YAML parser for Fantom



A set of non-exhaustive tests to ensure basic functionality.


A YamlObj whose content always has the type YamlObj[].


A YamlObj whose content always has the type YamlObj:YamlObj.


The base class for objects that represent nodes in a YAML hierarchy.


Converts YAML text into a hierarchy of YamlObjs, which can in turn be converted into native Fantom objects.


A YamlObj whose content always has the type Str.


A class used to convert parsed YamlObjs to native Fantom objects, and vice versa.



YamlTagErr indicates that a YAML tag is set that does not match its node's content.



This pod provides the means for parsing YAML files into native Fantom objects, as well as transforming Fantom objects back into YAML text. For information about YAML, see https://yaml.org/.

To do this, the intermediate class YamlObj is used, representing an abstract YAML node. Each node has a type (YamlScalar, YamlList, or YamlMap), a tag (e.g. tag:yaml.org,2002:str or ? for an non-specific tag), and content (which, depending on the node type, could contain further nodes). This can then be converted to a YAML string or a Fantom object.

This parser is fully compliant with the YAML 1.2.2 specification, other than disallowing nodes which contain themselves.

Reading YAML documents

First, YamlReader.parse parses YAML text from a stream into a YamlObj hierarchy. Then, YamlObj.decode finishes the process, transforming the YamlObj into a native Fantom object. (See the schema section on how to fine-tune this process if desired.)

Since multiple documents may be contained within a single YAML stream, the result of YamlReader.parse is always a YamlList tagged with tag:yaml.org,2002:stream which contains the list of documents as content. This way, you may call YamlObj.decode directly on the result, and it will correctly output the list of documents as a list of Fantom objects.

str := "---
        - This is
        - a list!
        - which is in a different
          document from...
        foo: 1
        inner list: [a,b,c] # comment
res := YamlReader(str.in).parse.decode

  ["This is", "a list!", "which is in a different document from..."],
  ["foo": 1, "map": null, "inner list": ["a","b","c"]]

Writing YAML documents

This time, the process occurs in reverse: native Fantom objects can be converted to YamlObjs, which can then be written to text.

To convert a Fantom object to a YamlObj, use YamlSchema.encode with a schema of your choice (YamlSchema.core is recommended). Then, to convert that YamlObj to text, call YamlObj.write:

map := ["one key": [1, 2, 3], "multiline\nkey": [:], ["complex", "key"].toImmutable: false]
yamlObj := YamlSchema.core.encode(map)

//written to standard out

? - complex
  - key
: false
"multiline\nkey": {}
one key:
 - 1
 - 2
 - 3

You may also use YamlObj.toStr to write this content into a string rather than an OutStream. Note that all objects written either way end with a newline character.

Also note that any serializable object can be encoded in YAML:

  • A simple is just encoded as a string.
  • A complex is encoded as a map, where each field is a map entry.
  • A collection is encoded like a complex, except there is an additional each field that contains the child items.

The object as a whole is then tagged as !fan/pod::Type.

Finally, note that this will always use YAML's block style to write objects, as it is not possible for some YAML objects to be written in flow style and still be preserved. If you would like to write objects in flow style anyway, use JsonOutStream.


Often, YAML documents contain plain, untagged text, such as Text or 1. (This is represented in YamlObjs by the ? tag.) Sometimes, we want these to be parsed as something other than a Str; for example, we might want 1 to be parsed as an Int, true as a Bool, and so on.

This is where the YamlSchema class comes in. Schemas are all about assigning and reading tags. Different schemas may assign different tags to ?-tagged nodes (reading) or Fantom objects (writing), and may treat tagged nodes differently when creating Fantom objects.

This lists the differences between the built-in schemas (where !! is short for tag:yaml.org,2002:):

| Schema      | Recognized tags => types | Main uses                           |
| YamlSchema. | !!str   => Str           | Useful when you want all text to be |
| failsafe    | !!seq   => List          | treated literally. For example,     |
|             | !!map   => Map           | empty keys will be treated as ""    |
|             |                          | instead of null.                    |
| YamlSchema. | failsafe's tags, +       | Useful if you want to parse JSON-   |
| json        | !!null  => null          | like text that may be mixed with    |
|             | !!bool  => Bool          | YAML's compact styles. Though it    |
|             | !!int   => Int           | won't notice anything style-        |
|             | !!float => Float         | related, it will error if there     |
|             |                          | are plain, untagged strings that    |
|             |                          | are not null/booleans/ints/floats.  |
| YamlSchema. | json's tags              | Useful in most cases (and thus      |
| core        |                          | used as the default schema). A wide |
|             |                          | variety of patterns can be used     |
|             |                          | to indicate non-Str content (e.g.   |
|             |                          | ints can be specified in hex or     |
|             |                          | octal). Unlike the JsonSchema, the  |
|             |                          | CoreSchema accepts plain strings.   |

See https://yaml.org/spec/1.2.2/#recommended-schemas for more information about each schema and exact definitions of each pattern.


str := "int: 1
        plain: Text
        large float: -2E+05
parsed   := YamlReader(str.in).parse
failsafe := YamlSchema.failsafe.decode(parsed)
core     := YamlSchema.core.decode(parsed)

//failsafe content - text is just literal
  ["int": "1", "empty": "", "plain": "Text", "large float": "-2E+05"]

//core content - text is interpreted in different ways
  ["int": 1, "empty": null, "plain": "Text", "large float": -20000.0f]

Creating custom schemas

To interpret additional tags, a custom schema must be created. To do this, it is recommended that you subclass the CoreSchema class (which is itself a subclass of YamlSchema), adding the extra tags you want from there. This makes it possible to keep all of the core schema's tags in use (e.g. 0x05 is still interpreted as the Int 5 without further work needed).

YamlSchemas all have three protected helper methods that you inherit: assignTag, validate, and isRecognized. The first is meant to be used for ?-tagged nodes, giving the node a proper tag, while the second is meant to be used for nodes with specified tags, ensuring that a !!int- tagged node does not contain "true" as content instead of, say, "5". The third just returns true if the schema recognizes a given tag. In each of these methods, if super.isRecognized would return true for a node (or super.assignTag assigns something other than !!str), you should use the superclass's method before defaulting to your own code; e.g. if a node contains the content true, calling super.assignTag will correctly return the !!bool tag. See the implementations of the JSON and core schemas in YamlSchema.fan for examples of this, both which inherit from the failsafe schema.