proposal: encoding/json/v2: MarshalFunc with reflect.Type

[torbenschinke]

Proposal Details

As recommended by Joe Tsai (go-json-experiment/json#168), I am opening this proposal.

The current v2 JSON API allows for customized generic marshalers and unmarshalers, such as:

// MarshalFunc constructs a type-specific marshaler that
// specifies how to marshal values of type T.
func MarshalFunc[T any](fn func(T) ([]byte, error)) *Marshalers

I attempted to adapt my enum package package to the new v2 API in order to support interface-based marshalling and unmarshalling using various styles (e.g., internally or externally tagged). However, I encountered a limitation: the current API only supports generic type parameters for defining custom marshalers and unmarshalers. Since my implementation relies on dynamic type discovery via reflect.Type, I cannot instantiate the required generic types.

To address this, I propose extending the API with additional functions that accept reflect.Type, for example:

// MarshalFuncOf constructs a type-specific marshaler that
// specifies how to marshal values of the given reflect.Type.
func MarshalFuncOf(t reflect.Type, fn func(reflect.Type) ([]byte, error)) *Marshalers

// UnmarshalFuncOf constructs a type-specific unmarshaler that
// specifies how to unmarshal values of the given reflect.Type.
func UnmarshalFuncOf(t reflect.Type, fn func([]byte, reflect.Type) error) *Unmarshalers

This approach would be consistent with how the reflect package provides both TypeOf(any) and TypeFor[T], enabling both static and dynamic use cases.

Alternatively, or in addition, I would also welcome native support in the v2 package for interface-based marshalling and unmarshalling, which would significantly improve flexibility for libraries relying on dynamic types.

[gabyhelp]

Related Issues

• proposal: encoding/json support Unmarshal type interface #23549 (closed)
• proposal: encoding/json: Unmarshal support for dynamic types #13338 (closed)

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