how to map cbor tag to golang basic type?

[agufagit]

it looks like I have to define a new type in order to map cbor tag to "string"

type NewString string

func (r *NewString) Unmarshal(...) {
  ...
}

is there a way to map cbor tag directly to golang basic type string without defining a new type?

[fxamacker]

Hi @agufagit,

is there a way to map cbor tag directly to golang basic type string without defining a new type?

We intentionally don't provide a direct mapping between CBOR tags and Go's basic types. We need to define new Go types to map to CBOR tags.

This approach prevents mistakes and other issues because a Go string type can be used with different CBOR tags. For example, CBOR tags defined in RFC8949 1, 32, 33, 34, and 36 all use data item of CBOR text string type.

BTW, if you just need out-of-box support for NewString with a CBOR tag, you don't need to implement NewString.Unmarshal. For example, https://go.dev/play/p/CTcLik_KSB9

// Example for disussion and issue at:
// - https://github.com/fxamacker/cbor/discussions/599
// - https://github.com/surrealdb/surrealdb.go/issues/169

package main

import (
	"fmt"
	"reflect"

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

type myString string

func main() {

	// Associate myString type with CBOR tag 100

	tagNum := uint64(100)
	myStringType := reflect.TypeOf(myString(""))

	tags := cbor.NewTagSet()
	err := tags.Add(cbor.TagOptions{EncTag: cbor.EncTagRequired, DecTag: cbor.DecTagRequired}, myStringType, tagNum)
	if err != nil {
		panic(fmt.Sprintf("TagSet.Add(%s, %d) returned error %v", myStringType, tagNum, err))
	}

	// Encode value of myString type

	em, _ := cbor.EncOptions{}.EncModeWithTags(tags)

	b, err := em.Marshal(myString("hello"))
	if err != nil {
		fmt.Printf("Marshal() returned error %v\n", err)
		return
	}

	edn, _ := cbor.Diagnose(b) // human-readable representation (used for discussing binary data)

	fmt.Printf("myString(\"hello\") is encoded to %x (EDN: %s)\n", b, edn)

	// Decode CBOR data item to v interface{} which is
	// automatically detected and decoded as a myString value.

	dm, _ := cbor.DecOptions{}.DecModeWithTags(tags)

	var v interface{}
	err = dm.Unmarshal(b, &v)
	if err != nil {
		fmt.Printf("Unmarshal() returned error %v\n", err)
		return
	}

	fmt.Printf("%x is decoded to \"%v\" (of type %T)\n", b, v, v)

	// Output:
	// myString("hello") is encoded to d8646568656c6c6f
	// d8646568656c6c6f is decoded to "hello" (of type main.myString)
}

[agufagit]

Hi @fxamacker,

Thank you for the answer.

I have one more question, for golang type *bool, will a cbor null or undefined value automatically unmarshal to golang nil value? Or I need to control this by code?

this question is regarding this issue surrealdb/surrealdb.go#176

[fxamacker]

Hi @agufagit,

for golang type *bool, will a cbor null or undefined value automatically unmarshal to golang nil value? Or I need to control this by code?

Yes, it is automatic.

• Decoding CBOR null or undefined to Go slice/map/pointer sets Go value to nil.
• Decoding CBOR null or undefined to other Go types is no-op (original value is not modified).

More info at https://github.com/fxamacker/cbor/blob/master/decode.go#L95-L98

// To unmarshal CBOR null (0xf6) and undefined (0xf7) values into a
// slice/map/pointer, Unmarshal sets Go value to nil.  Because null is often
// used to mean "not present", unmarshalling CBOR null and undefined value
// into any other Go type has no effect and returns no error.