Aeson parsing and Snap

SnapAeson.hs

import Data.Traversable (for)

import Data.Aeson (Value, json)
import Data.AttoParsec (parseOnly)
import Snap.Core

getPostJsonParam :: MonadSnap m => ByteString -> m (Maybe Value)
getPostJsonParam key = do
    bsOpt <- getPostParam key
    for bsOpt $ \bs ->
        case parseOnly json bs of
            Left e  -> fail e -- Of course, use a better way to handle that kind of error
            Right v -> return v