On Wed, Mar 26, 2008 at 05:07:10PM -0700, Don Stewart wrote:
droundy:
On Thu, Mar 27, 2008 at 01:09:47AM +0300, Bulat Ziganshin wrote:
-Collecting rendering stats is not easy without global variables. It occurs to me that it would be neat if there were some sort of write-only global variables that can be incremented by pure code but can only be read from within monadic code; that would be sufficient to ensure that the pure code wasn't affected by the values.
the code is called *pure* exactly because it has no side-effects and compiler may select either to call some function two times or reuse already computed result. actually, you can make sideeffects with unsafePerformIO, but there is no guarantees of how many times such code will be executed. try this:
plus a b = unsafePerformIO (modifyIORef counter (+1)) `seq` a+b
This is exactly what he wants to do. The point of putting traces into the code is precisely to figure out how many times it is called. The only trouble is that unsafePerformIO (I believe) can inhibit optimizations, since there are certain transformations that ghc won't do to unsafePerformIO code.
could we just use -fhpc or profiling here. HPC at least will tell you how many times top level things are called, and print pretty graphs about it.
It depends what the point is. I've found traces to be very helpful at times when debugging (for instance, to get values as well as counts). Also, I imagine that manual tracing is likely to be far less invasive (if you do it somewhat discretely) than profiling or using hpc. -- David Roundy Department of Physics Oregon State University