On % being a valid first character in a content line

To the YAML spec writers,

According to the document markers section in the current spec, 'It is  
valid to have a “%” character at the start of a line (e.g. as the  
first character of a top-level mapping key).' This does not seem to be  
true under either the current spec or PyYAML.

Implicit block mapping keys, in the case of plain scalars, are  
governed by ns-plain-one-line. This rule starts with ns-plain-first,  
which excludes c-indicator, which includes "%". In other words, the  
node cannot start with a "%". In fact, this is true for any plain  
scalar that begins with "%": it's not allowed.

The spec seems to give the following example as valid YAML containing  
a node starting with "%":
%YAML 1.2
---
%PS-Adobe-2.0: 777
According to InstantYAML (http://instantyaml.appspot.com/, uses PyYAML  
3.08), this is an invalid YAML 1.1 stream—it tries to parse "%PS- 
Adobe-2.0: 777" as a directive.

So:
- In YAML 1.1, were "%" characters allowed to start plain scalar nodes  
at the beginning of a line?
  (I cannot find a section that talks about this feature in the old  
YAML 1.1 spec.)

- Is InstantYAML's behavior correct? Is it correct for YAML 1.2 too?

- The spec's rules seem to contradict this feature. Is this actually so?

- Would it be possible to *change* this, so that non-directive lines  
cannot begin with "%"?
  Disallowing characters that are well known to be indicators, such as  
"%", "!", "?", etc. is
  reasonable and sane behavior—not just for the person writing the  
YAML parser, but also the
  person using the YAML documents. Making a strange exception just for  
the sake of letting a
  scalar beginning with "%" be a plain scalar places a burden on both  
people.

  This is, in my opinion, a higher cost than just requiring the  
scalars to be quoted, just like a
  scalar beginning with any other indicator. I ask this pleadingly,  
because the parser library I'm
  writing currently depends on this—allowing "%" to start a non- 
directive line would make
  parsing much more complicated.

Sincerely,
Joshua Choi


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