E.g. "Y", "yes", which are now (with yaml library) considered
boolean values, as well as "null".
This fixes a bug with roundtripping markdown -> markdown:
```
---
foo: "true"
...
```
- For LineEnding use lowercase constructors, e.g. `crlf`, `native`.
This was the original intent, but there was a bug in the
implementation.
- For HTMLSlideVariant use lowercase constructors.
- For ReaderOptions use e.g. `default-image-extension`
instead of `readerDefaultImageExtension` for field names.
- For Extension, use e.g. `tex_math_dollars` instead of
`Ext_tex_math_dollars` as constructor.
- For Extensions, use an array of Extensions, instead of
an object wrapping the tag `Extensions` and an integer.
(The representation is not supposed to be part of the
public API.)
- For Opt, use field names like `tab-stop` instead of `optTabStop`.
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.
Properties of Block values are marshalled lazily, which generally
improves performance considerably. Script users may also notice the
following differences:
- Block element properties can no longer be accessed by numerical
indexing of the `.c` field. The `.c` property now serves as an alias
for `.content`, so some filter that used this undocumented method
for property access may continue to work, while others will need to
be updated and use proper property names.
- The marshalled Block elements now have a `show` method, and a
`__tostring` metamethod. Both return the Haskell string
representation of the element.
- Block values now have the Lua type `userdata` instead of `table`.
John MacFarlane