Go-Structform - Structured Formatters

go-structform provides capabilities for serializing, desirializing, and transcoding structured data efficiently and generically.

The top-level package provides the common layer by which serializers and deserializers interact with each other. Serializers convert the input into a stream of events by calling methods on the Visitor interfaces. The Deserializers implement the Visitor interface to convert the stream of events into the target structure.

A type implementing the Visitor interface, but forwards calls to another visitor is called a Transformer or Filter. Transformers/Filters can manipulate the data stream or create/collect errors if some wanted validation fails.

Examples

Transcode a stream of JSON objects into a stream of CBOR objects:

func TranscodeJSON2CBOR(out io.Writer, in io.Reader) (int64, error) {
	bytesCount, err := json.ParseReader(in, cborl.NewVisitor(out))
	return bytesCount, err
}

Parse a stream of JSON objects:

	var in io.Reader = ...
	visitor, _ := gotype.NewUnfolder(nil)
	dec := json.NewDecoder(in, 2048, visitor)
	for {
		var to interface{}
		visitor.SetTarget(&to)
		if err := dec.Next(); err != nil {
			return err
		}

		// process `to`
	}

Encode a channel of go map[string]interface{} to a stream of cbor objects:

	var out io.Writer = ...
	var objects chan map[string]interface{} = ...

	visitor := cborl.NewVisitor(out)
	it, _ := gotype.NewIterator(visitor)
	for _, obj := range objects {
		if err := it.Fold(obj); err != nil {
			return err
		}
	}

	return nil

Convert between go types:

	var to map[string]int
	visitor, _ := gotype.NewUnfolder(&to)
	it, _ := gotype.NewIterator(visitor)

	st := struct {
		A int    `struct:"a"`
		B string `struct:",omit"`
	}{A: 1, B: "hello"}
	if err := it.Fold(st); err != nil {
		return err
	}

	// to == map[string]int{"a": 1}

Use custom folder and unfolder for existing go types:

type durationUnfolder struct {
	gotype.BaseUnfoldState // Use BaseUnfoldState to create errors for methods not implemented
	to                     *time.Duration
}

func foldDuration(in *time.Duration, v structform.ExtVisitor) error {
	return v.OnString(in.String())
}

func unfoldDuration(to *time.Duration) gotype.UnfoldState {
	return &durationUnfolder{to: to}
}

type durationUnfolder struct {
	gotype.BaseUnfoldState // Use BaseUnfoldState to create errors for methods not implemented
	to                     *time.Duration
}

func (u *durationUnfolder) OnString(_ gotype.UnfoldCtx, str string) error {
	d, err := time.ParseDuration(str)
	if err == nil {
		*u.to = d
	}
	return err
}


...


	visitor, _ := gotype.NewUnfolder(nil, gotype.Unfolders(
		unfoldDuration,
	))
	it, _ := gotype.NewIterator(visitor, gotype.Folders(
		foldDuration,
	))

	// store duration in temporary value
	var tmp interface{}
	visitor.SetTarget(&tmp)
	it.Fold(5 * time.Minute)

	// restore duration from temporary value
	var dur time.Duration
	visitor.SetTarget(&dur)
	it.Fold(tmp)

	// dur == 5 * time.Minute

Data Model

The data model describes by which data types Serializers and Deserializers interact. In this sense the data model provides a simplified type system, that supports a subset of serializable go types (for example channels or function pointers can not be serialized).

Go-structform provides a simplified, common data model via the Visitor interface.

Types

• primitives (See ValueVisitor interface): Bool, Byte, String, Int, Int8/16/32/64, Uint, Uint8/16/32/64, Float32, Float64, untyped Nil
• compound: objects (ObjectVisitor), arrays (ArrayVisitor)

Extended Data Model

The extended data model provides support for similar types to the Visitor interface, but provides a number of optimizations, allowing users to pass a set of common go values directly without having to serialize and deserialize those values.

The extended data model is provided by the ExtVisitor interface.

All features in the Extended Data Model can be seamlessly converted to the Data Model provided by the `Visitor interface.

Deserializers supporting the Extended Data Model must always implement the common Data Model as is required by the Visitor interface.

Serializers wanting to interact with the Extended Data Model should still accept the Visitor interface only and use EnsureExtVisitor in order to create an ExtVisitor. EnsureExtVisitor wraps the Visitor if necessarry, allowing the Visitor to implement only a subset of features in the Extended Data Model.

• extended primitives: The Visitor adds support for []byte values as strings. The value must be consumed immediately or copied, as the buffer is not guaranteed to be stable.
• slices: The visitor adds support for slice types, for each supported primitive type. For example []int8/16/32/64 can be passed as is.
• map: The visitor adds support for map[string]T types, for each supported primitive type. For example map[string]int8/16/32/64.

Data Formats

• JSON: the json package provides a JSON parser and JSON serializer. The serializer implements a subset of ExtVisitor.
• UBJSON: the ubjson packages provides a parser and serializer for Universal Binary JSON.
• CBOR: the cborl package supports a compatible subset of CBOR (for example object keys must be strings).
• Go Types: the gotype package provides a Folder to convert go values into a stream of events and an Unfolder to apply a stream of events to go values.