Type System

What are types:

• 意义的逻辑含义
• 一个契约，要求所有成员有共同的特性
• 所有值、变量都有type
• 所有对象object都有type
• 所有class、trait都是type

value types & reference types

• 有9种value types，它们不能为空：Byte Short Char Int Long Float Double Boolean Unit
• 其他都是reference types：objects functions 等等

Type Hierarchies

WEAK CONFORMANCE

• 和Java类似，前面type的都可以赋值转化成后面的type（typecast）
• 数、字符字面量，都可以赋值给 Byte Short and Char

Subtype

• 子类可以被当做父类

  val s: Student = new SmartStudent // 如此可以。s仍然是一个Student类，但它拥有SmartStudent的属性和方法
  

• S是T的子类，记作∶𝑆<∶𝑇, 读作 S is a T and satisfies all properties that T does，so S can be treated as T

可以把S赋值给T的三种情况：

1. 𝑆 primitive-widens to 𝑇 : widening is like Int converted to Double, is done implicitly
2. 𝑆 primitive-narrows to 𝑇 : narrowing is like Double to Int, needs conversion
3. 𝑆 <∶ 𝑇

Type Inference

Type inference is a technique for determining the type of some entity without explicitly specifying it

谨慎使用🤔

GENERICS, PARAMETRIC POLYMORPHISM & VARIANCE

想要通用化一个程序，可能是需要它可以处理不同的类型的参数。那么就可以把参数类型也作为一个参数传入，就是type parameter。表示为：

[T]

这样就会遇到subtype定义的问题。==a box of T should be able to both store and produce a T.==

假如GummyBear是Sweet的子类，但下面这两个都是错的：

val b: Box[GummyBear] = new Box[Sweet](new Sweet) // wrong
// 因为上述b需要可以 存 且 提供box中的类：
b.value = new GummyBear // ok
val a: GummyBear = b.value // not ok!

val b: Box[Sweet] = new Box[GummyBear](new GummyBear) // wrong
// 因为上述b需要可以 存 且 提供box中的类：
b.value = new Sweet // not ok!
val a: Sweet = b.value // ok

PECS Rule - Producer: Extends; Consumer: Super

1. if you need a box that Produces T, use a box of anything that Extends T

   def produceFromBox(b: Box[_ <: Sweet]): Sweet = b.value
   

   covariant: S<:TC[S]<:C[T]

   class Box[+T](val value: T) // make Box covariant in T
   val b: Box[Sweet] = new Box[GummyBear](new GummyBear) // ok now
   

2. if you need a box that Consumes T, use a box of anything that is Super of T

   def putInBox(b: Box[_ >: GummyBear], s: GummyBear) = { 
     b.value = s
   }
   

   contravariant: S<:TC[T]<:C[S]

   class Monster[-T] { // make Monster contravariant in T
       def eat(t: T) = println(t) 
   }
   val m: Monster[GummyBear] = new Monster[Sweet] // ok now
   

VARIANCE OF FUNCTION TYPES

I2<:I1,R2<:R1I1=>R1<:I2=>R2

for example, 一个function接收AnyRef，返回Subtring，那它就是这个function的子类：接收Sweet（<: AnyRef），返回String(>: Substring):

AnyRef => Substring <: Sweet => String