Struct Mapping

Use struct tags to automatically map environment variables to Go structs for type-safe configuration management.

Basic Mapping

Simple Example

package main

import (
    "fmt"
    "github.com/cybergodev/env"
)

type Config struct {
    Host string `env:"SERVER_HOST"`
    Port int64  `env:"SERVER_PORT"`
    Debug bool  `env:"DEBUG"`
}

func main() {
    env.Load(".env")

    cfg := Config{}
    if err := env.ParseInto(&cfg); err != nil {
        panic(err)
    }

    fmt.Printf("%+v\n", cfg)
}

Using a Loader Instance

loader, _ := env.New(env.DefaultConfig())
loader.LoadFiles(".env")

var cfg Config
if err := loader.ParseInto(&cfg); err != nil {
    panic(err)
}

Tag Syntax

env Tag

Specify the environment variable name:

type Config struct {
    Host string `env:"SERVER_HOST"`  // Maps SERVER_HOST
    Port int64  `env:"PORT"`         // Maps PORT
}

envDefault Tag

Set default values:

type Config struct {
    Host    string `env:"HOST" envDefault:"localhost"`
    Port    int64  `env:"PORT" envDefault:"8080"`
    Debug   bool   `env:"DEBUG" envDefault:"false"`
    Timeout int64  `env:"TIMEOUT" envDefault:"30"`
}

Ignoring Fields

Use env:"-" to skip a field:

type Config struct {
    Host    string `env:"HOST"`
    Ignored string `env:"-"`  // Will not be populated
}

Supported Types

Basic Types

type Config struct {
    String  string  `env:"STRING_VALUE"`
    Int     int     `env:"INT_VALUE"`
    Int8    int8    `env:"INT8_VALUE"`
    Int16   int16   `env:"INT16_VALUE"`
    Int32   int32   `env:"INT32_VALUE"`
    Int64   int64   `env:"INT64_VALUE"`
    Uint    uint    `env:"UINT_VALUE"`
    Uint64  uint64  `env:"UINT64_VALUE"`
    Float32 float32 `env:"FLOAT32_VALUE"`
    Float64 float64 `env:"FLOAT64_VALUE"`
    Bool    bool    `env:"BOOL_VALUE"`
}

Time Types

import "time"

type Config struct {
    Timeout  time.Duration `env:"TIMEOUT"`
    Interval time.Duration `env:"INTERVAL"`
}

Supported formats:

30s - 30 seconds
5m - 5 minutes
1h30m - 1 hour 30 minutes
100ms - 100 milliseconds

Slice Types

Slice fields are split by comma ,, with surrounding whitespace around the separator trimmed automatically.

type Config struct {
    Hosts []string `env:"HOSTS"`      // Comma-separated
    Ports []int64  `env:"PORTS"`      // Comma-separated
}

.env file:

bash

HOSTS=localhost,example.com,api.example.com
PORTS=80,443,8080

Nested Structs

Basic Nesting

type DatabaseConfig struct {
    Host string `env:"DB_HOST" envDefault:"localhost"`
    Port int64  `env:"DB_PORT" envDefault:"5432"`
    User string `env:"DB_USER"`
    Password string `env:"DB_PASSWORD"`
}

type RedisConfig struct {
    Host string `env:"REDIS_HOST" envDefault:"localhost"`
    Port int64  `env:"REDIS_PORT" envDefault:"6379"`
}

type AppConfig struct {
    Database DatabaseConfig
    Redis    RedisConfig
    Debug    bool `env:"DEBUG" envDefault:"false"`
}

Deep Nesting

type Credentials struct {
    Username string `env:"DB_USER"`
    Password string `env:"DB_PASSWORD"`
}

type Connection struct {
    Host        string      `env:"DB_HOST"`
    Port        int64       `env:"DB_PORT"`
    Credentials Credentials
}

type Database struct {
    Connection Connection
    Name       string `env:"DB_NAME"`
}

Pointer Types

Pointer fields are supported:

package main

import (
    "fmt"

