Reasons:
- Performance: HsYAML is around 20 times slower in parsing
large YAML bibliographies (#6084).
- An issue was submitted to HsYAML, but it hasn't gotten
any attention. HsYAML seems borderline unmaintained; it hasn't
had a commit in over a year.
- Unfortunately this goes back on our attempts to free ourselves
from C dependencies (#4535). But I don't see a better alternative
until a better pure Haskell parser is available.
Closes#6084.
Notes:
- We've removed the FromYAML instances for all types that had
them, since this is a HsYAML-specific typeclass [API change].
(The yaml package just uses From/ToJSON.)
- Unlike HsYAML (in the configuration we were using), yaml
parses 'Y', 'N', 'Yes', 'No', 'On', 'Off' as boolean values.
Users may need to quote these when they are meant to be
interpreted as strings. Similarly, 'null' is parsed as
a YAML null value (and will be treated as an empty string
by pandoc rather than the string 'null'). Quoting it will
force it to be interpreted as a string.
- Some tests had to be adjusted accordingly.
- Pandoc now behaves better when the YAML metadata contains
escaping errors: instead of just falling back on treating
the section as a table, it raises a YAML parsing error.
This fixes the citation number issue with ieee.csl and other
styles that do not explicitly sort bibliographies. (Pandoc
was numbering them by their order in the bibliography file,
rather than the order cited, as required by the CSL spec.)
Closes#6741.
