#2340: Improve Template Haskell error recovery
--------------------------------+-------------------------------------------
Reporter: simonpj | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.2
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
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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
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2340>
GHC <http://www.haskell.org/ghc/>
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