Simplify instance declarations

Many instance declarations can now be derived either by DerivingVia or
GeneralizedNewtypeDeriving.

XMonad/Layout/Mosaic.hs

@@ -1,4 +1,8 @@
-{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, PatternGuards #-}
+{-# LANGUAGE DeriveFoldable #-}
+{-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE PatternGuards #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  XMonad.Layout.Mosaic
@@ -179,16 +183,7 @@ normalize :: Fractional a => [a] -> [a]
 normalize x = let s = sum x in map (/s) x
 
 data Tree a = Branch (Tree a) (Tree a) | Leaf a | Empty
-
-instance Foldable Tree where
-   foldMap _f Empty = mempty
-   foldMap f (Leaf x) = f x
-   foldMap f (Branch l r) = foldMap f l `mappend` foldMap f r
-
-instance Functor Tree where
-   fmap f (Leaf x) = Leaf $ f x
-   fmap f (Branch l r) = Branch (fmap f l) (fmap f r)
-   fmap _ Empty = Empty
+  deriving (Functor, Show, Foldable)
 
 instance Semigroup (Tree a) where
     Empty <> x = x