feature: Option to specify map keys

[theory]

Is your feature request related to a problem? Please describe.

Currently, encoding and decoding a map[string]any results in a map[any]any. This is incompatible with JSON.

A clear and concise description of what the problem is.

I'm using JSON for may app, but CBOR for storage, as it's more compact. Users can submit any kind of JSON data through the API, including objects, which encoding/json marshals into map[string]any. A lot of JSON-based tooling expects JSON objects to be decoded in this way, e.g., jsonpath.

I gather that the CBOR standard allows any type of hashable value to be an object key, so when cbor decodes an encoded object, it creates a map[any]any. This is incompatible with JSON tooling that only recognizes map[string]any.

Describe the solution you'd like
A clear and concise description of what you want to happen.

I'd like some sort of configuration to tell cbor to use map[string]any instead of map[any]any when decoding objects.

May be similar or related to #439 and #441.

Describe alternatives you've considered

In my app, I can't pre-create a map, because the data is user-submitted, so might not be a map, and even if it was would almost certainly have texted map[string]any objects.

Thanks!

[theory]

Oh, just found that map[any]any doesn't work with encoding/json, either. Here's a failing test:

func TestMap(t *testing.T) {
	val := map[string]any{"name": "elise"}
	data, err := cbor.Marshal(val)
	if err != nil {
		t.Fatalf("Error marshaling cbor: %v", err)
	}

	var res any
	if err := cbor.Unmarshal(data, &res); err != nil {
		t.Fatalf("Error unmarshaling cbor: %v", err)
	}

	_, err = json.Marshal(res)
	if err != nil {
		t.Fatalf("Error marshaling json: %v", err)
	}
}

Output:

=== RUN   TestMap
    cbor_json_test.go:24: Error marshaling json: json: unsupported type: map[interface {}]interface {}

[theory]

Useful it from Section 5.6 of the spec:

The encoding and decoding applications need to agree on what types of keys are going to be used in maps. In applications that need to interwork with JSON-based applications, conversion is simplified by limiting keys to text strings only; otherwise, there has to be a specified mapping from the other CBOR types to text strings, and this often leads to implementation errors.

[theory]

Bah! I just found DefaultMapType. Trying it now…

[theory]

That does the trick. Fixed test:

func TestMap(t *testing.T) {
	val := map[string]any{"name": "elise"}
	data, err := cbor.Marshal(val)
	if err != nil {
		t.Fatalf("Error marshaling cbor: %v", err)
	}

	opts := cbor.DecOptions{}
	opts.DefaultMapType = reflect.TypeOf(map[string]any(nil))
	dec, err := opts.DecMode()
	if err != nil {
		t.Fatalf("Error creating decoder: %v", err)
	}

	var res any
	if err := dec.Unmarshal(data, &res); err != nil {
		t.Fatalf("Error unmarshaling cbor: %v", err)
	}

	_, err = json.Marshal(res)
	if err != nil {
		t.Fatalf("Error marshaling json: %v", err)
	}
}

Apologies for the ruckus.