salvia-0.0.4: src/Network/Salvia/Handlers/Fallback.hs

module Network.Salvia.Handlers.Fallback (hOr, hEither) where

import Control.Applicative
import Control.Monad.State
import Network.Salvia.Httpd
import Network.Protocol.Http

{- |
Until I figure out how to do this correctly, this handler somewow implements
the MonadPlus/Alternative instance using a custom function. When the first
handler fails the response will be reset and the second handler is executed.
-}

hOr :: Handler a -> Handler a -> Handler a
hOr h0 h1 = do
  a  <- h0
  st <- gets (status . response)
  if statusFailure st
    then reset >> h1
    else return a

hEither :: Handler a -> Handler b -> Handler (Either a b)
hEither h0 h1 = (Left <$> h0) `hOr` (Right <$> h1)