Stephane Bortzmeyer wrote:
minilang = do char 'a' try (optional (do {comma ; char 'b'})) optional (do {comma ; char 'c'}) eof return "OK"
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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?
Because 'try' can only help you if its argument fails. If the argument to 'try' succeeds, then it behaves as if it wasn't there. Now 'optional x' always succeeds, so the 'try' is useless where you placed it. You need to 'try' the argument to 'optional':
minilang = do char 'a' optional (try (do {comma ; char 'b'})) optional (do {comma ; char 'c'}) eof return "OK"
You could also factor your grammar or use ReadP, where backtracking is not an issue. Udo. -- Ours is a world where people don't know what they want and are willing to go through hell to get it. -- Don Marquis