bad-cli.go

package main

import (
	"errors"
	"fmt"
	"os"
	"reflect"
	"strconv"
	"strings"

	"github.com/fatih/structs"
	"github.com/spf13/cobra"
	"github.com/spf13/pflag"
)

type cmdArgs struct {
	SomeString string `short:"s"`
	SomeInt    int    `short:"i"`
}

func main() {
	args := cmdArgs{
		SomeString: "default string value",
		SomeInt:    9,
	}

	cmd := cobra.Command{
		Run: func(_ *cobra.Command, _ []string) {
			fmt.Printf("%+v\n", args)
		},
	}

	flags := cmd.PersistentFlags()
	flags.AddFlagSet(fromStruct(&args))

	if err := cmd.Execute(); err != nil {
		panic(err)
	}
}

func fromStruct(facePointer interface{}) *pflag.FlagSet {
	set := pflag.FlagSet{}

	fields := structs.Fields(facePointer)
	for _, f := range fields {
		if !f.IsExported() {
			continue
		}

		name := f.Tag("cli")
		if name == "" {
			name = f.Name()
		}

		short := f.Tag("short")
		if short == "" {
			short = string(name[0])
		}

		envVarName := f.Tag("envvar")
		if envVarName == "" {
			// TODO split on the camel case
			envVarName = strings.ToUpper(strings.Join(strings.Split(name, "-"), "_"))
		}
		fv := fieldValue(*f)

		defaultVal := os.Getenv(envVarName)
		if defaultVal != "" {
			if err := fv.Set(defaultVal); err != nil {
				panic(err)
			}
		}

		fmt.Println(name, fv.Type(), fv.String())
		set.AddFlag(&pflag.Flag{
			Name:      name,
			Shorthand: short,
			Value:     &fv,
		})
	}
	return &set
}

type fieldValue structs.Field

func (s *fieldValue) Set(val string) error {
	v := (*structs.Field)(s)
	switch v.Kind() {
	case reflect.String:
		return v.Set(val)
	case reflect.Int:
		intVal, err := strconv.Atoi(val)
		if err != nil {
			return err
		}
		return v.Set(intVal)
	}

	return errors.New("wtf")
}
func (s *fieldValue) Type() string {
	v := (*structs.Field)(s)
	return v.Kind().String()
}

func (s *fieldValue) String() string {
	v := (*structs.Field)(s)
	return fmt.Sprintf("%v", v.Value())
}