go-patterns/creational/singleton.md

#Singleton Pattern
Singleton creational design pattern restricts the instantiation of a type to a single object.

## Implementation
```go
package singleton

type singleton map[string]string

var once sync.Once

var instance *singleton

func New() *singleton {
	once.Do(func() {
		instance = make(singleton)
	})

	return instance
}
```

## Usage
```go
s := singleton.New()

s["this"] = "that"

s2 := singleton.New()

// s2["this"] == "that"
```

## Rules of Thumb
- Singleton pattern represents a global state and most of the time reduces testability.