Performance Timing

[Spencer Janssen]

I've written two versions of a prime number sieve, and I'm trying to 
figure out how to performance test them.  I've found functions to get 
the current date and time, and to subtract them, but when I put them in 
a do notation, I guess the laziness or something, makes the calculation 
happen first, then the two times are called, so I get almost 0 
difference between the two.

I guess I'm looking for something like this:
timer :: (a -> b) -> a -> IO TimeDiff

Spencer Janssen