    "github.com/cybergodev/env"
)

type Config struct {
    Host    *string `env:"HOST"`
    Port    *int64  `env:"PORT"`
    Enabled *bool   `env:"ENABLED"`
}

func main() {
    cfg := Config{}
    env.ParseInto(&cfg)

    if cfg.Port != nil {
        fmt.Println("Port:", *cfg.Port)
    }
}

Custom Types

Implementing the encoding.TextUnmarshaler Interface

Custom decoding of struct fields is done by implementing the standard library encoding.TextUnmarshaler interface — this is the interface that is actually invoked during per-field population.

package main

import (
    "fmt"

    "github.com/cybergodev/env"
)

type LogLevel string

// Implement encoding.TextUnmarshaler — invoked at the field level
func (l *LogLevel) UnmarshalText(text []byte) error {
    switch string(text) {
    case "debug", "info", "warn", "error":
        *l = LogLevel(text)
        return nil
    default:
        return fmt.Errorf("invalid log level: %s", string(text))
    }
}

type Config struct {
    Level LogLevel `env:"LOG_LEVEL"`
}

func main() {
    data := map[string]string{"LOG_LEVEL": "info"}

    var cfg Config
    if err := env.UnmarshalInto(data, &cfg); err != nil {
        panic(err)
    }

    fmt.Println(cfg.Level)
    // Output: info
}

Type Aliases with Validation

type Port int64

// Implement encoding.TextUnmarshaler with range validation during parsing
func (p *Port) UnmarshalText(text []byte) error {
    val, err := strconv.ParseInt(string(text), 10, 64)
    if err != nil {
        return err
    }
    if val < 1 || val > 65535 {
        return errors.New("port must be 1-65535")
    }
    *p = Port(val)
    return nil
}

About env.Marshaler / env.Unmarshaler interfaces

The env.Marshaler (MarshalEnv()) and env.Unmarshaler (UnmarshalEnv(map[string]string)) interfaces only take effect on the top-level value passed to env.Marshal / env.MarshalStruct / env.UnmarshalInto, and are not invoked by the per-field population logic of structs. To customize encoding/decoding of struct fields, implement the standard library encoding.TextMarshaler / encoding.TextUnmarshaler; they are recognized at the field level.

Configuration Validation

Struct Validation

package main

import (
    "errors"
    "log"

    "github.com/cybergodev/env"
)

type ServerConfig struct {
    Host string `env:"SERVER_HOST" envDefault:"0.0.0.0"`
    Port int64  `env:"SERVER_PORT" envDefault:"8080"`
}

func (c *ServerConfig) Validate() error {
    if c.Port < 1024 || c.Port > 65535 {
        return errors.New("port must be 1024-65535")
    }
    return nil
}

func main() {
    cfg := ServerConfig{}
    if err := env.ParseInto(&cfg); err != nil {
        log.Fatal(err)
    }

    if err := cfg.Validate(); err != nil {
        log.Fatal(err)
    }
}

Required Field Validation

type Config struct {
    APIKey    string `env:"API_KEY"`     // Required
    APISecret string `env:"API_SECRET"`  // Required
    Timeout   int64  `env:"TIMEOUT" envDefault:"30"`  // Optional
}

func (c *Config) Validate() error {
    if c.APIKey == "" {
        return errors.New("API_KEY is required")
    }
    if c.APISecret == "" {
        return errors.New("API_SECRET is required")
    }
    return nil
}

Practical Patterns

Centralized Configuration Management

// config/config.go
package config

import "github.com/cybergodev/env"

type Config struct {
    Server   ServerConfig
    Database DatabaseConfig
    Redis    RedisConfig
    Log      LogConfig
}

func Load() (*Config, error) {
    if err := env.Load(".env"); err != nil {
        return nil, err
    }

    cfg := &Config{
        Server:   ServerConfig{},
        Database: DatabaseConfig{},
        Redis:    RedisConfig{},
        Log:      LogConfig{},
    }

    if err := env.ParseInto(cfg); err != nil {
        return nil, err
    }

    return cfg, nil
}

Environment Separation

type BaseConfig struct {
    AppName string `env:"APP_NAME"`
    Version string `env:"APP_VERSION"`
}

type DevelopmentConfig struct {
    BaseConfig
    Debug bool `env:"DEBUG" envDefault:"true"`
}

type ProductionConfig struct {
    BaseConfig
    Debug bool `env:"DEBUG" envDefault:"false"`
}

func LoadConfig() interface{} {
    env.Load(".env")

    switch os.Getenv("GO_ENV") {
    case "production":
        cfg := ProductionConfig{}
        env.ParseInto(&cfg)
        return cfg
    default:
        cfg := DevelopmentConfig{}
        env.ParseInto(&cfg)
        return cfg
    }
}

