Why doesn't YAML.v3 marshal a struct based on the String() in golang?

I have a struct which contains a type based on an enum. I am trying to render it to a user friendly string. Here's minimum viable code:

package main

import (
	"fmt"

	"gopkg.in/yaml.v3"
)

type Job struct {
	Engine Engine `json:"Engine" yaml:"Engine"`
}

//go:generate stringer -type=Engine --trimprefix=Engine
type Engine int

const (
	engineUnknown Engine = iota // must be first
	EngineDocker
	engineDone // must be last
)

func main() {
	j := Job{Engine: EngineDocker}

	fmt.Printf("%+v\n\n", j)
	out, _ := yaml.Marshal(j)
	fmt.Println(string(out))
}

Here's the generated code:

// Code generated by "stringer -type=Engine --trimprefix=Engine"; DO NOT EDIT.

package main

import "strconv"

func _() {
	// An "invalid array index" compiler error signifies that the constant values have changed.
	// Re-run the stringer command to generate them again.
	var x [1]struct{}
	_ = x[engineUnknown-0]
	_ = x[EngineDocker-1]
	_ = x[engineDone-2]
}

const _Engine_name = "engineUnknownDockerengineDone"

var _Engine_index = [...]uint8{0, 13, 19, 29}

func (i Engine) String() string {
	if i < 0 || i >= Engine(len(_Engine_index)-1) {
		return "Engine(" + strconv.FormatInt(int64(i), 10) + ")"
	}
	return _Engine_name[_Engine_index[i]:_Engine_index[i+1]]
}

Here's the output:

{Engine:1}

Engine: 1

Here's what I'd like the output to be:

{Engine:Docker}

Engine: Docker

I thought the String() in the generated file would accomplish this. Is there any way to do this? Thanks!

yaml marshaler doesn't use String method. Instead YAML uses encoding.TextMarshaler and encoding.TextUnmarshaler interfaces. Actually, all other codec schemes - JSON, XML, TOML, etc. - use those interfaces to read/write the values. So, if you implement those methods for your type, you will receive all other codecs for free.

Here is an example how to make a human-readable encoding for your enum: https://go.dev/play/p/pEcBmAM-oZJ

type Engine int

const (
	engineUnknown Engine = iota // must be first
	EngineDocker
	engineDone // must be last
)

var engineNames []string
var engineNameToValue map[string]Engine

func init() {
	engineNames = []string{"Unknown", "Docker"}
	engineNameToValue = make(map[string]Engine)
	for i, name := range engineNames {
		engineNameToValue[strings.ToLower(name)] = Engine(i)
	}
}

func (e Engine) String() string {
	if e < 0 || int(e) >= len(engineNames) {
		panic(fmt.Errorf("Invalid engine code: %d", e))
	}
	return engineNames[e]
}

func ParseEngine(text string) (Engine, error) {
	i, ok := engineNameToValue[strings.ToLower(text)]
	if !ok {
		return engineUnknown, fmt.Errorf("Invalid engine name: %s", text)
	}
	return i, nil
}

func (e Engine) MarshalText() ([]byte, error) {
	return []byte(e.String()), nil
}

func (e *Engine) UnmarshalText(text []byte) (err error) {
	name := string(text)
	*e, err = ParseEngine(name)
	return
}

How it works:

func main() {
	j := Job{Engine: EngineDocker}

	fmt.Printf("%#v\n\n", j)
	out, err := yaml.Marshal(j)
	if err != nil {
		panic(err)
	}
	fmt.Printf("YAML: %s\n", string(out))

	var jj Job
	err = yaml.Unmarshal(out, &jj)
	if err != nil {
		panic(err)
	}
	fmt.Printf("%#v\n\n", jj)

	// == JSON ==
	out, err = json.Marshal(j)
	if err != nil {
		panic(err)
	}
	fmt.Printf("JSON: %s\n", string(out))

	var jjs Job
	err = json.Unmarshal(out, &jjs)
	if err != nil {
		panic(err)
	}
	fmt.Printf("%#v\n\n", jjs)

}

the output

main.Job{Engine:1}

YAML: Engine: Docker

main.Job{Engine:1}

JSON: {"Engine":"Docker"}
main.Job{Engine:1}

See? It writes and reads strings to both YAML and JSON without any extra effort.