Haskell IO reverse string input

0
votes

I keep getting an error: couldn't match expected type 'Bool' with actual type '[t0]'. I'm trying to get user inputs of string and then output however many strings in reversed ORDER.

Example input:

HI1
HI2

Example output:

HI2
HI1

My code:

    Back :: Int -> IO()
    Back x = do line <- sequence_[getLine|[1..x]]
                     mapM_ print (reverse line)
2
haskellmapiosequence
Do you want the inputs and outputs to occur one on each line, or all the inputs in a single line, and all the outputs in a single line? - Boris
Take a look at replicateM. - Vitus

2 Answers

8
votes

To expand on Vitus's comment, import Control.Monad, then

back count = do
    lines <- replicateM count getLine
    mapM_ putStrLn (reverse lines)

If this doesn't work for you, please say what error message you get, or give an example of incorrect output.

In this case, we can forego do notation fairly easily:

back count = mapM_ putStrLn . reverse =<< replicateM count getLine

Or

back count = mapM_ putStrLn =<< liftM reverse (replicateM count getLine)

You may or may not find either of those to be clearer.

Note that your function name must start with a lower case letter, e.g. back. Back as a function name is a syntax error.

Also note that indentation is significant. The indentation of the do block in your question is wrong; the do blocks in my and melpomene's answers are correctly indented.

3
votes
back :: Int -> IO ()
back x = do line <- sequence [getLine | _ <- [1 .. x]]
            mapM_ putStrLn (reverse line)