Error Handling

Parse Errors

cfg := Config{}
if err := env.ParseInto(&cfg); err != nil {
    var parseErr *env.ParseError
    if errors.As(err, &parseErr) {
        log.Fatalf("Parse error at %s:%d", parseErr.File, parseErr.Line)
    }
    log.Fatal(err)
}

Type Conversion Errors

type Config struct {
    Port int64 `env:"PORT"`  // If PORT is not a valid integer
}

cfg := Config{}
if err := env.ParseInto(&cfg); err != nil {
    // Type conversion failure returns an error
}

Complete Example

package main

import (
    "errors"
    "fmt"
    "log"
    "time"

    "github.com/cybergodev/env"
)

type ServerConfig struct {
    Host         string        `env:"SERVER_HOST" envDefault:"0.0.0.0"`
    Port         int64         `env:"SERVER_PORT" envDefault:"8080"`
    ReadTimeout  time.Duration `env:"READ_TIMEOUT" envDefault:"30s"`
    WriteTimeout time.Duration `env:"WRITE_TIMEOUT" envDefault:"30s"`
}

func (c *ServerConfig) Validate() error {
    if c.Port < 1024 || c.Port > 65535 {
        return errors.New("port must be 1024-65535")
    }
    return nil
}

type DatabaseConfig struct {
    Host     string `env:"DB_HOST" envDefault:"localhost"`
    Port     int64  `env:"DB_PORT" envDefault:"5432"`
    User     string `env:"DB_USER" envDefault:"postgres"`
    Password string `env:"DB_PASSWORD"`
    Name     string `env:"DB_NAME" envDefault:"myapp"`
}

func (c *DatabaseConfig) Validate() error {
    if c.Password == "" {
        return errors.New("DB_PASSWORD is required")
    }
    return nil
}

type Config struct {
    Server   ServerConfig
    Database DatabaseConfig
}

func LoadConfig() (*Config, error) {
    if err := env.Load(".env"); err != nil {
        return nil, fmt.Errorf("load env: %w", err)
    }

    cfg := &Config{
        Server:   ServerConfig{},
        Database: DatabaseConfig{},
    }

    if err := env.ParseInto(cfg); err != nil {
        return nil, fmt.Errorf("parse config: %w", err)
    }

    if err := cfg.Server.Validate(); err != nil {
        return nil, fmt.Errorf("server config: %w", err)
    }

    if err := cfg.Database.Validate(); err != nil {
        return nil, fmt.Errorf("database config: %w", err)
    }

    return cfg, nil
}

func main() {
    cfg, err := LoadConfig()
    if err != nil {
        log.Fatal(err)
    }

    fmt.Printf("Server: %s:%d\n", cfg.Server.Host, cfg.Server.Port)
    fmt.Printf("Database: %s@%s:%d/%s\n",
        cfg.Database.User,
        cfg.Database.Host,
        cfg.Database.Port,
        cfg.Database.Name,
    )
}

Package Functions - ParseInto - ParseInto function reference
Loader API - ParseInto - Loader method reference
Getting Started - Basic usage

Struct Mapping ​

Basic Mapping ​

Simple Example ​

Using a Loader Instance ​

Tag Syntax ​

env Tag ​

envDefault Tag ​

Ignoring Fields ​

Supported Types ​

Basic Types ​

Time Types ​

Slice Types ​

Nested Structs ​

Basic Nesting ​

Deep Nesting ​

Pointer Types ​

Custom Types ​

Implementing the encoding.TextUnmarshaler Interface ​

Type Aliases with Validation ​

Configuration Validation ​

Struct Validation ​

Required Field Validation ​

Practical Patterns ​

Centralized Configuration Management ​

Environment Separation ​

Error Handling ​

Parse Errors ​

Type Conversion Errors ​

Complete Example ​

Related Documentation ​