digitToInt, wrong datatypes and Unknown Exceptions - haskell

Question

I earlier posted almost the same code where I created a lot of errors with lines I didn't even need. previous post
Now I took away the type signatures and my problems were reduced to 3.
And maybe now I might be able to explain what my problems are.
What ghci shows:
C:\Users\MATZE\Desktop\star.hs:8:25:
Couldn't match expected type `IO b0' with actual type `Int'
In the return type of a call of `digitToInt'
In a stmt of a 'do' block: temp <- digitToInt n
In the expression:
do { temp <- digitToInt n;
if (((c < 0) && (temp < 6) && (temp > 0))
|| (bs !! c >= temp)) then
do { return temp }
else
do { putStrLn "Ung\252ltige Eingabe";
.... } }
C:\Users\MATZE\Desktop\star.hs:20:66:
Couldn't match expected type `Int' with actual type `[Char]'
In the second argument of `(!!)', namely `l'
In the first argument of `replicate', namely `(bs !! l)'
In the second argument of `($)', namely
`replicate (bs !! l) (" *")'
C:\Users\MATZE\Desktop\star.hs:36:17:
Couldn't match type `IO' with `[]'
Expected type: [()]
Actual type: IO ()
In the return type of a call of `putStrLn'
In a stmt of a 'do' block:
putStrLn ("#Spieler " ++ show (pl - 1) ++ "hat gewonnen!")
In the expression:
do { putStrLn ("#Spieler " ++ show (pl - 1) ++ "hat gewonnen!") }
When I try to load this:
import Data.Char
board = [5,4,3,2,1]
--getMove :: [Int] -> Int -> IO Int
getMove bs c = do
n <- getChar
if isDigit n then do
temp <- digitToInt n
if (((c<0) && (temp<6) && (temp > 0))|| (bs!!c >= temp)) then do
return temp
else do
putStrLn "Ungültige Eingabe"
getMove bs c
else do
putStrLn "Ungültige Eingabe"
getMove bs c
--drawBoard :: [Int] -> Int -> String -> IO String
drawBoard bs l = do
putStrLn (show ((l + 1)++":" ++ (concat $ replicate (bs!!l) (" *") )))
if l < 3 then do
drawBoard bs (l + 1)
else do
putStrLn "\n"
--mainloop :: [Int] -> Int -> IO()
mainloop bs pl = do
line <- (getMove bs (-1))-1
number <- getMove bs line
b1s <- take line bs ++ [ ((bs!!line) - number) ] ++ drop (line + 1) bs
drawBoard b1s 0
if ((sum b1s) /= 0) then do
mainloop b1s ((pl `mod` 2) + 1)
else do
putStrLn ("#Spieler "++ show(pl -1)++"hat gewonnen!")
--main :: IO()
main = do
mainloop board 1
Why is there a problem with the digitToInt ?
Why does ghci think "l" is a [char] ?
And what does the last Exception even mean.

Answer 1

The arrow in do notation binds the results of a monadic computation, so
do ...
temp <- digitToInt n
...
expects digitToInt n to have a monadic type, IO a in your case. Instead, the way to bind pure values is with let:
do ...
let temp = digitToInt n
...
The second error has to do with the type of l, which apparently is inferred to be String (a.k.a. [Char]). First of all, I recommend that you uncomment your type signature, to make sure that everything has the type that you think it does. In this case the signature for drawBoard should have one fewer argument,
drawBoard :: [Int] -> Int -> IO ()
The actual reason for the error is the mismatched parentheses show ((l + 1)++":"++..., here you concatenate (l + 1) with a string. You probably meant to write show (l + 1)++":"++....
The third error is similar to the first, you should write
let b1s = take line (bs ++ [ ((bs!!line) - number) ] ++ drop (line + 1) bs)
instead of using an arrow.
Finally you are trying to use ...-1 to subtract from a monadic computation that returns a number. You first need to bind the result of the computation, and then subtract 1. Or you can use fmap/liftM to do this.
As a stylistic suggestion: all of your code lives in the IO monad, while much of it doesn't have to. For example, you can draw a board to a String, and then print it in one go.