code.hs

import           System.IO.Unsafe

heavy y = unsafePerformIO $ do
    putStrLn "heavy"
    return y

f1 x = heavy 10 + x

f2 x = let y = heavy 10 in y + x

f3 x = y + x where y = heavy 10

main = do
    print $ map f1 [1..3]
    print $ map f2 [1..3]
    print $ map f3 [1..3]

output.txt

heavy
heavy
heavy
[11,12,13]
heavy
heavy
heavy
[11,12,13]
heavy
heavy
heavy
[11,12,13]