[Haskell-cafe] [Parsec] Backtracking with try does not work for me?

[Matthias Fischmann]

On Mon, Jul 31, 2006 at 09:04:32AM +0200, Stephane Bortzmeyer wrote:
> 
> minilang = do
>        char 'a'
>        try (optional (do {comma ; char 'b'}))
>        optional (do {comma ; char 'c'})
>        eof
>        return "OK"
> 
> parse error at (line 1, column 2):
> unexpected "c"
> expecting "b"
> 
> Apparently, "try" was used (do note that the column number indicates
> that there was backtracking) but the parser still fails for
> "a,c". Why?

minilang = do
       char 'a'
       try b <|> (return '-')
       optional c
       eof
       return "OK"
  where
  b = do { comma ; char 'b' }
  c = do { comma ; char 'c' }


The (return 'x') is needed for type consistency.  The (try) combinator
doesn't spare you the error, it merely resets the cursor on the input
stream.  To catch the parse error, you need to name a throwaway
alternative.

cheers,
matthias



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