[Haskell-beginners] Execution time of Mergesort

[Awsaf Rahman]

I did consider strictness and that is why the execution time has come down
from 16 seconds to 12 seconds! But I can't seem to find the issue anymore!

On Fri, Jul 6, 2018 at 4:06 AM, KC <kc1956 at gmail.com> wrote:

> I haven't looked at the code much yet
> But might want to consider strictness 😎
>
> --
> Sent from an expensive device which will be obsolete in a few months
> Casey
>
> On Thu, Jul 5, 2018, 7:01 PM Awsaf Rahman, <awsafrahman1704 at gmail.com>
> wrote:
>
>> Hello everyone!
>>
>> I am trying to find out the execution time of mergesort for a list size
>> of 1 million integers. I have done the same in Erlang as well and the
>> execution time in Erlang was around 0.93 seconds. I have implemented the
>> program in almost the same way in Haskell as well but for some reason the
>> Haskell implementation is taking around 12 seconds which doesn't seem
>> right.
>>
>> Here is the implementation in Haskell:
>>
>> *{-# LANGUAGE OverloadedStrings #-}*
>> *{-# LANGUAGE BangPatterns #-}*
>> *import Control.Exception*
>> *import Formatting*
>> *import Formatting.Clock*
>> *import System.Clock*
>> *import Control.DeepSeq*
>>
>> *mergesort [] = []*
>> *mergesort [x] = [x]*
>> *mergesort xs = let (lhalf, rhalf) = splitAt (length xs `div` 2) xs*
>> *               in merge' (mergesort lhalf) (mergesort rhalf)*
>>
>> *merge' lhalf rhalf = merge lhalf rhalf []*
>>
>> *merge [] [] acc = reverse acc*
>> *merge [] y acc = reverse acc ++ y*
>> *merge x [] acc = reverse acc ++ x*
>>
>> *merge (l:ls) (r:rs) acc*
>> *        | l < r = merge ls (r:rs) (l:acc)*
>> *        | otherwise = merge rs (l:ls) (r:acc)*
>>
>> *toList :: String -> [Integer]*
>> *toList input = read ("[" ++ input ++ "]")*
>>
>> *main = do*
>> *    file <- getLine*
>> *    contents <- readFile file*
>> *    let !unsortedlist = (toList contents)*
>> *    start <- getTime Monotonic*
>> *    evaluate(force (mergesort unsortedlist))*
>> *    end <- getTime Monotonic*
>> *    fprint (timeSpecs % "\n") start end*
>>
>>
>> What am I doing wrong?
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>
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