Preventing caching of computation in Criterion benchmark

Question

The following code (suggested by Reid Barton at Criterion causing memory consumption to explode, no CAFs in sight) has a benchmark time which scales proportional

Answer 1

The result is being cached in your lvl_r6yu. You can see that lst1 is [0..num] lifted out to the top level, and from $wlgo 0 lst1 it can be seen that the result of the summation is lifted out too.

It's easier to see what's happening if we add the top level definition foo = mysum . lst, and then look at the core for foo. You can see there that foo is a constant function returning the result of the summation.

If we add {-# OPTIONS -fno-full-laziness #-}, then subexpressions will not be lifted, and therefore the benchmark will work as intended.

It is a good idea in general when using criterion to control evaluation through the arguments supplied to whnf. In our case:

bench "mysum" $ whnf (\size -> mysum [1..size]) num

This works fine regardless of optimization or lifting.