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API Overview


NimYAML advocates parsing YAML input into native Nim types. Basic Nim library types like integers, floats and strings, as well as all tuples, enums and objects without private fields are supported out-of-the-box. Reference types are also supported, and NimYAML is able to detect if a reference occurs more than once and will serialize it accordingly. This means that NimYAML is able to dump and load potentially cyclic objects.

While loading into and dumping from native Nim types is the preferred way to use NimYAML, it also gives you complete control over each processing step, so that you can for example only use the parser and process its event stream yourself. The following diagram gives an overview of NimYAML's features based on the YAML processing pipeline. The items and terminology YAML defines is shown in italic, NimYAML's implementation name is shown in bold.

Intermediate Representation

The base of all YAML processing with NimYAML is the YamlStream. This is basically an iterator over YamlStreamEvent objects. Every proc that represents a single stage of the loading or dumping process will either take a YamlStream as input or return a YamlStream. Procs that implement the whole process in one step hide the YamlStream from the user. Every proc that returns a YamlStream guarantees that this stream is well-formed according to the YAML specification.

This stream-oriented API can efficiently be used to parse large amounts of data. The drawback is that errors in the input are only discovered while processing the YamlStream. If the YamlStream encounters an exception while producing the next event, it will throw a YamlStreamError which contains the original exception as parent. The caller should know which exceptions are possible as parents of YamlStream because they know the source of the YamlStream they provided.

Loading YAML

If you want to load YAML character data directly into a native Nim variable, you can use load. This is the easiest and recommended way to load YAML data. This section gives an overview about how load is implemented. It is absolutely possible to reimplement the loading step using the low-level API.

For parsing, a YamlParser object is needed. This object stores some state while parsing that may be useful for error reporting to the user. The parse proc implements the YAML processing step of the same name. All syntax errors in the input character stream are processed by parse, which will raise a YamlParserError if it encounters a syntax error.

Transforming a YamlStream to a native YAML object is done via construct. It skips the compose step for efficiency reasons. As Nim is statically typed, you have to know the target type when you write your loading code. This is different from YAML APIs of dynamically typed languages. If you cannot know the type of your YAML input at compile time, you have to manually process the YamlStream to serve your needs.

Dumping YAML

Dumping is preferredly done with dump, which serializes a native Nim variable to a character stream. As with load, the following paragraph describes how dump is implemented using the low-level API.

A Nim value is transformed into a YamlStream with represent. Depending on the AnchorStyle you specify, this will transform ref variables with multiple instances into anchored elements and aliases (for asTidy and asAlways) or write the same element into all places it occurs (for asNone). Be aware that if you use asNone, the value you serialize might not round-trip.

Transforming a YamlStream into YAML character data is done with present. You can choose from multiple presentation styles. psJson is not able to process some features of YamlStream s, the other styles support all features and are guaranteed to round-trip to the same YamlStream if you parse the generated YAML character stream again.

The Document Object Model

Much like XML, YAML also defines a document object model. If you cannot or do not want to load a YAML character stream to native Nim types, you can instead load it into a YamlDocument. This YamlDocument can also be serialized into a YAML character stream. All tags will be preserved exactly as they are when transforming from and to a YamlDocument. The only important thing to remember is that when a value has no tag, it will get the non-specific tag "!" for quoted scalars and "?" for all other nodes.

While tags are preserved, anchors will be resolved during loading and re-added during serialization. It is allowed for a YamlNode to occur multiple times within a YamlDocument, in which case it will be serialized once and referred to afterwards via aliases.

The document object model is provided for completeness, but you are encouraged to use native Nim types as start- or endpoint instead. That may be significantly faster, as every YamlNode is allocated on the heap and subject to garbage collection.