Puzzled: Clojure for loop with :while -> unexpected behaviour?

Question

I've been learning Clojure and puzzled by the following:

user=> (for [a (range 1 4) b (range 1 4)] [a b])
([1 1] [1 2] [1 3] [2 1] [2 2] [2 3] [3 1] [3 2] [3 3]); _no surprise here_

Let's add :while (not= a b), I expect to see an empty list as the loop should stop if the condition is false. In this case it's the very first item where a=b=1. Let's see:

user=> (for [a (range 1 4) b (range 1 4) :while (not= a b) ] [a b])

([2 1] [3 1] [3 2]) ; _surprise!_

Changing :while to :when to filter out (= a b) pairs

user=> (for [a (range 1 4) b (range 1 4) :when (not= a b) ] [a b])
([1 2] [1 3] [2 1] [2 3] [3 1] [3 2]); _expected_

Could anyone explain why (for [ ... :while ..] ...) behaves like this?

I'm using Clojure 1.3 on OS X.

Thank you and apologize for the lack of formatting. This is my virgin post on StackOverflow.

Answer 1

Let's look at each iteration.

a = 1
  b = 1 -> a == b, break because of while

a = 2
  b = 1 -> a != b, print [2 1]
  b = 2 -> a == b, break because of while

a = 3
  b = 1 -> a != b, print [3 1]
  b = 2 -> a != b, print [3 2]
  b = 3 -> a == b, break because of while

Answer 2

The :while condition in for only terminates the inner-most loop. I use for all the time, but :while so rarely that I never realized this; thanks for the great question!

Sadly I think the best you can do is wrap a take-while around the for, since you want a "global" stop-counter on the output sequence, not a stop-counter on one of the input sequences you're iterating over. For example:

(->> (for [a (range 1 4)
           b (range 1 4)]
       [a b])
     (take-while (fn [[a b]] (not= a b))))

()