Parsing simple molecule names with Attoparsec - parsing

Question

I find it extremely difficult to learn how to use Attoparsec, because the documentation is really just an API documentation and there are basically no tutorials around (except the one from FPComplete). If you know other places where I can learn Attoparsec, that'd be great.
I have to parse simple molecule names, in the following format: NaCl, CO2, H2O, HCN, H2O2.
An element name is an uppercase letter optionally followed by a lowercase one (I'm not considering those elements with a symbol longer than 2 characters).
An element can be followed by a number (that would be the subscript in a formula).
New version (thanks to Mark's and Tarmil's suggestions), which compiles but does not parse:
module Chem
where
import Data.Text (Text, pack)
import Control.Applicative ((<*>), (<$>))
import Data.Attoparsec.Text
data Element = Element String Int deriving (Eq, Ord, Show)
type Molecule = [Element]
parseString :: String -> Result Molecule
parseString = parse (many' parseElement) . pack
parseElement :: Parser Element
parseElement = do
el <- (++) <$> pClass "A-Z" <*> option "" (pClass "a-z")
n <- option 1 decimal
return $ Element el n
pClass :: String -> Parser String
pClass cls = (\c -> [c]) <$> satisfy (inClass cls)
Any suggestion is appreciated.
EDIT: I managed to get it running. Basically, a Partial continuation was returned, and to finish the parsing it's necessary to feed the parser with an empty bytestring. So the correct parseString would be:
parseString = flip feed empty . parse (many' parseElement) . pack
where empty is Data.Text.empty. However, since I don't need incremental parsing there is the useful function parseOnly, which does not wait for more input and returns an Either.
With that in mind, I rewrote the code like this (it works now):
module Chem
where
import Data.Text (Text, pack)
import Control.Applicative ((<*>), (<$>))
import Data.Attoparsec.Text
data Element = Element String Int deriving (Eq, Ord, Show)
type Molecule = [Element]
parseString :: String -> Either String Molecule
parseString = parseOnly (many' parseElement) . pack
parseElement :: Parser Element
parseElement = do
el <- (++) <$> pClass "A-Z" <*> option "" (pClass "a-z")
n <- option 1 decimal
return $ Element el n
pClass :: String -> Parser String
pClass cls = (\c -> [c]) <$> satisfy (inClass cls)

Answer 1

You have two problems in the letters parsing part:
inClass is not a parser, it is a function that is meant to be passed to satisfy.
<*> has type Parser (a -> b) -> Parser a -> Parser b, so the parser on the left should return a function. Typically, it is used like this:
pf <$> p1 <*> p2 <*> ... <*> pn
where pf is a function with n arguments.
So here you probably want something like this:
-- parse a character in the given class, and transform it to a single-char string
pClass cls = (\c -> [c]) <$> satisfy (inClass cls)
-- ...
el <- ((++) <$> pClass "A-Z" <*> pClass "a-z") <|> pClass "A-Z"
-- ...
I think this would be enhanced by using option, instead of duplicating the A-Z parser:
el <- (++) <$> pClass "A-Z" <*> option "" (pClass "a-z")