scala notes

https://www.safaribooksonline.com/library/view/scala-for-the/9780134510613/

for yield => map

for yield with guard clause => filter

reduceLeft

clojures - how are they implemented? In scala, they are objects which capture the method and bindings of free variables.

Expression evalutation (Recursive data structures)

abstract class Expr
case class Num(value: Int) extends Expr
case class Sum(left: Expr, right: Expr) extends Expr
case class Product(left: Expr, right: Expr) extends Expr

val e = Product(Num(3), Sum(Num(4), Num(5)))
def eval(e: Expr):Int = e match {
  case Num(v) => v
  case Sum(l,r) => eval(l) + eval(r)
  case Product(l,r) => eval(l) * eval(r)
}
eval(e)

Expression evalutation (Recursive data structures) - OOP version

abstract class Expr {
  def eval: Int
}

class Num(val data: Int) extends Expr {
  def eval: Int = data
}
class Product(val left: Expr, val right: Expr) extends Expr {
  def eval: Int = left.eval * right.eval
}
class Sum(val left: Expr, val right: Expr) extends Expr {
 def eval: Int = left.eval + right.eval
}

val e = new Product(new Num(3), new Sum(new Num(4), new Num(5)))
e.eval



So what to use, Polymorphism version or case classes.

Use case classes when your cases are bound. Like here. There is a finite set of expressions.

Use Polymorphism