fail mzero and Parser

csv_test.hs

import Data.Csv
import Control.Monad
import Control.Applicative

-- 2014-07-24 15:26:22 fail and mzero

data Test = A | B | C
    deriving (Show, Read)

instance FromField Test where
    parseField "A" = pure A
    parseField "B" = pure B
    parseField "C" = pure C
    --parseField _   = fail "wrong input"
    parseField _   = mzero

tp :: Parser Test
tp = parseField "Z"

test :: Either String Test
test = runParser tp