Attoparsec - ensure entire contents consumed with sepBy1

Question

I would like the below code to return [LoadInt 1,LoadDub 2.5,LoadInt 3], but it fails after parsing [LoadInt 1,LoadDub 2] and facing .5,3. How do I make it so it must parse all the way to the comma for a parse to succeed, and an int parse on 2.5 is a fail?

import qualified Data.Attoparsec.ByteString.Char8 as A
import Data.Attoparsec.ByteString.Char8 (Parser)
import Data.ByteString.Char8 (pack)
import Data.Attoparsec.Combinator
import Control.Applicative ((*>),(<$>),(<|>))
data LoadNum = LoadInt Int | LoadDub Double deriving (Show)

someFunc :: IO ()
someFunc = putStrLn . show $ A.parseOnly (lnParser <* A.endOfInput) (pack testString)


testString :: String
testString = "1,2.5,3"

lnParser :: Parser [LoadNum]
lnParser = (sepBy1' (ld <* A.atEnd) (A.char ','))

double :: Parser Double
double = A.double

int :: Parser Int
int = A.signed A.decimal

ld :: Parser LoadNum
ld = ((LoadInt <$> int ) <|> (LoadDub <$> double))

Answer 1

You could use a tiny bit of lookahead to decide whether you reached the end of a list element. So:

int :: Parser Int
int = do
    i <- A.signed A.decimal
    next <- A.peekChar
    case next of
        Nothing -> pure i
        Just ',' -> pure i
        _ -> fail "nah"