Hello Arie, Monday, August 21, 2006, 7:08:31 PM, you wrote:
getInfo :: Q Info getInfo = reify (mkName "Car")
-- Crashes if I try to print out the info -- info <- runQ getInfo
I seem to remember that 'reify' cannot be run in the IO monad.
oh, yes! runQ is just a way to test simple things, it's not a full-featured Q monad emulator. reify should be called at compile-time to get access to identifiers table and it crashes just because no exception handler was installed this is the appropriate part of TH library: class Monad m => Quasi m where qNewName :: String -> m Name qReport :: Bool -> String -> m () qRecover :: m a -> m a -> m a qReify :: Name -> m Info qCurrentModule :: m String qRunIO :: IO a -> m a Quasi class covers all operations of Q monad. It has two implementations: instance Quasi Q where qNewName = newName qReport = report qRecover = recover qReify = reify qCurrentModule = currentModule qRunIO = runIO this instance work for code generated at compile-time by $(...) splices and this instance used when you run computations in Q monad inside your program, at run-time, using runQ operation: instance Quasi IO where qNewName s = do { n <- readIORef counter ; writeIORef counter (n+1) ; return (mkNameU s n) } qReport True msg = hPutStrLn stderr ("Template Haskell error: " ++ msg) qReport False msg = hPutStrLn stderr ("Template Haskell error: " ++ msg) qReify v = badIO "reify" qCurrentModule = badIO "currentModule" qRecover a b = badIO "recover" -- Maybe we could fix this? qRunIO m = m badIO :: String -> IO a badIO op = do { qReport True ("Can't do `" ++ op ++ "' in the IO monad") ; fail "Template Haskell failure" } as you can see, it don't supports reification, recovery and information about currently compiled module just because there is no such information when program runs. and of course, you can't add new fields or new functions at run-time. isntead typical technique is: $(transform [d| data D = X {f::Int, g::String} |] ) where 'transform' parses declaration passed and generates something like this data D = X {unique::Int, f::Int, g::String} of course, you can also use reification, it's a matter of taste -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com