Use case for Free Applicatives

gergoerdi.hs

{-# LANGUAGE RankNTypes #-}

-- See http://gergo.erdi.hu/blog/2012-12-01-static_analysis_with_applicatives/

import Control.Applicative
import Data.Functor.HFree

data MarketData a
data MarketKey
request :: MarketData a -> MarketKey
request = undefined

data Market
getMarketData :: Market -> MarketData a -> a
getMarketData = undefined

type P a = HFree Applicative MarketData a

eval :: Applicative g => (forall b. f b -> g b) -> HFree Applicative f a -> g a
eval = rightAdjunct

get :: MarketData a -> P a
get = liftFree

dependencies :: P a -> [MarketKey]
dependencies = getConst . eval (Const . (:[]) . request)

runP :: (Applicative m, Monad m) => ([MarketKey] -> m Market) -> P a -> m a
runP loadMarket pricer = do
  let deps = dependencies pricer
  mkt <- loadMarket deps
  return $ iter (getMarketData mkt) pricer