creational/builder: implement the builder pattern

Author: tmrts

README.md

@@ -16,7 +16,7 @@ A curated collection of idiomatic design & application patterns for Go language.
 | Pattern | Description | Status |
 |:-------:|:----------- |:------:|
 | [Abstract Factory](/creational/abstract_factory.md) | Provides an interface for creating families of releated objects | ✘ |
-| [Builder](/creational/builder.md) | Builds a complex object using simple objects | ✘ |
+| [Builder](/creational/builder.md) | Builds a complex object using simple objects | ✔ |
 | [Factory Method](/creational/factory.md) | Defers instantiation of an object to a specialized function for creating instances | ✔ |
 | [Object Pool](/creational/object-pool.md) | Instantiates and maintains a group of objects instances of the same type | ✔ |
 | [Singleton](/creational/singleton.md) | Restricts instantiation of a type to one object | ✔ |

creational/builder.md

@@ -0,0 +1,61 @@
+# Builder Pattern
+
+Builder pattern separates the construction of a complex object from its
+representation so that the same construction process can create different
+representations.
+
+In Go, normally a configuration struct is used to achieve the same behavior,
+however passing a struct to the builder method fills the code with boilerplate
+`if cfg.Field != nil {...}` checks.
+
+## Implementation
+
+```go
+package car
+
+type Speed float64
+
+const (
+    MPH Speed = 1
+    KPH       = 1.60934
+)
+
+type Color string
+
+const (
+    BlueColor  Color = "blue"
+    GreenColor       = "green"
+    RedColor         = "red"
+)
+
+type Wheels string
+
+const (
+    SportsWheels Wheels = "sports"
+    SteelWheels         = "steel"
+)
+
+type Builder interface {
+    Color(Color) Builder
+    Wheels(Wheels) Builder
+    TopSpeed(Speed) Builder
+    Build() Interface
+}
+
+type Interface interface {
+    Drive() error
+    Stop() error
+}
+```
+
+## Usage
+
+```go
+assembly := car.NewBuilder().Paint(car.RedColor)
+
+familyCar := assembly.Wheels(car.SportsWheels).TopSpeed(50 * car.MPH).Build()
+familyCar.Drive()
+
+sportsCar := assembly.Wheels(car.SteelWheels).TopSpeed(150 * car.MPH).Build()
+sportsCar.Drive()
+```