When a module is imported, or used for the first time, are there things
that are not fully evaluated?
I see the same thing you do with the Data.Fixed methods (nice work by the
way!), but I also see similar (albeit less drastic) behaviour with other
datatypes, including some custom ones. It just always seems like the first
call to a modules method is always slower than subsequent calls. But I
can't for the life of me figure out why...
On Thu, Jul 9, 2015 at 5:45 PM, Roman Cheplyaka
This is a nice puzzle. However, I don't think it's related to IO.
So far I've traced this to Data.Fixed methods. If you add
evaluate $ floor (2/1 :: Fixed E12)
before the loop, you'll find that the delay decreases significantly (although doesn't vanish entirely).
How is this related to forcing a time value? Well, UTCTime represents the time of day as Fixed E12, and when you're evaluating the result of getCurrentTime, you're calling those methods, (/) and floor.
Using other methods of Fixed E12 before the loop, or replacing E12 with E9, kills or at least hurts the effect.
There might be some thunks referenced directly or indirectly by those methods, possibly involving specialization, but I don't see it so far.
I'll let someone else to take it from here.
On 10/07/15 00:07, Ben Gunton wrote:
Here is the same thing with no console usage, just forcing the time value. The first-iteration-delay seems to happen at the first IO-ish thing being evaluated.