f 0 = failWith "Postitive n required f n = [| \x -> x+n |] ....$(f 0).....

would elicit the error message

Foo.hs:8: Error in splice $(f 0): Positive n required

Is that what you want?

I'd like: 1) To be able to write the function that formats the error message. That is, the bit that adds in "Error in splice $(f 0):" Possible interface: --| Raise a TH exception -- First argument uses the splice string to produce -- an error string failWithEx :: (String -> Q String) -> Q a failWith msg = failWithEx (\ splice -> return ( "Error in splice " ++ splice ++ ":" ++ "\n" ++ msg) 2) To be able to catch errors resulting from compiling the spliced code. If we deliberately introduce an error: f n = [| \x -> x+'n' |] ....$(f 3).... we get this error message: Couldn't match `Int' against `Char' Expected type: Int Inferred type: Char In the second argument of `(+)', namely 'a' In a lambda abstraction: \ x'0 -> (x'0 + 'a') I'd like the option of replacing that message with: Foo.hs:8: Uncaught Template Greencard error while expanding '$(f 3)' Please report it as a bug The most obvious way to do this is to allow the exception to be caught. Possible interface: --| Though this has the type of a normal exception catching function, -- its semantics is (I think) a bit different because it catches -- exceptions triggered by splicing its first argument into the -- module and lets the handler offer up n alternative value to splice -- in. -- (For implementation reasons, it might be necessary to restrict -- its use to top-level splices? - I'm guessing that this is the only -- case where it's possible to unambiguously place the blame on -- the splice.) catchQ :: Q a -> (Exception -> Q a) -> Q a Using this, I'd rewrite f :: Int -> Q Exp f n = catchQ (\ ex -> failWithEx (\ splice -> return "Uncaught ...")) (f' n) f' 0 = failWith "Postitive n required f' n = [| \x -> x+n |] -- Alastair Reid www.haskell-consulting.com