Question: How to write a tag with a streamed content?

[ceving]

I would like to encode CSV like data using the tag 275. I did the following:

	// Create encoder
	var enc *cbor.Encoder
	enc = cbor.NewEncoder(writer)

	// Encode a map-like serialized array
	err = enc.Encode(cbor.RawTag{Number: 275})
	if err != nil {
		return fmt.Errorf("Can not encode tag 275: %w", err);
	}
	
	// Encode begin of indefinite array
	err = enc.StartIndefiniteArray()
	if err != nil {
		return fmt.Errorf("Can not encode start of indefinite array: %w", err)
	}

This generates:

• d9 01 13: Tag 275
• f6: Null, because content of RawTag is nil
• 9f: Start of indefinite array

How to get rid of the f6?

The struct RawTag seems to require always some content. This makes it impossible to stream the content.

[fxamacker]

Hi @ceving,

I would like to encode CSV like data using the tag 275. I did the following

Your example code tries to encode to an indefinite-length CBOR array with tag 275, so it wouldn't comply with tag 275 requirements.

The Tag 275 Specification is for encoding a map with text string keys:

This document describes a CBOR tag for maps having text-only keys.

Tag value: 275

Data item: Map (major type 5)

Semantics: Map contains only keys that are of type Text String (major type 3)

Using Tag 275 for Map with Text String Keys

To comply with Tag 275 requirements, you can encode a map with text string keys like this
https://go.dev/play/p/mHFxeSnkGZI

// Example to resolve https://github.com/fxamacker/cbor/issues/737
package main

import (
	"fmt"
	"reflect"

	"github.com/fxamacker/cbor/v2"
)

type TextKeyMap map[string]any

const CBORTextKeyMapTagNumber = 275

func main() {
	// NOTE: error handling is omitted for brevity.

	// Register tag number 275 with TextKeyMap type.
	tags := cbor.NewTagSet()
	_ = tags.Add(
		cbor.TagOptions{EncTag: cbor.EncTagRequired, DecTag: cbor.DecTagRequired},
		reflect.TypeOf(TextKeyMap{}),
		CBORTextKeyMapTagNumber,
	)

	// Create encoding mode with tags.
	em, _ := cbor.EncOptions{}.EncModeWithTags(tags)

	m := TextKeyMap{
		"ID":   1,
		"Name": "foo",
	}

	// Encode TextKeyMap.
	b, _ := em.Marshal(m)

	// Encoded CBOR: d90113a262494401644e616d6563666f6f
	fmt.Printf("Encoded CBOR: %x\n", b)

	d, _ := cbor.Diagnose(b)

	// EDN: 275({"ID": 1, "Name": "foo"})
	fmt.Printf("EDN: %s\n", d)

	// Create decoding mode with tags.
	dm, _ := cbor.DecOptions{}.DecModeWithTags(tags)

	var v any
	_ = dm.Unmarshal(b, &v)

	// Decode to map[ID:1 Name:foo] (main.TextKeyMap)
	fmt.Printf("Decode to %+v (%T)\n", v, v)
}

This uses cbor.Diagnose(b) to display the CBOR-encoded data using Extended Diagnostic Notation.

BTW, I will be adding a modern streaming API to this library, after refactoring the existing code first.