Dear all, I don't understand why the type of pBD defined in the where clause of pFG cannot be inferred to a general type in the presence of TypeFamilies. In particular I don't understand why nonetheless the type of pBD definied in the where clause of pF (only slightly simpler) *can* be inferred. Can anyone explain? Thanks Tom {-# LANGUAGE TypeFamilies #-} -- This code came up in the context of writing a parser, but that's -- not terribly important import Prelude hiding ((<$>)) data B = B data D f = F f | GF AP f | DF AM f data AM = AM data AP = AP pB :: Parser B pB = Parser -- Works fine pF :: Parser (D B) pF = pBD GF AP <|> pBD DF AM where pBD f p = f p <$> pB -- Shows the (presumably) inferred type for pBD pFWithType :: Parser (D B) pFWithType = pBD GF AP <|> pBD DF AM where pBD :: (t -> B -> b) -> t -> Parser b pBD f p = f p <$> pB -- One would hope this type would be inferred too pFGWithType :: Parser B -> Parser (D B) pFGWithType pBArg = pBD GF AP <|> pBD DF AM where pBD :: (t -> B -> b) -> t -> Parser b pBD f p = f p <$> pBArg -- But omitting it leads to a type error if TypeFamilies is enabled. -- There is no error if TypeFamilies is not enabled. pFG :: Parser B -> Parser (D B) pFG pBArg = pBD GF AP <|> pBD DF AM where pBD f p = f p <$> pBArg -- The specifics of the parser don't matter data Parser a = Parser (<|>) :: Parser a -> Parser a -> Parser a (<|>) _ _ = Parser (<$>) :: (a -> b) -> Parser a -> Parser b (<$>) _ _ = Parser