Recipes.hs

module Control.Applicative.Recipes where
-- base
import Control.Applicative
import Data.Foldable
import Data.Traversable
import Data.Monoid
-- containers
import Data.Set

data Recipe k v m a = Recipe
  { ingredients :: Set k
  , cook :: (k -> m v) -> m a
  }

instance Functor m => Functor (Recipe k v m) where
  fmap f (Recipe ks g) = Recipe ks (fmap f . g)

instance (Applicative m, Ord k) => Applicative (Recipe k v m) where
  pure v = Recipe Data.Set.empty (const $ pure v)
  Recipe ks f <*> Recipe ls g = Recipe (ks <> ls) (\x -> f x <*> g x)

using :: k -> Recipe k v m v
using k = Recipe (singleton k) ($ k)