Viper Unmarshal and Struct Mapping

Basic Unmarshal

type Config struct {
    Port     int    `mapstructure:"port"`
    Host     string `mapstructure:"host"`
    LogLevel string `mapstructure:"log_level"`
    Database struct {
        DSN     string `mapstructure:"dsn"`
        MaxConn int    `mapstructure:"max_conn"`
    } `mapstructure:"database"`
}

var cfg Config
if err := viper.Unmarshal(&cfg); err != nil {
    return fmt.Errorf("decoding config: %w", err)
}

mapstructure tags

Always use mapstructure tags. Without them, mapstructure falls back to case-insensitive field name matching, which works for simple cases but silently fails for:

Nested structs where the outer key uses an underscore (max_conn → MaxConn)
Unexported fields
Fields where the Go name does not match the config key

// ✓ Good — explicit and immune to rename surprises
type TLSConfig struct {
    CertFile string `mapstructure:"cert_file"`
    KeyFile  string `mapstructure:"key_file"`
    Enabled  bool   `mapstructure:"enabled"`
}

// ✗ Fragile — relies on case-folding; breaks when config key uses underscores
type TLSConfig struct {
    CertFile string  // viper key "certfile" or "CertFile", not "cert_file"
    KeyFile  string
    Enabled  bool
}

UnmarshalKey — extracting a sub-tree

type DatabaseConfig struct {
    DSN     string `mapstructure:"dsn"`
    MaxConn int    `mapstructure:"max_conn"`
}

var dbCfg DatabaseConfig
if err := viper.UnmarshalKey("database", &dbCfg); err != nil {
    return fmt.Errorf("decoding database config: %w", err)
}

Prefer UnmarshalKey over viper.Sub + Unmarshal — fewer nil checks and less boilerplate.

time.Duration

Viper's GetDuration parses duration strings ("1h30m", "500ms") from config files and env vars. When using Unmarshal, mapstructure does not know how to decode a duration string into time.Duration by default.

import "github.com/mitchellh/mapstructure"

var cfg Config
err := viper.Unmarshal(&cfg, func(dc *mapstructure.DecoderConfig) {
    dc.DecodeHook = mapstructure.ComposeDecodeHookFunc(
        mapstructure.StringToTimeDurationHookFunc(),
        mapstructure.StringToSliceHookFunc(","),
        dc.DecodeHook,
    )
})

StringToTimeDurationHookFunc handles "1h30m" → time.Duration. StringToSliceHookFunc(",") handles "a,b,c" → []string{"a", "b", "c"}.

net.IP and custom types

import "github.com/mitchellh/mapstructure"

func stringToIPHookFunc() mapstructure.DecodeHookFunc {
    return func(f reflect.Type, t reflect.Type, data interface{}) (interface{}, error) {
        if f.Kind() != reflect.String || t != reflect.TypeOf(net.IP{}) {
            return data, nil
        }
        ip := net.ParseIP(data.(string))
        if ip == nil {
            return nil, fmt.Errorf("invalid IP address: %s", data)
        }
        return ip, nil
    }
}

Squash for embedded structs

type BaseConfig struct {
    LogLevel string `mapstructure:"log_level"`
    Debug    bool   `mapstructure:"debug"`
}

type ServerConfig struct {
    BaseConfig `mapstructure:",squash"`  // merge BaseConfig fields at this level
    Port       int    `mapstructure:"port"`
}

Without ,squash, the base config must be nested under a baseconfig key in the config file.

Remain for unknown keys

type Config struct {
    Port     int                    `mapstructure:"port"`
    Remain   map[string]interface{} `mapstructure:",remain"`
}

Extra keys from the config file are collected in Remain instead of being silently dropped. Useful for forward compatibility.

Weak decoding

Enable weak type decoding (e.g., string "true" → bool true) when working with env vars that are always strings:

var cfg Config
err := viper.Unmarshal(&cfg, func(dc *mapstructure.DecoderConfig) {
    dc.WeaklyTypedInput = true
})

Use with caution — weak decoding can hide bugs where a wrong value type is silently converted.

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