Checklist

• Are you running the latest v3 release? The list of releases is here.
• Did you check the manual for your release? The v3 manual is here.
• Did you perform a search about this feature? Here's the GitHub guide about searching.

What problem does this solve?

The standard library's flag package has had flag.TextVar since 1.19 which allows any type that satisfies encoding.TextMarshaller and encoding.TextUnmarshaller to be set using flags. A good use for this is to set log level using strings such as slog.LevelVar.

Other logging packages such as go.uber.org/zap satisfies these interfaces for its atomic level type. You can have a logger before the log level has been set. It'll be ready before arguments have been parsed and output structured logs in case of a parse error.

Solution description

Implement TextValue and TextFlag.

Describe alternatives you've considered

Here's what I had to implement to get the functionality. It would be nice to see this being supported by this package.

package clix

import (
	"encoding"
	"strings"

	"github.com/urfave/cli/v3"
)

type TextMarshalUnMarshaller interface {
	encoding.TextMarshaler
	encoding.TextUnmarshaler
}

type TextFlag = cli.FlagBase[TextMarshalUnMarshaller, cli.StringConfig, TextValue]

type TextValue struct {
	Value  TextMarshalUnMarshaller
	Config cli.StringConfig
}

func (v TextValue) String() string {
	text, err := v.Value.MarshalText()
	if err != nil {
		return ""
	}

	return string(text)
}

func (v TextValue) Set(s string) error {
	if v.Config.TrimSpace {
		return v.Value.UnmarshalText([]byte(strings.TrimSpace(s)))
	}

	return v.Value.UnmarshalText([]byte(s))
}

func (v TextValue) Get() any {
	return v.Value
}

func (v TextValue) Create(t TextMarshalUnMarshaller, _ *TextMarshalUnMarshaller, c cli.StringConfig) cli.Value {
	return &TextValue{
		Value:  t,
		Config: c,
	}
}

func (v TextValue) ToString(t TextMarshalUnMarshaller) string {
	text, err := t.MarshalText()
	if err != nil {
		return ""
	}

	return string(text)
}

I can contribute a solution based on this if so desired.