Haskell Processing text from a file - string

Question

Hi Guys,
1. What do I want to do?
I get a 1-lined file with text
"Bangabang [Just 3, Nothing, Just 1, Nothing] [Nothing, Nothing, Nothing, Nothing] [Nothing, Nothing, Just 4, Nothing] [Nothing, Just 3, Nothing, Nothing]"
I want to read this text from a file and convert it to:
[[Just 3, Nothing, Just 1, Nothing], [Nothing, Nothing, Nothing, Nothing], [Nothing, Nothing, Just 4, Nothing], [Nothing, Just 3, Nothing, Nothing]]
Which is a [[Maybe Integer]] type.
2. What have I already done?
I can modify normal String to Maybe Integer
My String:
xxx = "Bangabang [Just 3, Nothing, Just 1, Nothing] [Nothing, Nothing, Nothing, Nothing] [Nothing, Nothing, Just 4, Nothing] [Nothing, Just 3, Nothing, Nothing]"
after executing stripChars ",]" $ drop 10 xxx I get:
"Just 31 Nothing Just 1 Nothing [Nothing Nothing Nothing Nothing [Nothing Nothing Just 4 Nothing [Nothing Just 3 Nothing Nothing"
after next commands map (splitOn " ") $ splitOn "[" I have:
[["Just","31","Nothing","Just","1","Nothing",""],["Nothing","Nothing","Nothing","Nothing",""],["Nothing","Nothing","Just","4","Nothing",""],["Nothing","Just","3","Nothing","Nothing"]]
Now I have to cut off that empty strings "" using cleany
And finally change [[String]] to [[Maybe Integer]] using cuty
[[Just 31,Nothing,Just 1,Nothing],[Nothing,Nothing,Nothing,Nothing],[Nothing,Nothing,Just 4,Nothing],[Nothing,Just 3,Nothing,Nothing]]
That is what I wanted to have!
3. The problem is...
...how can I execute this method:
parse xxx = cuty $ cleany $ map (splitOn " ") $ splitOn "[" $ stripChars ",]" $ drop 10 xxx
on text read from file (which is IO String type)?
This is my first Haskell project, so my functions may reinvent the wheel or do worse things :/
Used functions:
main do
text <- readFile "test.txt"
let l = lines
map parse . l
-- deletes unwanted characters from a String
stripChars :: String -> String -> String
stripChars = filter . flip notElem
-- converts String to Maybe a
maybeRead :: Read a => String -> Maybe a
maybeRead s = case reads s of
[(x,"")] -> Just x
_ -> Nothing
-- convert(with subfunction conv, because I don't know how to make it one function)
conv:: [String] -> [Maybe Integer]
conv[] = []
conv(x:xs) = if x == "Just" then conv xs
else maybeRead x: conv xs
convert:: [[String]] -> [[Maybe Integer]]
convert[] = []
convert(x:xs) = conv x : convert xs
-- cleany (with subfunction clean, because I don't know how to make it one function)
clean :: [String] -> [String]
clean [] = []
clean (x:xs) = if x == "" then clean xs
else x : clean xs
cleany :: [[String]] -> [[String]]
cleany [] = []
cleany (x:xs) = clean x : cleany xs

Answer 1

I'll assume you're ok with a parser that does zero to minimal error checking. Haskell has great libraries for parsing, and later I'll amend my answer with some alternatives you should look at.
Instead of using splitOn I would recommend writing these functions:
takeList :: String -> (String, String)
-- returns the match text and the text following the match
-- e.g. takeList " [1,2,3] ..." returns ("[1,2,3]", " ...")
takeLists :: String -> [String]
-- parses a sequence of lists separated by spaces
-- into a list of matches
I'll leave takeList as an exercise. I like to use span and break from Data.List for these kinds of simple parsers.
In terms of takeList, here is how you might write takeLists:
takeLists :: String -> [ String ]
takeLists str =
let s1 = dropWhile (/= '[') str
in if null s1
then []
else let (s2,s3) = takeList s1
in s2 : takeLists s3
For example, takeLists " [123] [4,5,6] [7,8] " will return:
[ "[123]", "[4,5,6]", "[7,8]" ]
Finally, to convert each string in this list to Haskell values, just use read.
answer :: [ [Int] ]
answer = map read (takeLists " [123] [4,5,6] [7,8] ")
Update
Using the ReadP and ReadS parsers available in the base libraries:
import Text.ParserCombinators.ReadP
bang :: ReadP [[Maybe Int]]
bang = do string "Bangabang"
skipSpaces
xs <- sepBy1 (readS_to_P reads) skipSpaces
eof
return xs
input = "Bangabang [Just 3, Nothing, Just 1, Nothing] [Nothing, Nothing, Nothing, Nothing] [Nothing, Nothing, Just 4, Nothing] [Nothing, Just 3, Nothing, Nothing]"
runParser p input = case (readP_to_S p) input of
[] -> error "no parses"
((a,_):_) -> print a
example = runParser bang input

Answer 2

You can use directly Read instance.
data Bangabang = Bangabang [Maybe Integer]
[Maybe Integer]
[Maybe Integer]
[Maybe Integer] deriving (Read, Show)
now, you can use all Read machinery (read, reads, readIO, ...), inferred from types. E.g.
readBangabang :: String -> Bangabang
readBangabang = read
If data came from file
readFile "foo.txt" >>= print . readBangabang