What does a single exclamation mark do in YAML?

Question

I'm working with the YamlDotNet library and I'm getting this error when loading a YAML file:

While parsing a tag, did not find expected tag URI.

The YAML file is supposed to be well-formed because it comes right from RoR. The error seems to be triggered by this code:

formats:   default: ! '%d-%m-%Y'   long: ! '%d %B, %Y'   short: ! '%d %b' 

I'm not an expert, but I see from the YAML spec that you can use an exclamation mark to indicate a custom object/type, and two exclamation marks to indicate an explicit built-in type.

obj1: !custom # whatever obj2: !!str "My string" 

However I haven't been able to find any reference to an exclamation mark used as above. What does that mean, and why the YAML library I use doesn't seem able to parse it? Note that if I remove those exclamation marks, the file is parsed fine.

Answer 1

That ! is the non-specific tag.

The YAML specification 1.2 (as well as the previous 1.1) says that :

By explicitly specifying a “!” non-specific tag property, the node would then be resolved to a “vanilla” sequence, mapping, or string, according to its kind.

Take a look here to the tag "grammar":

none    : Unspecified tag (automatically resolved by application). '!'     : Non-specific tag (by default, "!!map"/"!!seq"/"!!str"). '!foo'  : Primary (by convention, means a local "!foo" tag). '!!foo' : Secondary (by convention, means "tag:yaml.org,2002:foo"). '!h!foo': Requires "%TAG !h! <prefix>" (and then means "<prefix>foo"). '!<foo>': Verbatim tag (always means "foo"). 

Why is YamlDotNet throwing a error? I can't be 100% sure, but I think you found a bug.

YamlDotNet is a port of LibYAML, so it's easy to compare sources.

Line 2635 of scanner.c (LibYAML):

/* Check if the tag is non-empty. */ if (!length) { 

Line 2146 of Scanner.cs (YamlDotNet ):

// Check if the tag is non-empty. if (tag.Length == 0) 

I know, both looks very similar, but at this point length is 1 and tag.Length is 0. Original C code takes care of the initial "!" (whole length) but C# doesn't do it (just the tag "name" length).

File an issue to the project.