MatchOpt is applicative!

matchOpt.hs

{-# LANGUAGE TypeOperators, RankNTypes, DeriveFunctor #-}

-- Response to chapter 9 of http://paolocapriotti.com/assets/applicative.pdf


import Control.Applicative
import Control.Monad (guard)
import Data.Functor.HFree
import Data.Functor.Product
import Data.Monoid (Any(..))
import Data.Maybe (listToMaybe)


----- Code different from the paper -----

type FreeA = HFree Applicative

one :: Option a -> FreeA Option a
one = liftFree

raise :: Applicative g => (f :~> g) -> FreeA f :~> g
raise = rightAdjunct

matchOpt :: String -> String -> FreeA Option a -> Maybe (FreeA Option a)
matchOpt opt value = any2Maybe . raise findMatch
  where
    any2Maybe (Pair (Const (Any b)) fa) = guard b >> Just fa
    findMatch g = case ('-':'-':optName g == opt, optReader g value) of
      (True, Just b) -> Pair (Const (Any True)) (pure b)
      _              -> Pair (Const (Any False)) (one g)


----- Code from the paper -----

data User = User
  { username :: String
  , fullname :: String
  , id :: Int
  } deriving Show

data Option a = Option
  { optName :: String
  , optDefault :: Maybe a
  , optReader :: String -> Maybe a 
  } deriving Functor

userP :: FreeA Option User
userP = User
  <$> one (Option "username" Nothing Just)
  <*> one (Option "fullname" (Just "") Just)
  <*> one (Option "id" Nothing maybeRead)

maybeRead :: Read a => String -> Maybe a
maybeRead = fmap fst . listToMaybe . filter (null . snd) . reads

parserDefault :: FreeA Option a -> Maybe a
parserDefault = raise optDefault

runParser :: FreeA Option a -> [String] -> Maybe a
runParser p (opt : value : args) = do
  p' <- matchOpt opt value p
  runParser p' args
runParser p [] = parserDefault p
runParser _ _ = Nothing