====================== Serialization Overview ====================== Introduction ============ NimYAML tries hard to make transforming YAML characters streams to native Nim types and vice versa as easy as possible. In simple scenarios, you might not need anything else than the two procs `dump `_ and `load `_. On the other side, the process should be as customizable as possible to allow the user to tightly control how the generated YAML character stream will look and how a YAML character stream is interpreted. An important thing to remember in NimYAML is that unlike in interpreted languages like Ruby, Nim cannot load a YAML character stream without knowing the resulting type beforehand. For example, if you want to load this piece of YAML: .. code-block:: yaml %YAML 1.2 --- !nim:system:seq(nim:system:int8) - 1 - 2 - 3 You would need to know that it will load a ``seq[int8]`` *at compile time*. This is not really a problem because without knowing which type you will load, you cannot do anything useful with the result afterwards in the code. But it may be unfamiliar for programmers who are used to the YAML libraries of Python or Ruby. Supported Types =============== NimYAML supports a growing number of types of Nim's ``system`` module and standard library, and it also supports user-defined object, tuple and enum types out of the box. A complete list of explicitly supported types is available in `Schema `_. **Important**: NimYAML currently does not support polymorphism. This may be added in the future. This also means that NimYAML is generally able to work with object, tuple and enum types defined in the standard library or a third-party library without further configuration, given that all fields of the object are accessible at the code point where NimYAML's facilities are invoked. Scalar Types ------------ The following integer types are supported by NimYAML: ``int``, ``int8``, ``int16``, ``int32``, ``int64``, ``uint8``, ``uint16``, ``uint32``, ``uint64``. Note that the ``int`` type has a variable size dependent on the target operation system. To make sure that it round-trips properly between 32-bit and 64-bit operating systems, it will be converted to an ``int32`` during loading and dumping. This will raise an exception for values outside of the range ``int32.low .. int32.high``! If you define the types you serialize yourself, always consider using an integer type with explicit length. The same goes for ``uint``. The floating point types ``float``, ``float32`` and ``float64`` are also supported. There is currently no problem with ``float``, because it is always a ``float64``. ``string`` is supported and one of the few Nim types which directly map to a standard YAML type. NimYAML is able to handle strings that are ``nil``, they will be serialized with the special tag ``!nim:nil:string``. ``char`` is also supported. To support new scalar types, you must implement the ``constructObject()`` and ``representObject()`` procs on that type (see below). Collection Types ---------------- NimYAML supports Nim's ``array``, ``set``, ``seq``, ``Table`` and ``OrderedTable`` types out of the box. Unlike the native YAML types ``!!seq`` and ``!!map``, Nim's collection types define the type of all their contained items (or keys and values). So YAML objects with heterogeneous types in them cannot be loaded to Nim collection types. For example, this sequence: .. code-block:: yaml %YAML 1.2 --- !!seq - !!int 1 - !!string foo Cannot be loaded to a Nim ``seq``. For this reason, you cannot load YAML's native ``!!map`` and ``!!seq`` types directly into Nim types. However, you can use variant object types to process heterogeneous value lists, see below. Nim ``seq`` types may be ``nil``. This is handled by serializing them to an empty scalar with the tag ``!nim:nil:seq``. Reference Types --------------- A reference to any supported non-reference type (including user defined types, see below) is supported by NimYAML. A reference type will be treated like its base type, but NimYAML is able to detect multiple references to the same object and dump the structure properly with anchors and aliases in place. It is possible to dump and load cyclic data structures without further configuration. It is possible for reference types to hold a ``nil`` value, which will be mapped to the ``!!null`` YAML scalar type. ``ptr`` types are not supported because it seems dangerous to automatically allocate memory which the user must then manually deallocate. User Defined Types ------------------ For an object or tuple type to be directly usable with NimYAML, the following conditions must be met: - Every type contained in the object/tuple must be supported - All fields of an object type must be accessible from the code position where you call NimYAML. If an object has non-public member fields, it can only be processed in the module where it is defined. - The object may not have a generic parameter NimYAML will present enum types