Due to the diversity of implementations, many CSL files go beyond the CSL spec, and some even go beyond the CSL-M extension.
Therefore, it would be quite helpful if we provide better error messages for malformed CSL files, for both end authors and developers.

• Show the place of the error in the CSL file
• Show what data did not match any variant of the untagged enum
• Indicate whether the CSL file is really invalid or hayagriva does not support the feature yet
• (Optional) Ignore minor errors (take them as warnings) and continue parsing

Relates to typst/citationberg#22 and typst/citationberg#23, but this issue covers more.

Common errors

According to the survery, 222 of 302 (74%) CSL files in https://github.com/zotero-chinese/styles/tree/435cf756bf8bfaa193be236e27a347560ee39f54/src are considered malformed by hayagriva 799cfdc.
The error messages are as the follows.

• 196 (86%) × data did not match any variant of untagged enum Term
  (probably caused by space-et-al chinese et al. #291)
• 8 (4%) × unknown variant institution, expected one of name, et-al, label, substitute
  (might relate to Some entries in thesis and report bibliography items are not shown #112)
• 6 × data did not match any variant of untagged enum Variable
• 4 × data did not match any variant of untagged enum TextTarget
• 4 × missing field $value
• 2 × unknown variant monograph, expected one of article, article-journal, article-magazine, article-newspaper, bill, book, broadcast, chapter, classic, collection, dataset, document, entry, entry-dictionary, entry-encyclopedia, event, figure, graphic, hearing, interview, legal_case, legislation, manuscript, map, motion_picture, musical_score, pamphlet, paper-conference, patent, performance, periodical, personal_communication, post, post-weblog, regulation, report, review, review-book, software, song, speech, standard, thesis, treaty, webpage
• 1 × missing field term
• 1 × unknown variant last, expected first or all

The duplicate field layout error should be the most common one at present.
It's caused by CSL-M and tracked by typst/citationberg#5.
However, there are recent efforts improving it (e.g., #126), so I exclude this error by keeping only the last <layout> for <citation> and <bibliography> in CSL files.

Links

The following discussions (in Chinese) describe the discrepancies in the CSL world.

• 添加 Schema 以验证 CSL 语法 zotero-chinese/styles#518
• 请问`citation-range-delimiter`是什么？ zotero-chinese/styles#439