-- | Monoids with holes. The 'HoleyMonoid' allows building monoidal values of which certain components are to be filled in later. For example: -- -- > > let holey = now "x = " -- > . later show -- > . now ", y = " -- > . later show -- > > run holey 3 5 -- > "x = 3, y = 5" -- -- This module is intended to be imported in qualified fashion, e.g. -- -- > import qualified Data.HoleyMonoid as HM module Data.HoleyMonoid ( HoleyMonoid, run, now, later, map ) where import Prelude hiding (id, (.), map) import Control.Category import Data.Monoid -- | The type of a monoid with holes. The underlying monoid is represented by -- type parameter @m@. The @r@ is the result type and stays polymorphic until the -- very last moment when 'run' is called. The last argument @a@ is always a -- function with zero or more arguments, finally resulting in @r@. Ordering the -- arguments in this order allows holey monoids to be composed using `.`, stacking the -- expected arguments. Note that the `Monoid` constraint is only used in the -- identity 'HoleyMonoid' and in composing two 'HoleyMonoid's. newtype HoleyMonoid m r a = HoleyMonoid { runHM :: (m -> r) -> a } instance Monoid m => Category (HoleyMonoid m) where id = now mempty f . g = f `bind` \a -> g `bind` \b -> now (a `mappend` b) -- | Insert a constant monoidal value. now :: m -> HoleyMonoid m r r now a = HoleyMonoid ($ a) -- | Monadic indexed bind for holey monoids. bind :: HoleyMonoid m b c -> (m -> HoleyMonoid n a b) -> HoleyMonoid n a c m `bind` f = HoleyMonoid $ \k -> runHM m (\a -> runHM (f a) k) -- | Insert a monoidal value that is not specified until the computation is -- 'run'. The argument that is expected later is converted to the monoid type -- using the given conversion function. later :: (a -> m) -> HoleyMonoid m r (a -> r) later f = HoleyMonoid (. f) -- | Convert between underlying 'Monoid' types. map :: (m -> n) -> HoleyMonoid m r a -> HoleyMonoid n r a map g m = HoleyMonoid (\k -> runHM m (k . g)) -- | Run the computation, resulting in a function that still expects some -- arguments. The number of arguments that is still expected will be equal to the -- number of 'later's the computation is built of. run :: HoleyMonoid m m a -> a run m = runHM m id