Foldable & Traversable for Id

Ignore-this: 8a7ae889667ea7a6bb8f8b71c338160b

darcs-hash:20110226002430-fb517-806574abb4fe192984bfd754b080f44220e1557c.gz

src/Control/Compose.hs

@@ -583,7 +583,7 @@ instance (Applicative f, Monoid m) => Monoid (App f m) where
 
 
 {----------------------------------------------------------
-    Identity -- TODO: eliminate in favor of Data.Traversable.Id
+    Identity
 ----------------------------------------------------------}
 
 -- | Identity type constructor.  Until there's a better place to find it.
@@ -618,6 +618,16 @@ instance Applicative Id where
   pure  = Id
   (<*>) = inId2 ($)
 
+instance Foldable Id where
+  foldMap f (Id a) = f a
+
+instance Traversable Id where
+  sequenceA (Id fa) = fmap Id fa
+
+-- Id fa :: Id (f a)
+-- fa :: f a
+-- fmap Id fa = f (Id a)
+
 {----------------------------------------------------------
     Unary constructor pairing
 ----------------------------------------------------------}