如何在haskell的字符串列表中传递字符串

Question

How would I get this piece of code to accept a list of strings and output a frame around the outside. I understand the concept but just cannot execute the code in the final frame function.

minusdots :: Int -> String
minusdots 1    = "-."
minusdots n
   | n > 1     = "-." ++ (minusdots (n-1))
   | otherwise = error "please enter greater than 1"


bar :: Int -> String
bar n
    | even n    = minusdots (div n 2)
    | otherwise = (minusdots (div n 2)) ++ ['-']


 frame :: [String] -> IO String
 frame text = map putStrLn (bar m) ++ "\n" ++ textshown ++ "\n" ++ (bar m)
    where
    textshown = "- " ++ text ++ " -"
    m         = length textshown

I have worked on this all day and come up with this but there's still some bugs I need to work out 1. When I pass the border string into the frameM function, If I was to pass say "SS" is there any way I could make the S'S frame on top of each other,istead of side by side so the more letters i put into the first argument the bigger the total perimeter of the frame gets? heres what I've done:

minusdots ::  Int -> String -> String
minusdots 1  a =  a
minusdots  n a
   | n > 1     =  a ++ (minusdots (n-1) a)
   | otherwise = error "argument not addmissible"


bar :: String -> Int -> String
bar s n
   | even n    = minusdots (div n 2) s 
   | otherwise = (minusdots (div n 2) s) ++ s


frameM :: String -> String -> String
frameM a text = (bar a m) ++ "\n" ++ textshown ++ "\n" ++ (bar a m)  
    where
    textshown = b ++ text ++ b
    m         = length textshown
    b         = a

Answer 1

I believe the type of your frame should be frame :: String -> IO () — it takes a string a puts a "framed" version of it to stdout . Then you don't need map putStrLn and can just use putStrLn .

Now, consider this line:

putStrLn (bar m) ++ "\n" ++ textshown ++ "\n" ++ (bar m)

you are calling putStrLn (bar m) and then trying to append some stuff to the result of that (hint: use parentheses or $ ).