[dllthomas]

"This is because the true order of execution diverges greatly from what the program specifies in the do-notation. This has nothing to do with purity; it's a consequence of laziness."

Of course, that's not what the do notation specifies, but I agree that's somewhat subtle. As you say, it's a consequence of laziness. Replacing "return" with "evaluate" fixes this particular example.

In general, if you care about when some particular thing is evaluated - and for non-IO you usually don't - an IO action that you're sequencing needs to depend upon it. That can either be because looking at the thing determines which IO action is used, or it can be added artificially by means of seq (or conceivably deepSeq, if you don't just need WHNF).