Generic Value and SliceValue implementation

[mmatczuk]

Using Go generics it's possible to greatly streamline the process of binding custom value types.
I have created a library as a POC github.com/mmatczuk/anyflag.
To work with it users have to provide the type and a parser (aka fromString) function.

Example:

    var ba *url.Userinfo
    cmd.Flags().VarP(anyflag.NewValue[*url.Userinfo](nil, &ba, parseBasicAuth), "basic-auth", "", "basic auth")

    var bas []*url.Userinfo
    cmd.Flags().VarP(anyflag.NewSliceValue[*url.Userinfo](nil, &bas, parseBasicAuth), "basic-auth", "", "basic auth")

See full running example in example_test.go.

It's also possible to replace the concrete implementations here with the generic one largely reducing the code surface.

Please let me know what do you think.

[ljluestc]

package main

import (
	"fmt"
	"net/url"
	"strings"

	"github.com/mmatczuk/anyflag"
	"github.com/spf13/cobra"
	"github.com/spf13/pflag"
)

// parseBasicAuth parses a string like "username:password" into a *url.Userinfo.
// Returns nil and an error if the format is invalid.
func parseBasicAuth(s string) (*url.Userinfo, error) {
	parts := strings.SplitN(s, ":", 2)
	if len(parts) != 2 || parts[0] == "" || parts[1] == "" {
		return nil, fmt.Errorf("invalid basic auth format: expected 'username:password', got %q", s)
	}
	return url.UserPassword(parts[0], parts[1]), nil
}

// newCommand creates a Cobra command with flags for basic auth (single and slice).
func newCommand() *cobra.Command {
	var ba *url.Userinfo         // Single basic auth value
	var bas []*url.Userinfo      // Slice of basic auth values

	cmd := &cobra.Command{
		Use:   "auth",
		Short: "Demonstrate anyflag with basic auth",
		Run: func(cmd *cobra.Command, args []string) {
			// Print single basic auth value
			if ba != nil {
				username := ba.Username()
				password, _ := ba.Password()
				fmt.Fprintf(cmd.OutOrStdout(), "Single basic auth: username=%s, password=%s\n", username, password)
			} else {
				fmt.Fprintln(cmd.OutOrStdout(), "No single basic auth provided")
			}

			// Print slice of basic auth values
			if len(bas) > 0 {
				fmt.Fprintln(cmd.OutOrStdout(), "Multiple basic auth values:")
				for i, u := range bas {
					username := u.Username()
					password, _ := u.Password()
					fmt.Fprintf(cmd.OutOrStdout(), "  %d: username=%s, password=%s\n", i+1, username, password)
				}
			} else {
				fmt.Fprintln(cmd.OutOrStdout(), "No multiple basic auth values provided")
			}
		},
	}

	// Add flags using anyflag
	fs := cmd.Flags()
	fs.VarP(
		anyflag.NewValue[*url.Userinfo](nil, &ba, parseBasicAuth),
		"basic-auth", "",
		"basic auth in format username:password",
	)
	fs.VarP(
		anyflag.NewSliceValue[*url.Userinfo](nil, &bas, parseBasicAuth),
		"multi-auth", "",
		"multiple basic auth entries in format username:password (can be repeated)",
	)

	return cmd
}

func main() {
	if err := newCommand().Execute(); err != nil {
		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
		os.Exit(1)
	}
}

[tomasaschan]

@mmatczuk it seems to me that this is functionally equivalent to what #418 provides; would you agree?

[mmatczuk]

@tomasaschan the anyflag library more robust, it supports slice value given value parser, redacting.

[tomasaschan]

pflag currently has 0 dependencies, so I'm very reluctant to add one. But if you would consider contributing those types to this project, and move/copy them here in a PR, we'd consider it.

I think the main benefit I see to it is not additional features (even though automatic slice handling is nice!) but rather the opportunity for removing most of our custom Value implementations and use the generic one instead, so a PR to introduce these should preferably also do that - if not everywhere, then at least enough to demonstrate viability and gains.

[mmatczuk]

Sure, I can contribute it all. I'd sent a PR shortly.