how to force the evaluation of this expression?

Question

module Main where

import Control.Parallel(par,pseq)
import Text.Printf
import Control.Exception
import System.CPUTime
import Data.List
import IO
import Data.Char
import Control.DeepSeq

time :: IO t -> IO t
time a = do
   start <- getCPUTime
   v <- a
   end <- getCPUTime
   let diff = (fromIntegral (end - start)) / (10^12)
   printf "Computation time: %0.3f sec\n" (diff :: Double)
   return v

learquivo :: FilePath -> IO ([[Int]])
learquivo s = do
   content <- readFile s
   return (read content)

main :: IO ()
main = do
   t5 <- getCPUTime
   content <- learquivo "mkList1.txt"
   let !mapasort = rnf $ map sort content
   t6 <- getCPUTime
   let diffft6t5 = (fromIntegral (t6 - t5)) / (10^12)
   printf "Computation time Mapasort: %0.3f sec\n" (diffft6t5 :: Double)

How to tell if it evaluates all elements of content?

let !mapasort = rnf $ map sort content

I used the line in winghci:

*Main> let !mapasort = rnf $ map sort content  

But, returned:

*Main> mapasort ()  

Thanks

Answer 1

I see two questions:

1) Why is mapsort evaluating to unit, () .

Because the rnf function always returns () . See the documentation .

2) Is everything evaluated

Yes. The DeepSeq instance (which is where rnf lives) for list just calls the deepseq instance for each element in the list:

rnf [] = ()
rnf (x:xs) = rnf x `seq` rnf xs

Your elements all Ints, which have a correct NFData instance.

And I'd add two more questions:

3) How should this benchmarking be done correctly?

Using Criterion . There are many Criterion advocates here on SO, you can find answers that would serve as good examples with a search.

4) How should this evaluation be forced for non-benchmarking purposes?

Using the parallel package.

import Control.Parallel.Strategies
...
let !mapsort = (map sort content) `using` (evalList rdeepseq)

or still using rnf :

let mapsort = map sort content
    !_ = rnf mapsort