I need a custom stream that supports insertion of include files and expansions of macros. I also want to be able to give nice error messages (think of clang macro-expansion backtrace), so I cannot use the standard trick of concatenating included files and expanded macros to the current input with setInput/getInput (I think I can't maybe there's a way of keeping a more complex "position" and since the use in producing an error backtrac is rare, it migth be worth exploring; if anybody has ideas here, I'm listening) Assuming I need a more compelx stream, this is what I have (Macro and File both have a string argument, but it will be more compicated, a list of expansions for Macro for instance). Is there a better way for doing this? What are the performance implications with backtracking? I'll be benchmarking it, but if people see obvious problems, let me know. Thanks a lot, Maurizio {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE InstanceSigs #-} {-# LANGUAGE MultiParamTypeClasses #-} module Parsing where import Text.Parsec type Parser s m = ParsecT s () m data VStream = File String | Macro String deriving Show newtype StreamStack = StreamStack [VStream] deriving Show instance (Monad m) ⇒ Stream VStream m Char where uncons ∷ VStream -> m (Maybe (Char, VStream)) uncons (File (a:as)) = return $ Just (a, File as) uncons (File []) = return Nothing uncons (Macro (a:as)) = return $ Just (a, File as) uncons (Macro []) = return Nothing instance (Monad m) => Stream StreamStack m Char where uncons (StreamStack []) = return Nothing uncons (StreamStack (s:ss)) = case uncons s of Nothing → uncons $ StreamStack ss Just Nothing → uncons $ StreamStack ss Just (Just (c, File s')) → return $ Just (c, StreamStack (File s': ss)) Just (Just (c, Macro s')) → return $ Just (c, StreamStack (Macro s':ss))