Reading lines and reversing the text with Haskell - string

Question

My program should do the following:
Read lines from a file
For each line, reverse the text of each word ("bat cat hat" becomes "tab tac tah")
Print out the the reverse text
But it doesn't work and being a Haskell-newbie, I can't understand the errors.
Here is my code:
main = do
content <- readFile "test.txt"
let linesList = lines content
reverseLines
reverseWords :: String -> String
reverseWords = unwords . map reverse . words
reverseLines :: [String] -> IO ()
reverseLines inputList = do
if null inputList
then return ()
else do
line <- inputList!!0
if null line
then return ()
else do
putStrLn $ reverseWords line
reverseLines . tail inputList
And my errors:
readlines.hs:4:9:
Couldn't match expected type `IO b0'
with actual type `[String] -> IO ()'
In a stmt of a 'do' block: reverseLines
In the expression:
do { content <- readFile "test.txt";
let linesList = lines content;
reverseLines }
In an equation for `main':
main
= do { content <- readFile "test.txt";
let linesList = ...;
reverseLines }
readlines.hs:14:25:
Couldn't match type `[Char]' with `IO [Char]'
Expected type: [IO [Char]]
Actual type: [String]
In the first argument of `(!!)', namely `inputList'
In a stmt of a 'do' block: line <- inputList !! 0
In the expression:
do { line <- inputList !! 0;
if null line then
return ()
else
do { putStrLn $ reverseWords line;
.... } }
readlines.hs:19:33:
Couldn't match expected type `IO ()' with actual type `a0 -> IO ()'
In a stmt of a 'do' block: reverseLines . tail inputList
In the expression:
do { putStrLn $ reverseWords line;
reverseLines . tail inputList }
In a stmt of a 'do' block:
if null line then
return ()
else
do { putStrLn $ reverseWords line;
reverseLines . tail inputList }
readlines.hs:19:48:
Couldn't match expected type `a0 -> [String]'
with actual type `[String]'
In the return type of a call of `tail'
Probable cause: `tail' is applied to too many arguments
In the second argument of `(.)', namely `tail inputList'
In a stmt of a 'do' block: reverseLines . tail inputList

Answer 1

Firstly, your last line in your main function is of type [String] -> IO (), and you need it to be IO (), so you actually need to pass it the list of strings. Here is one way to do what you are requesting:
main :: IO ()
main = do
content <- readFile "test.txt"
let linesList = lines content
reverseLines linesList
reverseWords :: String -> String
reverseWords = unwords . map reverse . words
reverseLines :: [String] -> IO ()
reverseLines = mapM_ (putStrLn . reverseWords)
And here is a recursive version akin to what you wanted to do in your version
reverseLines2 :: [String] -> IO ()
reverseLines2 [] = return ()
reverseLines2 (x:xs) = do
putStrLn (reverseWords x)
reverseLines2 xs
I've also fixed your version with minimal changes, but bear in mind that this is not really the Haskell way to do things. Go with one of the options I provided above.
main = do
content <- readFile "test.txt"
let linesList = lines content
reverseLines linesList
reverseWords :: String -> String
reverseWords = unwords . map reverse . words
reverseLines :: [String] -> IO ()
reverseLines inputList = do
if null inputList
then return ()
else do
let line = head inputList
if null line
then return ()
else do
putStrLn $ reverseWords line
reverseLines $ tail inputList