Haskell Exception Non-exhaustive patterns on String

Question

My Function is

import System.IO

import Debug.Trace

main :: IO ()    
main = do     
    datei <- openFile  "palindrom.txt" ReadMode    
    palin <- hGetContents datei    
    putStrLn $ unlines [  check x | x <- lines palin]
    
check :: String -> String    
check x     
    | null x = ""    
    | trace ("call check "++ show x) False = x     
    | x == (reverse x) =  if null x then "" 
               else do x ++ " Palindrom length " ++ show (length x)

I get the Exception Non-exhaustive patterns in the function `check`.

How can I match the string to complete the pattern, I tried also the empty String "" or I am even not able to use this kind of pattern on a String in Haskell?

ps: The palindrom.txt is

a
aa
ab
aha
anna
anne
bry
bub

Answer 1

Let's try evaluating it by hand for a few steps (sans the trace call), shall we?

check "ab"
===
case null "ab" of True -> ""
   ; _ -> case "ab" == (reverse "ab") of True -> 
            "ab" ++ " Palindrom length " ++ show (length "ab")
===
case False of True -> ""
   ; _ -> case "ab" == "ba" of True -> 
            "ab Palindrom length " ++ show 2
===
case "ab" == "ba" of True -> 
            "ab Palindrom length " ++ "2"
===
case False of True -> 
            "ab Palindrom length " ++ "2"
===
ERROR: none of the cases matched.

So all the tests in guards having failed, the pattern match for x | ... fails a well, which is reported as "non-exhaustive patterns".

In GHCi:

> case False of True -> 1

*** Exception: <interactive>:789:1-23: Non-exhaustive patterns in case