CODEWITHSUNDEEP
scalapattern-matching
noahlz
noahlz

Reputation: 10311

Better solution to case class inheritance deprecation?

I am working through the (admittedly somewhat old) Programming Scala [Subramaniam, 2009] and encountered a troubling compiler warning when working through section 9.7 "Matching using case classes" (see below).

Here is the solution I devised based on my interpretation of the error message. How could I improve upon this code with a solution closer to the original intent of the book's example? Particularly if I wanted to use sealed case class functionality?

/** 
 * [warn] case class `class Sell' has case ancestor `class Trade'.  
 * Case-to-case inheritance has potentially dangerous bugs which 
 * are unlikely to be fixed.  You are strongly encouraged to instead use 
 * extractors to pattern match on non-leaf nodes.
 */

// abstract case class Trade()
// case class Buy(symbol: String, qty: Int) extends Trade
// case class Sell(symbol: String, qty: Int) extends Trade
// case class Hedge(symbol: String, qty: Int) extends Trade

object Side extends Enumeration {
  val BUY = Value("Buy")
  val SELL = Value("Sell")
  val HEDGE = Value("Hedge")
}

case class Trade(side: Side.Value, symbol: String, qty: Int)

def process(trade: Trade) :String = trade match {
  case Trade(_, _, qty) if qty >= 10000 => "Large transaction! " + trade
  case Trade(_, _, qty) if qty % 100 != 0 => "Odd lot transaction! " + trade 
  case _ => "Standard transaction: " + trade
}

Upvotes: 1

Views: 1312

Answers (1)

ramn
ramn

Reputation: 467

Inherit from a "sealed trait Trade" instead.

sealed trait Trade
case class Buy(symbol: String, qty: Int) extends Trade
case class Sell(symbol: String, qty: Int) extends Trade
case class Hedge(symbol: String, qty: Int) extends Trade

Upvotes: 4

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