On 18/10/16 09:29, Nickolay Kudasov wrote:

Hi Ruben,

I imagine, free applicative would allow you to easily insert whitespace/comment eaters afterwards. For instance, say you have Parser applicative for parsing. Then Ap Parser would represent the same parser, but with parsing combinators separated with Ap constructors. You would use Ap Parser when defining your grammar. Then you could "intersperse" whitespace eaters in between the combinators and "retract" the resulting Ap Parser into just Parser. That would probably be a cleaner approach compared to having every combinator wrapped in trimWhiteSpacesAndComments combinator.

I got the idea of what you said. Even if I want to intersperse `space` in the downgrade I end up with stuff on the wrong order. Probably has to do with the fact that Parsec already has a Applicative instance which I am using, instead of just Functor. import Text.Parsec import Control.Applicative hiding (many) import Control.Applicative.Free {- prints: Left (line 1, column 1): unexpected "h" expecting space -} main :: IO () main = print $ example2 -- Works example :: Either ParseError String example = parse query "" "hi number 5" where query = many letter *> space *> many letter *> space *> many digit -- Works in wrong order example2 :: Either ParseError String example2 = parse (down query) "" "hi number 5" where query = liftAp (many letter) *> liftAp (many letter) *> liftAp (many digit) down :: Ap (Parsec String u0) a -> Parsec String u0 a down (Pure a) = pure a down (Ap fa ap) = down ap <* space <*> fa {- to help understanding instance Applicative (Ap f) where pure = Pure Pure f <*> y = fmap f y Ap x y <*> z = Ap x (flip <$> y <*> z) Ap x y :: (Ap f (a -> b)) z :: (Ap f a) x :: (f c) y :: (Ap f (c -> a -> b)) flip <$> y <*> z :: Ap f (c -> b) flip <$> y :: Ap f (a -> c -> b) liftAp space :: Ap Parser Char liftAp space = Ap space (Pure id) liftAp letter :: Ap Parser Char liftAp letter = Ap letter (Pure id) liftAp space *> liftAp letter :: Ap Parser Char = (id <$ (Ap space (Pure id))) <*> Ap letter (Pure id) = Ap space (Pure (const id)) <*> Ap letter (Pure id) = Ap space (flip <$> (Pure (const id)) <*> Ap letter (Pure id)) = Ap space ( Pure (\a _ -> a)) <*> Ap letter (Pure id) ) = Ap space ( Ap letter (Pure (const id)) ) -} -- -- Ruben