[Haskell-cafe] announce: Glome.hs raytracer

[David Roundy]

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