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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.