Haskell High Performance Programming

Inspecting time and space usage

It is often necessary to have numbers about the time and space usage of Haskell programs, either to have an indicator of how well the program performs or to identify unnecessary allocations. The GHC Runtime System flag -s enables printing allocation and garbage-collection statistics when the program finishes.

Let's try this with an example program, which naively calculates the covariance of two lists:

-- file: time_and_space.hs import Data.List (foldl') sum' = foldl' (+) 0 mean :: [Double] -> Double mean v = sum' v / fromIntegral (length v) covariance :: [Double] -> [Double] -> Double covariance xs ys = sum' (zipWith (\x y -> (x - mean xs) * (y - mean ys)) xs ys) / fromIntegral (length xs) main = do let xs = [1, 1.1 ...