finished exercises on Course.Compose

README.markdown

@@ -27,8 +27,8 @@ After this, the following progression of modules is recommended:
 * `Course.State` -> Done
 * `Course.StateT` -> Done
 * `Course.Extend` -> Done
-* `Course.Comonad`
-* `Course.Compose`
+* `Course.Comonad` -> Done
+* `Course.Compose` -> Done
 * `Course.Traversable`
 * `Course.ListZipper`
 * `Course.Parser` *(see also `Course.Person` for the parsing rules)*

src/Course/Compose.hs

@@ -16,20 +16,19 @@ newtype Compose f g a =
 -- Implement a Functor instance for Compose
 instance (Functor f, Functor g) =>
     Functor (Compose f g) where
-  (<$>) =
-    error "todo: Course.Compose (<$>)#instance (Compose f g)"
+  (<$>) func (Compose futor) = Compose( (func <$>) <$> futor)
 
 instance (Applicative f, Applicative g) =>
   Applicative (Compose f g) where
 -- Implement the pure function for an Applicative instance for Compose
-  pure =
-    error "todo: Course.Compose pure#instance (Compose f g)"
+  pure a = Compose (pure (pure a))
 -- Implement the (<*>) function for an Applicative instance for Compose
-  (<*>) =
-    error "todo: Course.Compose (<*>)#instance (Compose f g)"
+  (<*>) (Compose cfa) (Compose fa) = Compose z
+    where
+      z = ((<*>) <$> cfa) <*> fa
 
 instance (Monad f, Monad g) =>
   Monad (Compose f g) where
 -- Implement the (=<<) function for a Monad instance for Compose
   (=<<) =
-    error "todo: Course.Compose (<<=)#instance (Compose f g)"
+    error "This is the exercise which cannot be solved"