Getting [Char] instead of IO() String - string

Question

I'm a new to Haskell. I'm reading an input string (a) and want to return a string when I find a character (e) inside. Now my whole source code:
a = "b,b,b,b/b,b,b/b,b,b,b/e,e,e/w,w,w,w/w,w,w/w,w,w,w"
n = length a
simpleCase n =
case n of
'a' -> "hey cutie"
eLoopCase i =
if i < n
then do
let char = a !! i
case char of
'e' -> putStr $ "(" ++ "found an e" ++ "),"
'w' -> return ()
'b' -> return ()
',' -> eLoopCase (i+1)
'/' -> eLoopCase (i+1)
if (char == ',' || char == '/') == False
then eLoopCase (i+1)
else return ()
else return ()
simpleCase gives me back a string but eLoopCase gives me an IO() back. It works, however I'd like for eLoopCase to give an String back so I can work on it.
:t simpleCase
simpleCase :: Char -> [Char]
:t eLoopCase
eLoopCase :: Int -> IO ()
I understand that this has something to do with monads,then do and putStr, this is where my understanding ends. Removing do gives a parse error.

Answer 1

eLoopCase is returning nothing because you have return () at all the "ends" of it. I think you're after something like this (Notice appending the current character to the result of the recursive call in the x == 'w' || x == 'b' branch):
a :: String
a = "b,b,b,b/b,b,b/b,b,b,b/e,e,e/w,w,w,w/w,w,w/w,w,w,w"
eLoopCase :: String -> IO String
eLoopCase [] = return []
eLoopCase (x:xs)
| x == 'e' = do
putStrLn "(found an e)"
return [x]
| x == ',' || x == '/' =
eLoopCase xs
| x == 'w' || x == 'b' = do
rest <- eLoopCase xs
return (x : rest)
| otherwise = do
putStrLn ("Encountered invalid character: " ++ show x)
return []
General problems with your code:
The program you wrote is very imperative in style. This is something I would urge you to try to move away from while writing Haskell.
Prefer pattern matching on lists over indexing with !!.
I should note that the function I provided has flaws of its own. It presupposes that the strings provided to it will only consist of a couple different chars. A way to improve it would be to add an otherwise branch to the guards. (Edit: Added to the snippet above)
I think it's also worth pointing out that this function really need not depend on IO at all to work. Look into other "pure" alternatives for doing error handling/reporting such as Either, Maybe, and Writer.

Answer 2

I'm glad you've decided to try to learn Haskell. You can do a lot better than eLoopCase. I'll tell you how, but first I'll explain the problem you were having in your original question and how you fixed it.
As originally written, eLoopCase is has a type Int -> IO (), meaning it is a function which takes an Int and returns an input output action. IOW, the type tells you that if you give it a number it will do something. If you look at the code you can see that that something is just printing out strings, but it could have been almost anything.
In your answer, you rewrote eLoopCase to construct strings directly via the ++ operator, instead of printing them out to the terminal via putStr. This is the reason your rewritten function has the type that it does. It also explains why you no longer need the return () statements, which are the "Do nothing while returning a () value" IO actions.
I hope that clears things up a little. That said, eLoopCase can be tremendously improved.
Haskell programs are efficiently written when the control logic matches the data structure. For instance, this program is iterating through a list. A list is defined as the data which is either an empty list or an element and more list, as seen in the declaration below.
data List a = []
| a : List a
Consequently, programs which iterate through a list will be based on decisions for those two constructors. For example (using the synonym String for [Char]), eLoopCase can be rewritten as
eLoopCase' :: String -> String
eLoopCase' [] = ""
eLoopCase' (a:as) = evalCase a ++ eLoopCase' as
evalCase a = case a of
'e' -> "(" ++ "found an e" ++ "),"
'w' -> ""
'b' -> ""
',' -> ""
'/' -> ""
_ -> ""
Notice eLoopCase' needs to be fed a as an input as it is no longer hard coded into the body of the function--a good thing. It also does away with the index i and errors arising from using !! (try calling eLoopCase (-1)).
Having practice in writing recursive functions such as eLoopCase' is a good start. An evolution in programming is to see what you are intending to do with that loop and applying appropriate patterns.
For instance, since you want to evaluate every element in a list by a certain function, use a map. Specifically map evalCase to go from a list of characters to your list of strings, then use concat to append all those lists together:
eLoopCase'' = concat . map evalCase

Answer 3

Actually I've answered my own question. The following seems to work:
a = "b,b,b/b,b,b/b,b,b,b/e,e,e/w,w,w,w/w,w,w/w,w,w,w"
n = length a
eLoopCase i =
if i < n
then
case a !! i of
'e' -> "(" ++ "found an e" ++ ")," ++ eLoopCase (i+1)
'w' -> "" ++ eLoopCase (i+1)
'b' -> "" ++ eLoopCase (i+1)
',' -> eLoopCase (i+1)
'/' -> eLoopCase (i+1)
else ""
I'm still curious as to what happened.