on the way to a poor man's coerce...

src/Coerce.hs

@@ -1,37 +1,43 @@
 {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
 {-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
 module Coerce where
+import Data.Monoid (Sum (..), getSum)
 import Data.Functor.Product             -- Product of Functors
 import Data.Functor.Compose             -- Composition of Functors
 import Data.Functor.Const               -- Const Functor
 import Control.Applicative              -- WrappedMonad
 import Control.Monad.State.Lazy         -- State Monad
+import Data.Typeable
   
 -- | This module provides explicit coercion.
 --   Just in case you want to know what's behind the "magic" Data.Coerce.coerce 
 class Coerce a b | a -> b where
-    down :: a -> b
+    unwrap :: a -> b
-    up   :: b -> a
+    wrap   :: b -> a
 
 instance Coerce (Const a b) a where
-    down = getConst
+    unwrap = getConst
-    up   = Const
+    wrap   = Const
+
+instance Coerce (Sum a) a where
+    unwrap = getSum
+    wrap   = Sum
 
 instance (Coerce(m a) b, Coerce(n a) c) => Coerce((Product m n) a) (b,c) where
-    down mnx = (down (pfst mnx), down(psnd mnx)) where
+    unwrap mnx = (unwrap (pfst mnx), unwrap(psnd mnx)) where
         pfst (Pair fst _) = fst
         psnd (Pair _ snd) = snd
-    up (x,y) = Pair (up x) (up y)
+    wrap (x,y) = Pair (wrap x) (wrap y)
 
 instance (Functor m, Functor n, Coerce(m b)c, Coerce(n a)b) => Coerce((Compose m n) a) c where
-    down = down . fmap down . getCompose
+    unwrap = unwrap . fmap unwrap . getCompose
-    up   = Compose . fmap up . up    
+    wrap   = Compose . fmap wrap . wrap    
 
 instance Coerce (m a) c => Coerce (WrappedMonad m a) c where
-    down = down . unwrapMonad
+    unwrap = unwrap . unwrapMonad
-    up   = WrapMonad . up
+    wrap   = WrapMonad . wrap
 
 instance Coerce (State s a) (s -> (a,s)) where
-    down = runState
+    unwrap = runState
-    up   = state
+    wrap   = state