Monad, probs

src/GoodList.hs

@@ -22,9 +22,7 @@ instance Applicative GoodList where
 -- Needs to be proven
 instance Monad GoodList where
   return = pure
-  Empty          >>= _ = Empty
-  Singleton x    >>= f = f x
-  Multiple a b c >>= f = append (append (f a) (b >>= f)) (f c)
+  x >>= f = join $ fmap f x
 
 instance Semigroup (GoodList a) where
   (<>) = append
@@ -60,3 +58,8 @@ append (Singleton x)     ys              = cons x ys
 append  xs               Empty           = xs
 append (Multiple a b c) (Singleton y)    = Multiple a (snoc b c) y
 append (Multiple a b c) (Multiple d e f) = Multiple a (snoc b c `append` cons d e) f
+
+join :: GoodList (GoodList a) -> GoodList a
+join Empty = Empty
+join (Singleton x) = x
+join (Multiple a b c) = append (a `append` join b) c