Why is this Haskell program so much slower than an equivalent Python one?

Question

As part of a programming challenge, I need to read, from stdin, a sequence of space-separated integers (on a single line), and print the sum of those integers to stdout

Answer 1

read is slow. For bulk parsing, use bytestring or text primitives, or attoparsec.

I did some benchmarking. Your original version ran in 23,9 secs on my computer. The version below ran in 0.35 secs:

import qualified Data.ByteString.Char8 as B
import Control.Applicative
import Data.Maybe
import Data.List
import Data.Char

main = print . sum =<< getIntList

getIntList :: IO [Int]
getIntList =
    map (fst . fromJust . B.readInt) . B.words <$> B.readFile "test.txt"

By specializing the parser to your test.txt file, I could get the runtime down to 0.26 sec:

getIntList :: IO [Int]          
getIntList =
    unfoldr (B.readInt . B.dropWhile (==' ')) <$> B.readFile "test.txt"