Monadic & Non monadic

gistfile1.scala

// sample snippet that does not use monads
// note explicit null checks and imperative flow

String param(String key) {
  //.. fetch value from request/session
  return value;
}

Trade queryTrade(String ref) {
  //.. query from db
  return trade
}

public static void main(String[] args) {
  String key;
  //.. set key

  String refNo = param(key);
  if (refNo == null) {
    //.. exception processing
  }

  Trade trade = queryTrade (refNo);
  if (trade == null) { //.. explicit null check
    //.. exception processing
  }
}

// monadic version

def param(key: String): Option[String] = { //.. monadic return
  //..
}
def queryTrade(ref: String): Option[Trade] = { //.. monadic return
  //..
}
  
def main(args: Array[String]) {

  // monadic comprehension block
  // automatic sequencing plus NO explicit null handling
  // makes program structure look better
  val trade =
    (
      for {   
        r <- param("refNo") 
        t <- queryTrade (r)
      } 
      yield t
  ) getOrElse error("not found")

//..