creational/factory: implement factory method pattern

Author: tmrts

README.md

@@ -17,7 +17,7 @@ A curated collection of idiomatic design & application patterns for Go language.
 |:-------:|:----------- |:------:|
 | [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 | ✘ |
-| [Factory Method](/creational/factory.md) | Defers instantiation of an object to a specialized function for creating instances | ✘ |
+| [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/factory.md

@@ -0,0 +1,58 @@
+# Factory Method Pattern
+
+Factory method creational design pattern allows creating objects without having
+to specify the exact type of the object that will be created.
+
+## Implementation
+
+The example implementation shows how to provide a data store with different
+backends such as in-memory, disk storage.
+
+### Types
+
+```go
+package data
+
+import "io"
+
+type Store interface {
+    Open(string) (io.ReadWriteCloser, error)
+}
+```
+
+### Different Implementations
+
+```go
+package data
+
+type StorageType int
+
+const (
+    DiskStorage StorageType = 1 << iota
+    TempStorage
+    MemoryStorage
+)
+
+func NewStore(t StorageType) Store {
+    switch t {
+    case MemoryStorage:
+        return newMemoryStorage( /*...*/ )
+    case DiskStorage:
+        return newDiskStorage( /*...*/ )
+    default:
+        return newTempStorage( /*...*/ )
+    }
+}
+```
+
+## Usage
+
+With the factory method, the user can specify the type of storage they want.
+
+```go
+s, _ := data.NewStore(data.MemoryStorage)
+f, _ := s.Open("file")
+
+n, _ := f.Write([]byte("data"))
+defer f.Close()
+```