Go - Type Assertion

• X.(T)  X is interface, T is concrete type.
• When T is a concrete type, it assert dynamic type of X is identical to T. Result is the dynamic value with type of T
  
• Left hand of type assertion must be an interface, otherwise panic
  
• Panic if assert type fail
  
• If T is an interface, it check is X satisfy T interface. Result is the type T
  
• If two result are expected, type assertion will return boolean but not panic.
  w, ok := x.(T)
• If result is false, result type will be nil