Haskell application debugging

Question

After learning a few basics, I wanted to try the “real world application” in Haskell, started with the Bittorrent client. Following the explanation of this post, I did NOT use the Attoparsec Parser Compiler Library . Instead, through the Huttons book , I started writing Parser Combiners. This is the code that I still have (still at the parsing stage, a long way to go):

module Main where

import System.Environment (getArgs)
import qualified Data.Map as Map
import Control.Monad (liftM, ap)
import Data.Char (isDigit, isAlpha, isAlphaNum, ord)
import Data.List(foldl')

main :: IO ()
main = do
    [fileName] <- getArgs
    contents <- readFile fileName
    download . parse $ contents

parse :: String -> Maybe BenValue
parse s = case runParser value s of
    []       -> Nothing
    [(p, _)] -> Just p

download :: Maybe BenValue -> IO ()
download (Just p) = print p
download _        = print "Oh!! Man!!"

data BenValue = BenString String
    | BenNumber Integer
    | BenList [BenValue]
    | BenDict (Map.Map String BenValue)
    deriving(Show, Eq)

-- From Hutton, this follows: a Parser is a function 
-- that takes a string and returns a list of results
-- each containing a pair : a result of type a and 
-- an output string. (the string is the unconsumed part of the input).
newtype Parser a = Parser (String -> [(a, String)])

-- Unit takes a value and returns a Parser (a function)
unit :: a -> Parser a
unit v = Parser (\inp -> [(v, inp)])

failure :: Parser a
failure = Parser (\inp -> [])

one :: Parser Char
one = Parser $ \inp -> case inp of
        []      -> []
        (x: xs) -> [(x, xs)]

runParser :: Parser a -> String -> [(a, String)]
runParser (Parser p) inp = p inp

bind :: Parser a -> (a -> Parser b) -> Parser b
bind (Parser p) f = Parser $ \inp -> case p inp of
    []         -> []
    [(v, out)] -> runParser (f v) out

instance Monad Parser where
    return  = unit
    p >>= f = bind p f

instance Applicative Parser where
    pure  = unit
    (<*>) = ap

instance Functor Parser where
    fmap = liftM

choice :: Parser a -> Parser a -> Parser a
choice p q = Parser $ \inp -> case runParser p inp of
    [] -> runParser q inp
    x  -> x

satisfies :: (Char -> Bool) -> Parser Char
satisfies p = do
    x <- one
    if p x 
    then unit x
    else failure

digit :: Parser Char
digit = satisfies isDigit

letter :: Parser Char
letter = satisfies isAlpha

alphanum :: Parser Char
alphanum = satisfies isAlphaNum

char :: Char -> Parser Char
char x = satisfies (== x)

many :: Parser a -> Parser [a]
many p = choice (many1 p) (unit [])

many1 :: Parser a -> Parser [a]
many1 p = do
    v <- p
    vs <- many p
    unit (v:vs)

peek :: Parser Char
peek = Parser $ \inp -> case inp of
    []       -> []
    v@(x:xs) -> [(x, v)]    

taken :: Int -> Parser [Char]
taken n = do
    if n > 0
    then do
        v <- one
        vs <- taken (n-1)
        unit (v:vs)
    else unit []

takeWhile1 :: (Char -> Bool) -> Parser [Char]
takeWhile1 pred = do
    v <- peek
    if pred v
    then do
        one
        vs <- takeWhile1 pred
        unit (v:vs)
    else unit []

decimal :: Integral a => Parser a
decimal = foldl' step 0 `fmap` takeWhile1 isDigit
    where step a c = a * 10 + fromIntegral (ord c - 48)

string :: Parser BenValue
string = do
    n <- decimal
    char ':'
    BenString <$> taken n

signed :: Num a => Parser a -> Parser a
signed p = (negate <$> (char '-' *> p) )
    `choice` (char '+' *> p)
    `choice` p

number :: Parser BenValue
number = BenNumber <$> (char 'i' *> (signed decimal) <* char 'e')

list :: Parser BenValue
list = BenList <$> (char 'l' *> (many value) <* char 'e')

dict :: Parser BenValue
dict = do
    char 'd'
    pair <- many ((,) <$> string <*> value)
    char 'e'
    let pair' = (\(BenString s, v) -> (s,v)) <$> pair
    let map' = Map.fromList pair'
    unit $ BenDict map'

value = string `choice` number `choice` list `choice` dict

, / , . download " ", . , download .

• torrent.:( , , , . , - .
• "", , combinatorrent
• , Debian/Ubuntu .., .
• , , GHCI , :trace/:history , document, :-).
• : " !!":-)
• , .

.

Answer 1

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, GHCi, , , , , - , , , , . , .

- . , Nothing - , . , , . attoparsec. , attoparsec ByteString - . , .

, , , . ByteString - - . OverloadedStrings ByteString, , .