[Swift] Type Inference vs Type Annotation

Swift is a type-safe language. A type safe language encourages you to be clear about the types of values your code can work with. If part of your code expects a String, you can’t pass it an Int by mistake.

var welcomeMessage: String
welcomeMessage = 22 // this would create an error because you  
//already specified that it's going to be a String

Type Inference

If you don’t specify the type of value you need, Swift uses type inference to work out the appropriate type. Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.

var meaningOfLife = 42 // meaningOfLife is inferred to be of type Int
meaningOfLife = 55 // it Works, because 55 is an Int

var meaningOfLife = 42 // 'Type inference' happened here, we didn't specify that this an Int, the compiler itself found out.
meaningOfLife = 55 // it Works, because 55 is an Int
meaningOfLife = "SomeString" // Because of 'Type Safety' ability you will get an 
//error message: 'cannot assign value of type 'String' to type 'Int'' 

Optimize Compiler Performance(Type Annotation)

Type Inference takes more time than Type Annotation.

To optimize compiler performance, try to write the code like below.

let names : [String] = ["John", "Ali", "Jane", " Taika"]

References

Official Swift Document - The Basics (opens new window)

개발자 소들이 - 타입 추론(Type Inference) vs 타입 어노테이션(Type Annotation) (opens new window)