Text flags

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.
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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.

@somebadcode

@somebadcode Yes please !!!!

feat: add TextFlag

@somebadcode

@somebadcode This is what I have and it works

package main

import (
	"context"
	"log"
	"log/slog"
	"os"

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

type textValue struct {
	lv slog.LevelVar
}

func (tv *textValue) String() string {
	return tv.lv.String()
}

func (tv *textValue) Set(value string) error {
	if err := tv.lv.UnmarshalText([]byte(value)); err != nil {
		return err
	}
	return nil
}

func (tv *textValue) Get() any {
	return tv.lv.Level()
}

func newTextValue() *textValue {
	p := &textValue{
		lv: slog.LevelVar{},
	}
	return p
}

func ptr(v cli.Value) *cli.Value {
	return &v
}

func main() {
	nv := newTextValue()

	cmd := &cli.Command{
		Name: "foo",
		Flags: []cli.Flag{
			&cli.GenericFlag{
				Name:        "level",
				Usage:       "log-level",
				Value:       nv,
				Destination: ptr(nv),
			},
		},
		Action: func(_ context.Context, c *cli.Command) error {
			log.Printf("%v", c.Value("level").(slog.Level))
			return nil
		},
	}

	cmd.Run(context.TODO(), os.Args)
}

Only difference is that it uses the GenericFlag instead of a new TextFlag. While I like the TextFlag approach I think the current plumbing of cli allows you to achieve what you want without a new flag in the mix.

$ go run . --level INFO
2025/04/18 20:45:03 INFO

feat: add TextFlag

Add TextFlag for encoding.TextMarshaler/TextUnmarshaler values

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Text flags #2051

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What problem does this solve?

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Describe alternatives you've considered

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Text flags #2051

Description

Checklist

What problem does this solve?

Solution description

Describe alternatives you've considered

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dearchap commented on Feb 5, 2025

dearchap commented on Apr 19, 2025

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