First, import a library to catch fatal errors as follows:

import Control.Exception (catch, SomeException)

Next, import a library to get command-line arguments so that the file path is dynamic. We use the following line of code to do this:

import System.Environment (getArgs)

Continuing as before, define and implement main as follows:

main :: IO ()
main = do

Define a fileName string depending on the user-provided argument, defaulting to input.txt if there is no argument. The argument is obtained by retrieving an array of strings from the library function, getArgs :: IO [String], as shown in the following steps:

args <- getArgs
  let filename = case args of
    (a:_) -> a
        _ -> "input.txt"

Now apply readFile on this path, but catch any errors using the library's catch :: Exception e => IO a -> (e -> IO a) -> IO a function. The first argument to catch is the computation to run, and the second argument is the handler to invoke if an exception is raised, as shown in the following commands:

  input <- catch (readFile fileName)
    $ \err -> print (err::SomeException) >> return ""

The input string will be empty if there were any errors reading the file. We can now use input for any purpose using the following command:

  print $ countWords input

Don't forget to define the countWords function as follows:

countWords input = map (length.words) (lines input)