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))