Re: Partition A Stream Error

On 2015-08-08 8:58 PM, [email protected] wrote:
> created the partitioning function:
> |scala>defmatchOrders(o :Order,s 
> :Stream[Order])=s.contains(o)matchOrders:(o:Order,s:Stream[Order])Boolean|
>
> then tried to apply this to stream:
>
> |scala>vals 
> :(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?
>

You cannot, logically, use circular programming 
<https://en.wikibooks.org/wiki/Haskell/Fix_and_recursion> like this, 
because contains may reach the end of s._1, and at the very least, even 
if you circularly-program this with the right primitive (lazy val not 
val 
<https://github.com/scalaz/scalaz/blob/v7.1.3/core/src/main/scala/scalaz/std/Function.scala#L160>), 
you don't know where the end of s._1 is, so contains cannot terminate.  
In other words, matchOrders is strict on the s argument's spine.  (You 
may be tempted to try making that argument to matchOrders by-name; try 
to work out why that won't make a difference.)

I have included links with working examples of this kind of programming; 
I suggest using the lazy val or fix function to port these to Scala.  
The simplest is a basic infinite stream of a given value.

-- 
Stephen Compall
If anyone in the MSA is online, you should watch this flythrough.

-- 
You received this message because you are subscribed to the Google Groups "scala-language" group.
To unsubscribe from this group and stop receiving emails from it, send an email to [email protected].
For more options, visit https://groups.google.com/d/optout.