Strange (?) for comprehension evaluation in Scala

Question

Now, it took me a while to figure out why my recursion is somehow managing to blow the stack. Here it is, the part causing this problem:

scala> for {
     |   i <- List(1, 2, 3)
     |   j = { println("why am I evaluated?"); 10 } if false
     | } yield (i, j)
why am I evaluated?
why am I evaluated?
why am I evaluated?
res0: List[(Int, Int)] = List()

Isn't this, like, insane? Why at all evaluate j = ... if it ends in if false and so will never be used?

What happens when instead of { println ... } you have a recursive call (and recursion guard instead of if false), I have learned. :<

Why?!

Answer 1

If you structure your loop like this, it will solve your problem:

scala> for {
     |   i <- List(1, 2, 3)
     |   if false
     |   j = { println("why am I evaluated?"); 10 }
     | } yield (i, j)
res0: List[(Int, Int)] = List()

Scala syntax in a for-loop treats the if statement as a sort of filter; this tutorial has some good examples.

One way to think of it is to walk through the for loop imperatively, and when you reach an if statement, if that statement evaluates to false, you continue to the next iteration of the loop.