A parser of simple typed sums

parserkm.hs

import Text.ParserCombinators.Parsec
import Text.Parsec.Numbers

data Value = Kilometer Double | Mile Double

value = try valKm <|> valMile

valKm, valMile :: GenParser Char st Value

valKm = do 
    spaces
    num <- parseFloat
    char 'k'
    char 'm'
    return $ Kilometer num

valMile = do
    spaces
    num <- parseFloat
    char 'm'
    char 'l'
    return $ Mile num

expression = do
    result <- value `sepBy` (char '+')
    eof
    return result

toKm (Kilometer a) = a
toKm (Mile a) = a * 1.619

eval expr = foldl evaler 0 expr 
    where evaler accum value = accum + (toKm value)

parseE input = parse expression "(unknown)" input

main = do
    l <- getLine
    let e = parseE l
    case e of
        Right exp -> print $ eval exp
        _ -> print "Parse Error"