lixenwraith/config LLM Usage Guide

This guide details the lixenwraith/config package for thread-safe Go configuration. It supports multiple sources (files, environment, CLI), type-safe struct population, and live reconfiguration.

Quick Start: Recommended Usage

The recommended pattern uses the Builder with a target struct. This provides compile-time type safety and eliminates the need for runtime type assertions.

package main

import (
	"fmt"
	"log"
	"os"
    
	"github.com/lixenwraith/config"
)

// 1. Define your application's configuration struct.
type AppConfig struct {
	Server struct {
		Host string `toml:"host"`
		Port int    `toml:"port"`
	} `toml:"server"`
	Debug bool `toml:"debug"`
}

func main() {
	// 2. Create a target instance with defaults.
	// Method A: Direct initialization (cleaner for simple defaults)
	target := &AppConfig{}
	target.Server.Host = "localhost"
	target.Server.Port = 8080

	// Method B: Use WithDefaults() for explicit separation (shown below)

	// 3. Use the builder to configure and load all sources.
	cfg, err := config.NewBuilder().
		WithTarget(target).               // Enable type-safe mode and register struct fields.
		// WithDefaults(&AppConfig{...}), // Optional: Override defaults from WithTarget.
		WithFile("config.toml").          // Load from file (supports .toml, .json, .yaml).
		WithEnvPrefix("APP_").            // Load from environment (e.g., APP_SERVER_PORT).
		WithArgs(os.Args[1:]).            // Load from command-line flags (e.g., --server.port=9090).
		Build()                           // Build the final config object.
	if err != nil {
		log.Fatalf("Config build failed: %v", err)
	}

	// 4. Access the fully populated, type-safe struct.
	// The `target` variable is now populated with the final merged values.
	fmt.Printf("Running on %s:%d\n", target.Server.Host, target.Server.Port)

    // Or, retrieve the updated struct at any time (e.g., after a live reload).
    latest, _ := cfg.AsStruct()
    latestConfig := latest.(*AppConfig)
    fmt.Printf("Debug mode: %v\n", latestConfig.Debug)
}

Supported Formats: TOML, JSON, YAML

The package supports multiple file formats. The format is auto-detected from the file extension (.toml, .json, .yaml, .yml) or file content.

To specify a format explicitly, use Builder.WithFileFormat("json").
The default struct tag for field mapping is toml, but can be changed with Builder.WithTagName("json").

Builder Pattern

The Builder is the primary way to construct a Config instance.

// NewBuilder creates a new configuration builder.
func NewBuilder() *Builder
// Build finalizes configuration; returns the first of any accumulated errors.
func (b *Builder) Build() (*Config, error)
// MustBuild is like Build but panics on fatal errors.
func (b *Builder) MustBuild() *Config

Builder Methods

WithTarget(target any): (Recommended) Enables type-safe mode. Registers fields from the target struct and allows access via AsStruct(). The target's initial values are used as defaults unless WithDefaults is also called.
WithDefaults(defaults any): Explicitly sets a struct containing default values. Overrides any defaults from WithTarget.
WithFile(path string): Sets the configuration file path.
WithFileDiscovery(opts FileDiscoveryOptions): Enables automatic config file discovery (searches CLI flags, env vars, XDG paths, and current directory).
WithArgs(args []string): Sets the command-line arguments to parse (e.g., --server.port=9090).
WithEnvPrefix(prefix string): Sets the global environment variable prefix (e.g., MYAPP_).
WithSources(sources ...Source): Overrides the default source precedence order.
WithTypedValidator(fn any): (Recommended for validation) Adds a type-safe validation function that runs after the target struct is populated. The function signature must be func(c *YourConfigType) error.
WithValidator(fn ValidatorFunc): Adds a validation function that runs before type-safe population, operating on the raw *Config object.
WithTagName(tagName string): Sets the primary struct tag for field mapping ("toml", "json", "yaml").
WithPrefix(prefix string): Adds a prefix to all registered paths from a struct.
WithFileFormat(format string): Explicitly sets the file format ("toml", "json", "yaml", "auto").
WithSecurityOptions(opts SecurityOptions): Sets security options for file loading (path traversal, file size limits).
WithEnvTransform(fn EnvTransformFunc): Sets a custom environment variable mapping function.
WithEnvWhitelist(paths ...string): Limits environment variable loading to a specific set of paths.

Type-Safe Access & Population

These are the preferred methods for accessing configuration data.

AsStruct() (any, error): After using Builder.WithTarget(), this method returns the populated, type-safe target struct. This is the primary way to access config after initialization or live reload.
Scan(basePath string, target any): Populates a struct with values from a specific config path (e.g., cfg.Scan("server", &serverConf)).
GetTyped[T](c *Config, path string) (T, error): Retrieves a single value and decodes it to type T, handling type conversion automatically.
ScanTyped[T](c *Config, basePath ...string) (*T, error): A generic wrapper around Scan that allocates, populates, and returns a pointer to a struct of type T.

Live Reconfiguration

Enable automatic reloading of configuration when the source file changes.

AutoUpdate(): Enables file watching and automatic reloading with default options.
AutoUpdateWithOptions(opts WatchOptions): Enables reloading with custom options (e.g., poll interval, debounce).
StopAutoUpdate(): Stops the file watcher.
Watch() <-chan string: Returns a channel that receives the paths of changed values.
WatchFile(filePath string, formatHint ...string): Switches the watcher to a new file at runtime.
IsWatching() bool: Returns true if the file watcher is active.

The watch channel also receives special notifications: "file_deleted", "permissions_changed", "reload_error:...".

Source Precedence

The default order of precedence (highest to lowest) is:

CLI: Command-line arguments (--server.port=9090)
Env: Environment variables (MYAPP_SERVER_PORT=8888)
File: Configuration file (config.toml)
Default: Values registered from a struct.

This order can be changed via Builder.WithSources() or Config.SetPrecedence().

Dynamic / Legacy Value Access

These methods are for dynamic key-value access and should be avoided when a type-safe struct can be used. They require runtime type assertions.

Get(path string) (any, bool): Retrieves the final merged value. The bool indicates if the path was registered. Requires a type assertion, e.g., port := val.(int64).
Set(path string, value any): Updates a value in the highest priority source. The path must be registered first.
GetSource(path string, source Source) (any, bool): Retrieves a value from a specific source layer.
SetSource(path string, source Source, value any): Sets a value for a specific source layer.

API Reference Summary

Core Types

Config: The primary thread-safe configuration manager.
Source: A configuration source (SourceCLI, SourceEnv, SourceFile, SourceDefault).
LoadOptions: Options for loading configuration from multiple sources.
Builder: Fluent API for constructing a Config instance.

Core Methods

New() *Config: Creates a new Config instance.
Register(path string, defaultValue any): Registers a path with a default value.
RegisterStruct(prefix string, structWithDefaults any): Recursively registers fields from a struct using toml tags.
Validate(required ...string): Checks that all specified required paths have been set from a non-default source.
Save(path string): Atomically saves the current merged configuration state to a file.
Clone() *Config: Creates a deep copy of the configuration state.
Debug() string: Returns a formatted string of all values for debugging.

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