Improve Template Haskell error recovery

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

Trac metadata

Trac field	Value
Version	6.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
CC	marco-oweber@gmx.de
Operating system	Unknown
Architecture	Unknown