## Introduction
**config** is a simple golang library and designed to read configurations from JSON, Yaml files, environment variables and command line. **config** depends on [go-yaml](https://github.com/go-yaml/yaml) to anlayze Yaml file and uses built-in golang library to handle JSON file.

## Installation
1. Install [Yaml](https://github.com/go-yaml/yaml) library first:
```
go get gopkg.in/yaml.v2
```

2. Install **config** library:
```
go get github.com/eschao/config
```

## Usage
### I. Define configuration name in structure tags
Like JSON, Yaml, **config** uses tags to define configurations:

| Tag | Example | Function |
|-----|---------|------|
| json | Host string `json:"host"` | Maps `Host` to a JSON field: **host** |
| yaml | Host string `yaml:"host"` | Maps `Host` to a Yaml field: **host** |
| env | Host string `env:"HOST"` | Maps `Host` to a Environment variable: **HOST** |
| cli | Host string `cli:"host database host"` | Maps `Host` to a command line argument: **-host** or **--host** |
| default | Port int `default:"8080"` | Defines the port with default value: **8080** |


#### 1. Data types
 **config** supports the following golang data types:
  * bool
  * string
  * int8, int16, int, int32, int64
  * uint8, uint16, uint, uint32, uint64
  * float32, float64
  * slice type. e.g: []string, []int ...
  
#### 2. Defines **default** values
Using **default** keyword in structure tags to define default value. Example codes:
```golang
  type Log struct {
    Path  string `default:"/var/logs"`
    Level string `default:"debug"`
  }
```

After that, calls ```config.ParseDefault(interface{})``` to set structure instance with default values. Example codes:
```golang
  logConfig := Log{}
  config.ParseDefault(&logConfig)
```

#### 3. Defines configruation name for JSON
Like parsing JSON object, using **json** keyword to define configuration name
```golang
  type Database struct {
    Host string     `json:"host"`
    Port int        `json:"port"`
    Username string `json:"username" default:"admin"`
    Password string `json:"password" default:"admin"`
    Log Log         `json:"log"`
  }
```

#### 4. Defines configuration name for Yaml
Like parsing Yaml object, using **yaml** keyword to define configuration name
```golang
  type Database struct {
    Host string     `yaml:"host"`
    Port int        `yaml:"port"`
    Username string `yaml:"username" default:"admin"`
    Password string `yaml:"password" default:"admin"`
    Log Log         `yaml:"log"`
  }
```

#### 5. Defines configuration name for Environment variable
Using **env** keyword to define configuration name
```golang
  type Database struct {
    Host string     `env:"DB_HOST"`
    Port int        `env:"DB_PORT"`
    Username string `env:"DB_USER" default:"admin"`
    Password string `env:"DB_PASSWORD" default:"admin"`
    Log Log         `env:"DB_LOG_"`
  }
```

#### 6. Defines configuration name for Command line
Using **cli** keyword to define configuration name
```golang
  type Database struct {
    Host string     `cli:"host database host name"`
    Port int        `cli:"port database port"`
    Username string `cli:"username database username" default:"admin"`
    Password string `cli:"password database password" default:"admin"`
    Log Log         `cli:"log database log configurations"`
  }
```

#### III. Reads configurations from JSON and Yaml files
##### 1. Using ```json``` to define configuration name


##### 2. Using ```yaml``` to define configuration name
```golang
  type Database struct {
    Host string     `yaml:"host"`
    Port int        `yaml:"port"`
    Username string `yaml:"username" default:"admin"`
    Password string `yaml:"password" default:"admin"`
    Log Log         `yaml:"log"`
  }
```

Parsing configurations from JSON:
```golang
  dbConfig := Database{}
  config.ParseConfigFile(&dbConfig, "config.json")
```