Sometimes the representation of a value as a request parameter will be different from the representation required by Read. We can use checkRq to lift a custom parsing function into RqData.
#ifdef HsColour
> checkRq :: (Monad m, HasRqData m) => m a -> (a -> Either String b) -> m b
#endif
In this example we create a type Vote with a custom parsing function:
> module Main where
>
> import Control.Applicative ((<$>), (<*>))
> import Happstack.Server (ServerPart, badRequest, nullConf, ok, simpleHTTP)
> import Happstack.Server.RqData (RqData, checkRq, getDataFn, look, lookRead)
>
> data Vote = Yay | Nay deriving (Eq, Ord, Read, Show, Enum, Bounded)
>
> parseVote :: String -> Either String Vote
> parseVote "yay" = Right Yay
> parseVote "nay" = Right Nay
> parseVote str = Left $ "Expecting 'yay' or 'nay' but got: " ++ str
>
> votePart :: ServerPart String
> votePart =
> do r <- getDataFn (look "vote" `checkRq` parseVote)
> case r of
> (Left e) ->
> badRequest $ unlines e
> (Right i) ->
> ok $ "You voted: " ++ show i
>
> main :: IO ()
> main = simpleHTTP nullConf $ votePart
[Source code for the app is here.]
Now if we visit http://localhost:8000/?vote=yay, we will get the message:
You voted: Yay
If we visit http://localhost:8000/?vote=yes, we will get the error:
Expecting 'yay' or 'nay' but got: yes