YAML is more complex that most people tend to realize. (This was brought up in a 2011 discussion about possibly standardizing a metadata section for Markdown documents which sadly went nowhere. [1])

Take a look at example 2.11 in the YAML spec [2], for example, and see if you can make heads or tails of it.

[1]: https://pairlist6.pair.net/pipermail/markdown-discuss/2011-A...

[2]: http://www.yaml.org/spec/1.2/spec.html#id2760395

pyYAML has this collection of problems with the spec: http://pyyaml.org/wiki/BugsInTheYAMLSpecification

(and pyYAML itself can't always parse its own output correctly...)

You don't need most of those features. A pared down YAML with the cruft removed (implicit typing, flow style, tag tokens, node anchor & references) is actually pretty simple as well as less "gotcha-y".

Sure, but most language YAML parsers support all or most of the spec. That can be a problem if you aren't expecting it.

I believe it has even created security issues. Didn’t Rails have at least one YAML-based vuln?

You need to restrict YAML to SecureLoad, with manually adding allowed typed and classes.

At least perl doesn't support this, so it's inherently insecure there, but you can always use YAML::Syck which didn't go this way.

From hjson.org:

"YAML expresses structure through whitespace. Significant whitespace is a common source of mistakes that we shouldn't have to deal with."

Okay, does anyone actually believe that causes a problem?

I can think of ONE time when that causes a problem, and that's with indentation with multi-line strings. Oh look, HJSON included that feature. That's like throwing the baby out and keeping all the bathwater.

I can't find a specific example off the top of my head but I'll say I've been managing a Jekyll site for a while now and whitespace errors in frontmatter and data files cause all kinds of problems. I'm not sure I could explain the details but it's a legit criticism of YAML. IMO part of the problem is that YAML looks very straightfowrard and is until it suddenly isn't. Whitespace is part of that problem.

As an anecdote on the flip side, I've been building Middleman sites for a while now and can't remember ever having an issue with whitespace in the front matter or local data.

Yeah I'm with you. Developers should have no problem dealing with whitespace and the result is you get a easier to read format.

Although admittedly I haven't had to work with YAML a lot but I have liked it when i've touched it.

yaml has any number of ambiguous cases

Could you provide some examples?

Unquoted strings are valid in yaml just like this format. There are at least 2 ways to specify a list of things. There are some super bizarre looking possible formats for lists of mapping types.

There are a number of others given the length of the spec. Yaml is a complicated beast that generally has more than one way to do any given thing