Flag / Env / Config / Default argument ordering

[AlephEleven]

Are there any solutions to prioritize args as

Command-line flag (e.g., --debug true)
Environment variable (e.g., DEBUG=true)
Configuration file (e.g., debug: true in config.yaml)
A sensible default (e.g., false)

The current solution I have swaps env / file:
flag
file
env
default

package main

import (
	"log"

	"github.com/alecthomas/kong"
	kongyaml "github.com/alecthomas/kong-yaml"
)

type Cli struct {
	Config string `help:"Path to the configuration file"`
	Debug  bool   `help:"Enable debug mode." default:"false" env:"DEBUG"`
}

func main() {
	var cli Cli

	kong.Parse(&cli)

	if cli.Config != "" {
		kong.Parse(&cli,
			kong.Configuration(kongyaml.Loader, cli.Config))

	}

	log.Printf("Debug mode: %v", cli.Debug)

Saw this discussion on https://danielms.site/zet/2023/kong-is-an-amazing-cli-for-go-apps/ and wanted to bring it up.

Thanks :)

[costela]

FYI, there's a concrete attempt at changing this here: #664

[tevinch]

For the requested order, you can keep Kong's environment decoding and make the YAML resolver return no value when that flag has a configured environment variable present. Returning nil, nil lets the already-decoded environment value stand; explicit CLI flags still take precedence.

Use kong.ConfigFlag to select the file and parse once:

var cli struct {
    Config kong.ConfigFlag `help:"Path to the YAML configuration file."`
    Debug  bool            `default:"false" env:"DEBUG"`
}
kong.Parse(&cli, kong.Configuration(envFirstYAML))

The complete loader and runnable example use Kong v1.16.1 and kong-yaml v0.2.0. The checks reproduce the original file-over-environment behavior, then verify flags > environment > YAML > defaults, including actual CLI output, false, 0, empty strings, typed values and repeated parses. No dependency patch is needed.

Presence matters here, so the loader uses os.LookupEnv, not a nonempty-value check. Invalid environment values still fail according to Kong's native behavior; on this version even an explicit CLI override does not hide an invalid environment value. Other custom resolvers retain their own precedence, so use the loader for each YAML layer you want covered.