BoolDefault return the result of searching an env var, if the env var value is empty, return a default value as bool.

DurationDefault return the result of searching an env var, if the env var value is empty, return a default value as time.Duration.

IntDefault return the result of searching an env var, if the env var value is empty, return a default value as int.

Must return the result of searching an env var, if the env var value is empty, then return a fatal error.

OrDefault return the result of searching an env var, if the env var value is empty, return a default value.