Marc Weber wants better error recovery in TH's Q monad: http://www.haskell.org/pipermail/template-haskell/2008-May/000666.html

The Quasi monad is defined thus:

class (Monad m, Functor m) => Quasi m where
	-- Fresh names
  qNewName :: String -> m Name

	-- Error reporting and recovery
  qReport  :: Bool -> String -> m ()	-- Report an error (True) or warning (False)
					-- ...but carry on; use 'fail' to stop
  qRecover :: m a -> m a -> m a		-- Recover from the monadic 'fail'
					-- The first arg is the error handler
 
	-- Inspect the type-checker's environment
  qReify :: Name -> m Info
  qLocation :: m Loc

	-- Input/output (dangerous)
  qRunIO :: IO a -> m a

A sensible change (but it would be a change) would be to change qRecover thus:

  qRecover :: m a -> ([(Bool,String)] -> m a) -> m a
  -- (qRecover q f) runs the action `q`, collecting all the messages
  -- emitted by `qReport`.  If the action `q` calls the monad `fail`,
  -- these messages are passed to `f`.  If the action `q` does not call
  -- `fail`, the messages are retained in the monad, just as if the 
  -- `qRecover` was not there.

Comments? This would be quite easy to implement in 6.10.

Simon