as YAML scalars, and tuple and object types as YAML maps. Some of the conditions above may be loosened in future releases. Variant Object Types .................... A *variant object type* is an object type that contains one or more ``case`` clauses. NimYAML supports variant object types. Only the currently accessible fields of a variant object type are dumped, and only those may be present when loading. The value of a discriminator field must be loaded before any value of a field that depends on it. Therefore, a YAML mapping cannot be used to serialize variant object types - the YAML specification explicitly states that the order of key-value pairs in a mapping must not be used to convey content information. So, any variant object type is serialized as a list of key-value pairs. For example, this type: .. code-block:: nim type AnimalKind = enum akCat, akDog Animal = object name: string case kind: AnimalKind of akCat: purringIntensity: int of akDog: barkometer: int will be serialized as: .. code-block:: yaml %YAML 1.2 --- !nim:custom:Animal - name: Bastet - kind: akCat - purringIntensity: 7 You can also use variant object types for processing heterogeneous data sets. For example, if you have a YAML document which contains differently typed values in the same list like this: .. code-block:: yaml %YAML 1.2 --- - 42 - this is a string - !!null You can define a variant object type that can hold all types that occur in this list in order to load it: .. code-block:: nim import yaml type ContainerKind = enum ckInt, ckString, ckNone Container = object case kind: ContainerKind of ckInt: intVal: int of ckString: strVal: string of ckNone: discard markAsImplicit(Container) var list: seq[Container] s = newFileStream("in.yaml") load(s, list) ``markAsImplicit`` tells NimYAML that you want to use the type ``Container`` implicitly, i.e. its fields are not visible in YAML, and are set dependent on the value type that gets loaded into it. The type ``Container`` must fullfil the following requirements: - It must contain exactly one ``case`` clause, and nothing else. - Each branch of the ``case`` clause must contain exactly one field, with one exception: There may be at most one branch that contains no field at all. - It must not be a derived object type (this is currently not enforced) When loading the sequence, NimYAML writes the value into the first field that can hold the value's type. All complex values (i.e. non-scalar values) *must* have a tag in the YAML source, because NimYAML would otherwise be unable to determine their type. The type of scalar values will be guessed if no tag is available, but be aware that ``42`` can fit in both ``int8`` and ``int16``, so in the case you have fields for both types, you should annotate the value. When dumping the sequence, NimYAML will always annotate a tag to each value it outputs. This is to avoid possible ambiguity when loading. If a branch without a field exists, it is represented as a ``!!null`` value. Tags ==== NimYAML uses local tags to represent Nim types that do not map directly to a YAML type. For example, ``int8`` is presented with the tag ``!nim:system:int8``. Tags are mostly unnecessary when loading YAML data because the user already defines the target Nim type which usually defines all types of the structure. However, there is one case where a tag is necessary: A reference type with the value ``nil`` is represented in YAML as a ``!!null`` scalar. This will be automatically detected by type guessing, but if it is for example a reference to a string with the value ``"~"``, it must be tagged with ``!!string``, because otherwise, it would be loaded as ``nil``. As you might have noticed in the example above, the YAML tag of a ``seq`` depends on its generic type parameter. The same applies to ``Table``. So, a table that maps ``int8`` to string sequences would be presented with the tag ``!n!tables:Table(tag:nimyaml.org,2016:int8,tag:nimyaml.org,2016:system:seq(tag:yaml.org,2002:string))``. These tags are generated on the fly based on the types you instantiate ``Table`` or ``seq`` with. You may customize the tags used for your types by using the template `setTagUri `_. It may not be applied to scalar and collection types implemented by NimYAML, but you can for example use it on a certain ``seq`` type: .. code-block:: nim setTagUri(seq[string], "!nim:my:seq") Customizing Field Handling ========================== NimYAML allows the user to specify special handling of certain object fields. This configuration will be applied at compile time when NimYAML generates the (de)serialization code for an object type. It is important that the configuration happens before any YAML operations (e.g. ``load`` or ``dump``) are executed on a variable of the object type. Transient Fields ---------------- It may happen that certain fields of an object type are transient, i.e. they are used in a way that makes (de)serializing them unnecessary. Such fields can be marked