For a real use case, consider using a parser library. If you ignore the boilerpl...

For a real use case, consider using a parser library. If you ignore the boilerplate import stuff, it's IMHO rather short and elegant - and not very imperative:

  {-# LANGUAGE OverloadedStrings #-}

  import Data.Text (Text)
  import Data.Void (Void)
  import qualified Data.Text as T
  import Text.Megaparsec
  import Text.Megaparsec.Char
  import qualified Text.Megaparsec.Char.Lexer as L

  type Parser = Parsec Void Text

  numParser :: Parser [Integer]
  numParser = L.decimal `sepBy` space1

  -- just for demonstration purposes - this will print an ugly debug string, it can be customised
  main = putStrLn $ show $ parse numParser "some-source-file.txt" "10 20  30 x"

  -- this would print "Right [10, 20, 30]"
  -- main = putStrLn $ show $ parse numParser "some-source-file.txt" "10 20  30"