep the issue open though.

[jdufresne]

Upstream issue eemeli/yaml#242

[eemeli]

Yeah, this is a bug in yaml. There it's already fixed in its master branch, via the new lexer + parser that's a major reason for the library's ongoing v2 overhaul.

So this poses a bit of a problem, as I've got my hands full in that work, and I'd been hoping to avoid needing to do more v1 patch releases. For most users, v2 should work perfectly fine without any changes, but underneath, the CST changes completely. And as I recall, that's what @ikatyang's yaml-unist-parser uses.

Posting about this here to make sure that you have visibility into this; not sure yet about how to resolve this.

[eemeli]

I resolved the issue by releasing a patch update yaml@1.10.1.

[thorn0]

Thanks @eemeli. This way we can fix it without touching yaml-unist-parser.

However, I noticed something. Is this expected?

Prettier pr-10516
Playground link

Input:

key1: &default
# This key
  subkey1: value1

key2:
  <<: *default

Output:

SyntaxError: All collection items must start at the same column (1:1)
> 1 | key1: &default
    | ^^^^^^^^^^^^^^
> 2 | # This key
    | ^^^^^^^^^^
> 3 |   subkey1: value1
    | ^^^^^^^^^^
> 4 |
    | ^^^^^^^^^^
> 5 | key2:
    | ^^^^^^^^^^
> 6 |   <<: *default
    | ^^^^^^^^^^^^^^^

[eemeli]

No, that's not expected. Bother. But I'm pretty sure I can fix this pretty easily. You may expect a yaml@1.10.2 shortly.

[eemeli]

Yah, it was what I thought. yaml@1.10.2 is out with this fixed.

[jdufresne]

Thank you @eemeli and @thorn0 for the very prompt responses and fixes! 🎉 🚀

[thorn0]

@jdufresne You aren't going to like the resulting formatting though. As can be seen in the snapshots in the PR, the comment after the anchor doesn't stay on its own line. That'll be a separate issue.