Modifying the list while using for comprehension in Scala - scala

Question

I have a list that contains all possible deals between clients, and every sell-buy combination needs to be compared. With the comparison, another list need to be modified.
I have the following code:
def res(input_orders: List[Order], input_clients: List[Client]): List[Client] = {
for {
order1 <- input_orders
order2 <- input_orders if check_order(order1, order2)
firstClient = input_clients.find(_.name == order1.client)
secondClient = input_clients.find(_.name == order2.client)
tmp <- if (order1.operation == 's') {
input_clients.map { case `firstClient` => sell(firstClient.get, order1); case x => x }
input_clients.map { case `secondClient` => buy(secondClient.get, order2); case x => x }
} else {
input_clients.map { case `firstClient` => buy(firstClient.get, order1); case x => x }
input_clients.map { case `secondClient` => sell(secondClient.get, order2); case x => x }
}
} yield tmp
}
But it returns the list of clients as it is and does not modify it.
I suppose that the problem is in this "modification" block:
input_clients.map { case `firstClient` => sell(firstClient.get, order1); case x => x }
input_clients.map { case `secondClient` => buy(secondClient.get, order2); case x => x }
} else {
input_clients.map { case `firstClient` => buy(firstClient.get, order1); case x => x }
input_clients.map { case `secondClient` => sell(secondClient.get, order2); case x => x }
What is wrong with it?

Answer 1

Recall that the map function is immutable which means it doesn't do any modification to the datastructure but rather returns a new object which content has been updated.
In the block you're mentioning, the first calls of mapare useless because the return value is never used. A proper way of achieving the task you initially wanted to do would be:
val order = order1.operation == 's'
input_clients.map {
case `firstClient` => if (order) sell(firstClient.get, order1) else buy(firstClient.get, order1)
case `secondClient` => if (order) buy(secondClient.get, order2) else sell(secondClient.get, order2)
case x => x
}
Moreover, avoid using the get accessor of an Option[T] (imagine what would happen if the option was None). Prefer safer operations like pattern matching or flatMap / for-comprehension instead.