Question

Should the following be valid?

parent:
- child
- child

So what we have is a sequence of values inside a mapping.

The specific question is about whether the indentation for the 2nd and 3rd lines is valid. The Ruby YAML.dump generated this code, but the Yaml parser here rejects it, because the child lines are not indented.

i.e. it wants something like:

parent:
  - child
  - child

Who is right?

Looking at the YAML spec, it’s certainly not obvious, and the line

The “-”, “?” and “:” characters used to denote block collection entries are perceived by people to be part of the indentation

doesn’t help much.

Answer 1

Yes, that is legal YAML. The relevant text from the spec is here:

Since people perceive the “-” indicator as indentation, nested block sequences may be indented by one less space to compensate, except, of course, if nested inside another block sequence (block-out context vs. block-in context).

and the subsequent example 8.22:

sequence: !!seq
- entry
- !!seq
 - nested
mapping: !!map
 foo: bar