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// Package layout for a small web service:
//
// myapp/
//   go.mod
//   main.go               (package main)
//   cmd/
//     server/
//       main.go           (package main — the binary entrypoint)
//   internal/
//     config/
//       config.go         (package config — unexported to outside module)
//     handler/
//       user.go           (package handler)
//   pkg/
//     validate/
//       validate.go       (package validate — public reusable library)

// internal/config/config.go
package config

import "os"

// Config holds application configuration.
type Config struct {
    Port     string
    DSN      string
    LogLevel string
}

// Load reads configuration from environment variables.
func Load() Config {
    port := os.Getenv("PORT")
    if port == "" {
        port = "8080"
    }
    return Config{
        Port:     port,
        DSN:      os.Getenv("DATABASE_URL"),
        LogLevel: os.Getenv("LOG_LEVEL"),
    }
}

// pkg/validate/validate.go
package validate

import (
    "errors"
    "strings"
)

// Email returns an error if s is not a valid email address.
func Email(s string) error {
    if !strings.Contains(s, "@") {
        return errors.New("validate: invalid email address")
    }
    return nil
}