Question

import System.IO
import Data.Char
edastaNr n
   = do
      putStrLn "Anna rida:"
      line <- getLine
      if null line
         then return () 
         else do return length(line)

can anybody explain why the last line is incorrect and how it is possible to fix it? I do not realize.. error is:

Couldn't match expected type `IO ()' with actual type `[a0] -> Int'

Answer 1

You’re missing parentheses.

...
else do return (length line)

However, there are type errors as well. What is the type of your edasaNr function?
It must do some IO and return an Int, when given an argument.

So it would be better written as:

edastaNr :: a -> IO Int
edastaNr n = do
    putStrLn "Anna rida:"
    line <- getLine
    if null line
     then return 0
     else return $ length line

which also reveals that you don’t use the n argument (so it can have any type).