couldn't match expected type [a0] with actual type IO ()

Question

What is wrong in my code:

insertValue file x = 
    if x == \"10\" then \"ok\"
    else do putStrLn \"Error\"; file

Answer 1

You need to use return :: a -> IO a to "lift" your strings into IO String:

insertValue file x = 
    if x == "10"
      then return "ok"
      else do putStrLn "Error"
              return file

But are you sure you don't want to call putStrLn "ok" (instead of return "ok") and return a Maybe value? Otherwise you are returning file or "ok" and your caller could never determine if there was an error when calling insertValue on a file named "ok".