How to re-use a struct from an third-party package, while changing the marshalling behavior for a single field?

Question

Let's say I want to marshal a struct into YAML, and the struct already has all of its YAML tags defined, except there is one tag that I want to change. How can I change the behavior for this single field without changing the struct itself? Assume the struct is coming from a third-party package.

Here's an example to demonstrate, along with my best attempt. Let's assume that the User struct (and its associated Secret struct) are coming from a third-party package, so we can't modify them.

package main

import (
    "fmt"

    "gopkg.in/yaml.v2"
)

type User struct {
    Email    string  `yaml:"email"`
    Password *Secret `yaml:"password"`
}

type Secret struct {
    s string
}

// MarshalYAML implements the yaml.Marshaler interface for Secret.
func (sec *Secret) MarshalYAML() (interface{}, error) {
    if sec != nil {
        // Replace `"<secret>"` with `sec.s`, and it gets the desired
        // behavior. But I can't change the Secret struct:
        return "<secret>", nil
    }
    return nil, nil
}

func (sec *Secret) UnmarshalYAML(unmarshal func(interface{}) error) error {
    var st string
    if err := unmarshal(&st); err != nil {
        return err
    }
    sec.s = st
    return nil
}

// My best attempt at the solution:
type SolutionAttempt struct {
    User
}

func (sol *SolutionAttempt) MarshalYAML() (interface{}, error) {
    res, err := yaml.Marshal(
        struct {
            // I don't like having to repeat all these fields from User:
            Email    string `yaml:"email"`
            Password string `yaml:"password"`
        }{
            Email:    sol.User.Email,
            Password: sol.User.Password.s,
        },
    )
    if err != nil {
        return nil, err
    }
    return string(res), nil
}

func main() {
    user := &User{}
    var data = `
  email: [email protected]
  password: asdf
`
    err := yaml.Unmarshal([]byte(data), user)
    if err != nil {
        fmt.Printf("errors! %s", err)
        return
    }

    buf, err := yaml.Marshal(user)
    if err != nil {
        fmt.Printf("errors! %s", err)
        return
    }

    // Without touching User or Secret, how can I unmarshall an
    // instance of User that renders the secret?
    fmt.Printf("marshalled output:\n%s\n", buf)

    ///////////////////////////////////////////////////////
    // attempted solution:
    ///////////////////////////////////////////////////////
    sol := &SolutionAttempt{}
    var data2 = `
user:
    email: [email protected]
    password: asdf
`
    err = yaml.Unmarshal([]byte(data2), sol)
    if err != nil {
        fmt.Printf("errors! %s", err)
        return
    }

    buf, err = yaml.Marshal(sol)
    if err != nil {
        fmt.Printf("errors! %s", err)
        return
    }
    fmt.Printf("attempted solution marshalled output:\n%s\n", buf)
}

Here is a Go playground link of the code above: https://go.dev/play/p/ojiPv4ylCEq

Answer 1

How can I change the behavior for this single field without changing the struct itself? Assume the struct is coming from a third party package.

This simply is impossible.

Your "best attempt" is the way to Go.