scala Stream::filterNot not lazy

Question

A simple example like following:

val set = Set(0, 1, 2)
Stream.from(0).filterNot(set).head

it seems loop forever; but the following code works:

Stream.from(0).filter(!set(_)).head

I am using

Scala code runner version 2.11.4 -- Copyright 2002-2013, LAMP/EPFL

Is this a bug? or any reason for this?

Answer 1

According to the ticket: https://issues.scala-lang.org/browse/SI-8627 the implementation of filterNot which applies to Streams is not lazy, which would manifest in what you are seeing.

So, appears to be a bug and according to the ticket, no way has been found to fix it without either significant performance penalties to other collections or else breaking binary compatibility, hence it seems to be targeted at a major release (2.12)