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.

[stepcut]

crud. I've often wished for a PVP verification tool. I'm working on the more grotesque approach of using hydra to build all the things from git and against multiple versions of GHC. But, at the moment this is the only package in my hydra config. I will take the steps needed to bring this back in compliance with the PVP, though it will likely take a couple days.