How can I parse fixed-length, non-delimited integers with attoparsec? - parsing

Question

I'm trying to parse two integers from 3 characters using attoparsec. A sample input might look something like this:
341
... which I would like to parse into:
Constructor 34 1
I have two solutions that work but which are somewhat clunky:
stdK :: P.Parser Packet
stdK = do
P.char '1'
qstr <- P.take 2
let q = rExt $ P.parseOnly P.decimal qstr
n <- P.decimal
return $ Std q n
stdK2 :: P.Parser Packet
stdK2 = do
P.char '1'
qn <- P.decimal
let q = div qn 10
let n = rem qn 10
return $ Std q n
There must be a better way to achieve something as simple as this. Am I missing something?

Answer 1

Your code snippet is far from being self-contained (in particular, imports and the definition of your Packet data type are missing), but you seem to be overcomplicating things.
First, define a parser for one-digit integers. Then, use the latter as a building block for a parser for two-digit integers. After that, use applicative operators to combine those two parsers and define a parser for your custom Packet data type. See below.
Note that you don't need the full power of monads; applicative parsing is sufficient, here.
-- test_attoparsec.hs
{-# LANGUAGE OverloadedStrings #-}
import Control.Applicative ((<$>))
import Data.Attoparsec.Text
import Data.Char
data Packet = Std {-# UNPACK #-} !Int
{-# UNPACK #-} !Int
deriving (Show)
stdK :: Parser Packet
stdK = char '1' *> (Std <$> twoDigitInt <*> oneDigitInt)
twoDigitInt :: Parser Int
twoDigitInt = timesTenPlus <$> oneDigitInt <*> oneDigitInt
where
timesTenPlus x y = 10 * x + y
oneDigitInt :: Parser Int
oneDigitInt = digitToInt <$> digit
Tests in GHCi:
λ> :l test_attoparsec.hs
[1 of 1] Compiling Main ( test_attoparsec.hs, interpreted )
Ok, modules loaded: Main.
λ> :set -XOverloadedStrings
λ> parseOnly stdK "1341"
Right (Std 34 1)
λ> parseOnly stdK "212"
Left "1: Failed reading: satisfyWith"