Lens.hs: Complete the easier part of this Lens exercise

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>

src/Lets/Lens.hs

@@ -111,8 +111,7 @@ fmapT ::
   (a -> b)
   -> t a
   -> t b
-fmapT =
-  error "todo: fmapT"
+fmapT f = getIdentity . traverse (Identity . f)
 
 -- | Let's refactor out the call to @traverse@ as an argument to @fmapT@.
 over ::
@@ -120,8 +119,7 @@ over ::
   -> (a -> b)
   -> s
   -> t
-over =
-  error "todo: over"
+over t f = getIdentity . t (Identity . f)
 
 -- | Here is @fmapT@ again, passing @traverse@ to @over@.
 fmapTAgain ::
@@ -129,8 +127,7 @@ fmapTAgain ::
   (a -> b)
   -> t a
   -> t b
-fmapTAgain =
-  error "todo: fmapTAgain"
+fmapTAgain = over traverse
 
 -- | Let's create a type-alias for this type of function.
 type Set s t a b =
@@ -143,22 +140,19 @@ type Set s t a b =
 sets ::
   ((a -> b) -> s -> t)
   -> Set s t a b
-sets =
-  error "todo: sets"
+sets f g = Identity . f (getIdentity . g)
 
 mapped ::
   Functor f =>
   Set (f a) (f b) a b
-mapped =
-  error "todo: mapped"
+mapped f g = Identity (getIdentity . f <$> g)
 
 set ::
   Set s t a b
   -> s
   -> b
   -> t
-set =
-  error "todo: set"
+set f s b = over f (const b) s
 
 ----
 
@@ -170,8 +164,7 @@ foldMapT ::
   (a -> b)
   -> t a
   -> b
-foldMapT =
-  error "todo: foldMapT"
+foldMapT f = getConst . traverse (Const . f)
 
 -- | Let's refactor out the call to @traverse@ as an argument to @foldMapT@.
 foldMapOf ::
@@ -179,8 +172,7 @@ foldMapOf ::
   -> (a -> r)
   -> s
   -> r
-foldMapOf =
-  error "todo: foldMapOf"
+foldMapOf t f = getConst . t (Const . f)
 
 -- | Here is @foldMapT@ again, passing @traverse@ to @foldMapOf@.
 foldMapTAgain ::
@@ -188,8 +180,7 @@ foldMapTAgain ::
   (a -> b)
   -> t a
   -> b
-foldMapTAgain =
-  error "todo: foldMapTAgain"
+foldMapTAgain = foldMapOf traverse
 
 -- | Let's create a type-alias for this type of function.
 type Fold s t a b =
@@ -206,14 +197,12 @@ folds ::
   -> (a -> Const b a)
   -> s
   -> Const t s
-folds =
-  error "todo: folds"
+folds f g s = let t = (f (getConst . g) s) in Const t
 
 folded ::
   Foldable f =>
   Fold (f a) (f a) a a
-folded =
-  error "todo: folded"
+folded = folds foldMap
 
 ----
 
@@ -227,8 +216,7 @@ get ::
   Get a s a
   -> s
   -> a
-get =
-  error "todo: get"
+get f = getConst . f Const
 
 ----
 
@@ -243,20 +231,20 @@ type Traversal s t a b =
 -- | Traverse both sides of a pair.
 both ::
   Traversal (a, a) (b, b) a b
-both =
-  error "todo: both"
+-- both :: (a -> f b) -> (a , a) -> f (b , b)
+both f (a, b) = (,) <$> f a <*> f b
 
 -- | Traverse the left side of @Either@.
 traverseLeft ::
   Traversal (Either a x) (Either b x) a b
-traverseLeft =
-  error "todo: traverseLeft"
+traverseLeft f (Left a) = Left <$> f a
+traverseLeft _ (Right x) = pure $ Right x
 
 -- | Traverse the right side of @Either@.
 traverseRight ::
   Traversal (Either x a) (Either x b) a b
-traverseRight =
-  error "todo: traverseRight"
+traverseRight _ (Left x) = pure $ Left x
+traverseRight f (Right b) = Right <$> f b
 
 type Traversal' a b =
   Traversal a a b b
@@ -286,8 +274,7 @@ type Prism s t a b =
 
 _Left ::
   Prism (Either a x) (Either b x) a b
-_Left =
-  error "todo: _Left"
+_Left = _
 
 _Right ::
   Prism (Either x a) (Either x b) a b