Null Pointer When Partitioning A Stream

Hi,


I'm not sure if this is possible but I want to partition a stream

based on some condition that depends on the current output

state of the stream. It will make sense with an example I think.


I will create a bunch of orders which I will stream since the 

actual use case is a stream of orders coming in so it is not 

known up front what the next order will be or even the full list 

of orders:


scala> case class Order(item : String, qty : Int, price : Double)
defined class Order

scala> val orders = List(Order("bike", 1, 23.34), Order("book", 3, 2.34), Order("lamp", 1, 9.44), Order("bike", 1, 23.34))
orders: List[Order] = List(Order(bike,1,23.34), Order(book,3,2.34), Order(lamp,1,9.44), Order(bike,1,23.34))


Now I want to partition/group these orders into one set which 

contain exactly the same order and another set which contains unique 

orders. So in the above example, when I force the stream it should 

create two streams: one with the two orders for a bike (since they are the 
same) 

and another stream containing all the other orders.

I tried the following:


Created the partitioning function:


scala> def matchOrders(o : Order, s : Stream[Order]) = s.contains(o)
matchOrders: (o: Order, s: Stream[Order])Boolean


then tried to apply this to stream:


scala> val s : (Stream[Order], Stream[Order]) = orders.toStream.partition(matchOrders(_, s._1))


I got a null pointer exception since I guess the s._1 is empty initially?? 

I'm not sure. I've tried other ways but I'm not getting very far. Is there 
a 

way to achieve this partitioning?


Regards

Iftikhar

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