On Monday 08 August 2005 09:43, Simon Peyton-Jones wrote: | | > Are you saying, that the second error causes the first one? Thus, | | > everything is caused by 'printer' being recursively defined? | | | | Yes, that's what I believe. I'm not sure why it shows the first | | error message at all; in my experience, strange things happen when | | ghc continues after an error message that you shouldn't pay too | | much attention to. Perhaps ghc-6.4 just assumes identical contexts | | in recursive groups without checking the type signatures. | | After an error in a function definition for f, GHC carries on | assuming the function has type | | f :: forall a. a | | That very seldom leads to subsequent errors, but maybe it did this | time. The way to find out is to show the program that elicits this | message but not the other. As Thomas correctly guessed, this happens if the type signatures of both 'printer' and 'printCatalog': \begin{code} class Catalog c where traverse :: c -> Viewer -> IO () instance Catalog Int where traverse i v = viewShowable v i type View a = a -> IO () data Viewer = Viewer { viewShowable :: forall s. Show s => View s, viewCatalog :: forall c. Catalog c => View c } --printer :: Viewer --printer = Viewer { -- viewCatalog = \x -> traverse x printer, -- viewShowable = putStrLn . show } printer = Viewer { viewCatalog = printCatalog, viewShowable = putStrLn . show } --printCatalog :: forall c. Catalog c => View c printCatalog x = traverse x printer data X = X { cat :: Int } instance Catalog X where traverse x v = do viewCatalog v (cat x) main = do let x = X { cat = 20 } traverse x printer \end{code} Ben