as transient. This will cause them not to be serialized to YAML. They will also not be accepted when loading the object. Example: .. code-block:: nim type MyObject: object storable: string temporary: string markAsTransient(MyObject, temporary) Default Values -------------- When you load YAML that has been written by a human, you might want to allow the user to omit certain fields, which should then be filled with a default value. You can do that like this: .. code-block:: nim type MyObject: object required: string optional: string setDefaultValue(MyObject, optional, "default value") Whenever ``MyObject`` now is loaded and the input stream does not contain the field ``optional``, that field will be set to the value ``"default value"``. Customize Serialization ======================= It is possible to customize the serialization of a type. For this, you need to implement two procs, ``constructObject̀`` and ``representObject``. If you only need to process the type in one direction (loading or dumping), you can omit the other proc. constructObject --------------- .. code-block:: nim proc constructObject*(s: var YamlStream, c: ConstructionContext, result: var MyObject) {.raises: [YamlConstructionError, YamlStreamError.} This proc should construct the type from a ``YamlStream``. Follow the following guidelines when implementing a custom ``constructObject`` proc: - For constructing a value from a YAML scalar, consider using the ``constructScalarItem`` template, which will automatically catch exceptions and wrap them with a ``YamlConstructionError``, and also will assure that the item you use for construction is a ``yamlScalar``. See below for an example. - For constructing a value from a YAML sequence or map, you **must** use the ``constructChild`` proc for child values if you want to use their ``constructObject`` implementation. This will check their tag and anchor. Always try to construct child values that way. - For non-scalars, make sure that the last value you remove from the stream is the object's ending event (``yamlEndMap`` or ``yamlEndSequence``) - Use `peek `_ for inspecting the next event in the ``YamlStream`` without removing it. - Never write a ``constructObject`` proc for a ``ref`` type. ``ref`` types are always handled by NimYAML itself. You can only customize the construction of the underlying object. The following example for constructing from a YAML scalar value is the actual implementation of constructing ``int`` types: .. code-block:: nim proc constructObject*[T: int8|int16|int32|int64]( s: var YamlStream, c: ConstructionContext, result: var T) {.raises: [YamlConstructionError, YamlStreamError].} = var item: YamlStreamEvent constructScalarItem(s, item, name(T)): result = T(parseBiggestInt(item.scalarContent)) The following example for constructiong from a YAML non-scalar is the actual implementation of constructing ``seq`` types: .. code-block:: nim proc constructObject*[T](s: var YamlStream, c: ConstructionContext, result: var seq[T]) {.raises: [YamlConstructionError, YamlStreamError].} = let event = s.next() if event.kind != yamlStartSequence: raise newException(YamlConstructionError, "Expected sequence start") result = newSeq[T]() while s.peek().kind != yamlEndSequence: var item: T constructChild(s, c, item) result.add(item) discard s.next() representObject --------------- .. code-block:: nim proc representObject*(value: MyObject, ts: TagStyle = tsNone, c: SerializationContext, tag: TagId): {.raises: [].} This proc should push a list of tokens that represent the type into the serialization context via ``c.put``. Follow the following guidelines when implementing a custom ``representObject`` proc: - You can use the helper template `presentTag `_ for outputting the tag. - Always output the first token with a ``yAnchorNone``. Anchors will be set automatically by ``ref`` type handling. - When outputting non-scalar types, you should use the ``representObject`` implementation of the child types, if possible. - Always use the ``tag`` parameter as tag for the first token you generate. - Never write a ``representObject`` proc for ``ref`` types. The following example for representing to a YAML scalar is the actual implementation of representing ``int`` types: .. code-block:: nim proc representObject*[T: int8|int16|int32|int64](value: T, ts: TagStyle, c: SerializationContext, tag: TagId) {.raises: [].} = ## represents an integer value as YAML scalar c.put(scalarEvent($value, tag, yAnchorNone)) The following example for representing to a YAML non-scalar is the actual implementation of representing ``seq`` and ``set`` types: .. code-block:: nim proc representObject*[T](value: seq[T]|set[T], ts: TagStyle, c: SerializationContext, tag: TagId) {.raises: [YamlStreamError].} = ## represents a Nim seq as YAML sequence let childTagStyle = if ts == tsRootOnly: tsNone else: ts c.put(startSeqEvent(tag)) for item in value: representChild(item, childTagStyle, c) c.put(endSeqEvent())