Challenge 2: A Pure Core With a Thin IO Shell — Possible Solution
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Main.hs:

calculateTotal :: [Int] -> Int    -- completely free of IO
calculateTotal = sum

main :: IO ()
main = do
  line <- getLine
  let numbers = map read (words line) :: [Int]
      total = calculateTotal numbers
  putStrLn ("Total: " ++ show total)

Sample run (input: "3 5 7 10"):

    Total: 25

Explanation:
calculateTotal's type, [Int] -> Int, contains no IO anywhere -- it's
a pure function, fully testable on its own with an ordinary function
call like calculateTotal [3, 5, 7, 10], no IO mocking or setup
required at all. All the actual IO -- reading the line, parsing it,
printing the result -- lives entirely inside main's own do block,
which is the thin shell responsible for getting real-world input to
the pure core and printing its result back out. calculateTotal itself
never touches getLine, putStrLn, or anything IO-flavored.

WHY THIS WORKS AS AN ANSWER
------------------------------
This implements the exact pure-core/IO-shell pattern the chapter
recommends, with calculateTotal kept genuinely free of IO in its own
type while main handles all the actual reading and printing around
it.