I figured it out, but it's not pretty. The problem is that the eof parser had no awareness of the showTok function. To fix the problem, I had to replace eof with its definition in terms of notFollowedBy, then replace notFollowedBy with its definition in terms of try and unexpected. Then, I changed the "show [c]" into "showToken c". Passing a token shower to the token function isn't a very robust way of guaranteeing your tokens display properly in error messages, because the other combinators don't take the same option. Of course, you can implement a Show instance for your tokens as you like. But, if you make the Show instance show the pretty version for the user, you lose the ability to see the real structure you get from a derived Show instance. In my real code, I want debugging to show tokens using a derived Show instance, so I can see all the structure. But when I show them to the user, I don't want them to see the embedded SourcePos, or the constructor names - I just want them to see a representation of what was lexed in order to produce that token. I think there should be a class called Token, with a method called showToken, or unlex, or display, or displayInError, something like that. This class should be a precondition of all the GenParser combinators. It should use the provided method to show the token in error messages. Here's the working version: import Text.ParserCombinators.Parsec import Text.ParserCombinators.Parsec.Pos(newPos) showToken (pos,t) = "<" ++ show t ++ ">" myToken :: (Eq t, Show t) => (t -> Bool) -> GenParser (SourcePos,t) () (SourcePos,t) myToken q = token showToken posFromTok testTok where posFromTok (pos,t) = pos testTok (pos,t) = if (q t) then Just (pos,t) else Nothing main = do putStrLn "" case parse the123Parser "" [(newPos "" 1 n, n) | n <- [1,2,3,4]] of (Left err) -> putStrLn (show err) (Right _) -> putStrLn "parsed correctly" putStrLn "" case parse the123Parser "" [(newPos "" 1 n, n) | n <- [1,3,4]] of (Left err) -> putStrLn (show err) (Right _) -> putStrLn "parsed correctly" the123Parser = do myToken (==1) myToken (==2) myToken (==3) try (do{ c <- myToken (const True); unexpected (showToken c) } <|> return ()) notFollowedBy (myToken (==4)) return 123 - Lyle