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 -- Institute of Information Systems, Humboldt-Universitaet zu Berlin web: http://www.wiwi.hu-berlin.de/~fis/ e-mail: fis@wiwi.hu-berlin.de tel: +49 30 2093-5742 fax: +49 30 2093-5741 office: Spandauer Strasse 1, R.324, 10178 Berlin, Germany pgp: AD67 CF64 7BB4 3B9A 6F25 0996 4D73 F1FD 8D32 9BAA