How to use Criterion to measure performance of Haskell programs?

Question

I'm trying to measure the performance of a simple Haar DWT program using the Criterion framework. (It is erroneously slow, but I'll leave that for another question). I can't find any good documentation on the web, unfortunately. My two primary problems are

How can one pass data from one benchmark to another? I want to time each stage of the program.
How does the sampling work, and avoid lazy evaluation reusing its previous computations?

This source is relatively pared down; the first function getRandList generates a list of random numbers; haarStep transforms an input signal into differences and sums, and haarDWT calls the former and recurses on the sums. I'm trying to pass the getRandList to the haarDWT via lazy evaluation, but perhaps my usage is incorrect / unsupported. The timings don't seem to make sense.

{-# LANGUAGE ViewPatterns #-}

import Control.Arrow
import qualified Data.Vector.Unboxed as V
import System.Random
import Criterion.Main

invSqrt2 = 0.70710678118654752440

getRandList :: RandomGen g => g -> Int -> [Float]
getRandList gen 0 = []
getRandList gen n = v:rest where
    (v, gen') = random gen
    rest = getRandList gen' (n - 1)

haarStep :: V.Vector Float -> (V.Vector Float, V.Vector Float)
haarStep = (alternatingOp (-) &&& alternatingOp (+)) where
    alternatingOp op x = V.generate (V.length x `div` 2) (\i ->
        ((x V.! (2 * i)) `op` (x V.! (2 * i + 1))) * invSqrt2)

haarDWT :: V.Vector Float -> V.Vector Float
haarDWT xl@(V.length -> 1) = xl
haarDWT (haarStep -> (d, s)) = haarDWT s V.++ d

main = do
    gen <- getStdGen
    inData <- return $ getRandList gen 2097152
    outData <- return $ haarDWT (V.fromList inData)

    defaultMain [
        bench "get input" $ nf id inData,
        bench "transform" $ nf V.toList outData
        ]
    writeFile "input.dat" (unlines $ map show inData)
    writeFile "output.dat" (unlines $ map show $ V.toList outData)

Finally, I'm getting an error when I try to call it with -s 1; maybe this is just a Criterion bug.

Main: ./Data/Vector/Generic.hs:237 ((!)): index out of bounds (1,1)

Thanks in advance!

How to use Criterion to measure performance of Haskell programs?

1 Answers