Type System
Overview
- Option
- Either
- List
- Tuple
- Map
- Try
1. Option
What is Option?
Option: The Option type is the standard library uses case classes to express values that might or might not be present.
When use Option?
def div(x:Double, y:Double):Double = x/y
div(1,2) // 0.5
div(1,0) // error
def div(x:Double, y:Double):Option[Double] = if(y==0) None else Some(x/y)
div(1,2) // Some(0.5)
div(1,0) // None
How to use Option value?
val a:Option[Int] = None
a.get //ERROR:None.get
a.getOrElse(0) //0
a.map(_+1) // None
a.flatMap(x => div(x, 1)) // None
a.flatMap(x => div(x, 0)) // None
val b:Option[Int] = Some(1)
b.get //1
b.getOrElse(0) //1
b.map(_+1) // Some(2)
b.flatMap(x => div(x, 1)) // Some(1)
b.flatMap(x => div(x, 0)) // None
val a = for {
x1 <- div(1,2)
x2 <- div(1,2)
x3 <- div(1,2)
} yield x1 + x2 + x3 // Some(1.5)
val b = for {
x1 <- div(1,2)
x2 <- div(1,2)
x3 <- div(1,0)
} yield x1 + x2 + x3 // None
2. Either
When use Either?
def div(x:Double, y:Double):Double = x/y
div(1,2) // 0.5
div(1,0) // error
def div(x:Double, y:Double):Either[String, Double] = if(y==0) Left("y is 0") else Right(x/y)
div(1,2) // Right(0.5)
div(1,0) // Left("y is 0")
How to use Either value?
val a:Either[String, Int] = Left("Some error happen")
a.map(_+1) // Left("Some error happen")
a.flatMap(x => div(x, 1)) // Left("Some error happen")
a.flatMap(x => div(x, 0)) // Left("Some error happen")
val b:Either[String, Int] = Right(1)
b.map(_+1) // Right(2)
b.flatMap(x => div(x, 1)) // Right(1)
b.flatMap(x => div(x, 0)) // Left("Some error happen")
val a = for {
x1 <- div(1,2)
x2 <- div(1,2)
x3 <- div(1,2)
} yield x1 + x2 + x3 // Right(1.5)
val b = for {
x1 <- div(1,2)
x2 <- div(1,2)
x3 <- div(1,0)
} yield x1 + x2 + x3 // Left("y is 0")
3. List
What is List?
List: In Scala, a list is either Nil(that is, empth) or an object with a head element and a tail that is again a list.
How to use List?
val a = List(1,2,3)
val b = 1::List(4,2) //The :: operator makes a new list from a given head and tail
val b' = 1::4::2::Nil
val c = List(1) ::: List(1,2,3)
val d = List(1) :: List(1,2,3) //List(List(1), 1, 2, 3)
//(:: prepends a single item whereas ::: prepends a complete list)
List(1,2,3).map(_+1) //List(2, 3, 4)
List(1,2,3).flatMap(x=>List(x,x)) //List(1, 1, 2, 2, 3, 3)
List(3,2,1).sorted //List(1, 2, 3)
List(3,2,1).filter(_%2==1) //List(3, 1)
List(3,2,1).max //3
List(3,2,1).min //1
List(3,2,1).sum //6
List(3,2,1).fold(0)((x,y)=>x+y) //6
4. Tuple
What is Tuple?
Tuple: Aggregrates of n objects, not necessarily of the same type.
Pair: Simply a tuple with n = 2.
When use Tuple?
Tuples are useful for functions that returns more than one value.
"New York".partition(_.isUpper) //("NY","ew ork")
How to use Tuple?
val tuple = (1, 3.14, "Fred") //(Int, Double, String)
val second = tuple._2
val (first, second, _) = tuple //use pattern matching to get at the components of a tuple
5. Map
What is Map?
Tuple: Aggregrates of n objects, not necessarily of the same type.
Pair: Simply a tuple with n = 2.
Map: Collection of pairs.
How to use Map?
val immutalbeScores = Map("Alice" -> 10, "Bob" -> 3, "Cindy" -> 8)
val mutalbeScores = scala.collection.mutable.Map("Alice" -> 10, "Bob" -> 3, "Cindy" -> 8)
val scores = Map(("Alice",10), ("Bob", 3), ("Cindy", 8))
val bobScore = scores("Bob") //will throw an exception is doesn't contain value
val bobScore = scores.getOrElse("Bob", 0) // return an option object
Update Mutalbe Map
mutableScores("Bob") = 10
mutableScores("Fred") = 7
mutalbeScores += ("Bob" -> 10, "Fred" -> 7) //use += opration to add multiple associations
mutalbeScores -= "Alice"
"Update" Immutable Map
Obtion a new map that has the desire update:
val newScores = immutalbeScores + ("Bob" -> 10, "Fred" -> 7)
for((name, score) <- scores) yield (score, name)
6. Try
What is Try?
Try: A Try[T] instance is either a Success(v), where v is a value of type T or a Failure(ex), where ex is a Throwable.
How to use Try?
t match {
case Success(v) => println(s"The answer is $v")
case Failure(ex) => println(s"Failed. Reason: ${ex.getMessage}")
}
if (t.isSuccess) println(s"The answer is ${t.get}")
if (t.isFailure) println(t.failed.get.getMessage)
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