On 31 October 2010 16:53, Nils Schweinsberg wrote:

Am 31.10.2010 17:27, schrieb Stephen Tetley:

...

Left factoring! :-)

Stupid question: Whats that? :)

Actually a good question... Its a standard grammar transformation - if you have two alternative productions that share a common prefix A -> a b | a c Left factoring produces: A -> A_part (b | c) A_part -> a Left factoring can quickly lead to the grammar getting quite convoluted, but using 'try' can lead to very fragile parsers that stop working in mysterious ways when you extend them. Because parser combinators are just functions, you can do questionable hacks to make left factoring a bit less tedious: parseA :: Parser A parseA = do a <- a_part (parseB a <|> parseC a) parseB :: a -> Parser A parseB a = do b <- b_part return $ B a b parseC :: b -> Parser A parseC a = do c <- c_part return $ C a c