Test.StrictBench

A library to benchmark how long it takes to fully evaluate a value. Can be useful to identify the slow part of an algorithm, since Haskell's lazy evaluation can make it hard to see where the bottleneck lies.

Full evalution of a value is achieved by the rnf function, which requires that the data type of the value being tested is an instance of NFData. Making a data type an instance of NFData is trivially done by applying rnf to all of its fields and seq-ing those together.

Example:

  data Tree3 a = Leaf a | Branch (Tree3 a) (Tree3 a) (Tree3 a)
  
  instance NFData a => NFData (Tree3 a) where
      rnf (Leaf x) = rnf x
      rnf (Branch l c r) = rnf l `seq` rnf c `seq` rnf r
      
  main = bench . take 13 $ iterate (\x -> Branch x x x) (Leaf 'a')

  Output:
  765.625 ms

If a data constructor has no fields you can suffice with (), e.g.:

  data Answer = Yes | No

  instance NFData Answer where
      rnf Yes = ()
      rnf No  = ()

Documentation

bench :: NFData a => a -> IO ()

Print how long it takes to strictly evaluate the given value.

Example:

  main = bench [1..10000000 :: Integer]

  Output:
  515.625 ms

benchDesc :: NFData a => String -> a -> IO ()

Like bench, benchDesc prints the time needed to fully evaluate the given value. Additionally, it prefixes the time taken with the provided string, which can be useful to distinguish between different benchmarks.

Example:

  main = benchDesc "Long string" $ replicate 10000000 'a'

  Output:
  Long string: 375.0 ms

time :: NFData a => a -> IO Double

The function used by bench and benchpress to determine how long (in milliseconds) the value takes to calculate. You can use this function for instance if you wish to sum the time of several different values.

Example:

  main = do t1 <- time $ filter (< 10) $ take 1000000 $ repeat (9 :: Int)
            t2 <- time $ reverse $ take 1000000 $ cycle "StrictBench"
            print $ t1 + t2
  
  Output:
  562.5