[Haskell-cafe] help w/ improving custom Stream for parsec

[Malcolm Wallace]

I think what you really need is a two-pass parser.  The first parser consumes the input stream, and copies it to the output stream with files inserted, and macros expanded.  The second parser consumes the already-preprocessed stream, and does whatever you like with it.

Regards,
    Malcolm

On 7 Apr 2015, at 17:25, Maurizio Vitale wrote:

> 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))
> 
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