Option sugar

[JonathanAldrich]

See the design at https://github.com/wyvernlang/wyvern/wiki/Sugar-for-Options

[sychoo]

An option type sugar design of Kotlin can be found in https://kotlinlang.org/docs/reference/null-safety.html

[JonathanAldrich]

Pull request #359 covers the following:

• Supported T? as the sugar for option.Option[T].
• Supported a built-in None value that works for all T.
  • NONE = option.None[Dyn] <: option.None[T] for all T.
  • Note that NONE is equivalent to option.None[Dyn].

Still to implement:

• The implicit conversion from T to T?.

  // this will convert "Hello" to option.Some[String]("Hello").
  val str: String? = "Hello"

• Support "!" operator that explicitly convert T? to T, raise an error if str is NONE.

  // this will convert type of str from "String?" to the type of str2 "String".
  val str2: String = str!

• Propagate None using ?. operator.

  type Nested
     val str: String?
  
  val n1: Nested? = new
     val str: String? = "Hello"
  
  // this will print "Hello" successfully.
  stdout.print(n1?.str.get())
  
  val n2: Nested? = NONE
  
  // this will print "NONE", since n2 is of NONE type, "?." operator is used to propagate NONE.
  stdout.print(n2?.str.get())
  
  val n3: Nested? = new
     val str: String? = NONE
  
  // this will print "NONE, since n3.str is of NONE type, "?." operator is used to propagate NONE.
  stdout.print(n3?.str.get())