Release 7.4.3 introduces breaking API changes

[snoyberg]

Pinging @gbaz, as this was discovered in jmacro-rpc-happstack breakage. Version 7.4.3 of happstack-server introduces the following API breakage at a few places:

-instance (Monad m) => Monoid (ServerPartT m a) where
+instance (Monad m, MonadPlus m) => Monoid (ServerPartT m a) where

This leads to downstream packages no longer compiling, e.g.:

Network/JMacroRPC/Happstack.hs:101:55:
    Could not deduce (Control.Monad.MonadPlus m)
      arising from a use of ‘mappend’
    from the context (Monad m, Functor m, MonadIO m)
      bound by the type signature for
                 panelToPage :: (Monad m, Functor m, MonadIO m) =>
                                Maybe String
                                -> String -> Panel (ServerPartT m) -> ServerPartT m Response
      at Network/JMacroRPC/Happstack.hs:100:16-123
    Possible fix:
      add (Control.Monad.MonadPlus m) to the context of
        the type signature for
          panelToPage :: (Monad m, Functor m, MonadIO m) =>
                         Maybe String
                         -> String -> Panel (ServerPartT m) -> ServerPartT m Response
    In the expression: dir "jrpcs" updateHandler `mappend` drawHandler
    In an equation for ‘panelToPage’:
        panelToPage jqLoc title p
          = dir "jrpcs" updateHandler `mappend` drawHandler
          where
              (updateHandler, drawHandler)
                = panelToPageGen
                    (serveRpcs $ const (return ()))
                    (return . toResponse . H.preEscapedToHtml)
                    jqLoc
                    title
                    p

This change requires a major version bump. I'll need to temporarily block version 7.4.3 from Stackage until either the problem is resolved or downstream packages update to require the MonadPlus constraint as well.