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+Day 5
+- Homework from day 4
+- Higher order programming
+  - Anonymous functions
+  
+```λ> let f = \x y -> x `div` y```
+  - Function composition
+  
+```haskell
+λ> let isEvenLengthTree t = even (length (inOrder t))
+λ> let compose f g = \x -> f (g x)
+λ> :t compose
+compose :: (b -> c) -> (a -> b) -> a -> c
+λ> let isEvenLengthTree t = (even `compose` length `compose` inOrder) t
+λ> :t isEvenLengthTree
+isEvenLengthTree :: BST -> Bool
+λ> :t (.)
+(.) :: (b -> c) -> (a -> b) -> a -> c
+λ> let isEvenLengthTree t = (even . length . inOrder) t
+λ> :t ($)
+($) :: (a -> b) -> a -> b
+λ> length $ [1..10]
+10
+λ> :i ($)
+($) :: (a -> b) -> a -> b 	-- Defined in ‘GHC.Base’
+infixr 0 $
+λ> let isEvenLengthTree t = even . length . inOrder $ t
+λ> :t isEvenLengthTree
+isEvenLengthTree :: BST -> Bool
+```
+  - Currying
+  - Partial application
+```haskell
+λ> let t = fromList [1..10]
+λ> :t insert t
+insert t :: Int -> BST
+λ> let insertIntoT = insert t
+λ> t
+Node (Node (Node (Node (Node (Node (Node (Node (Node (Node EmptyNode 1 EmptyNode) 2 EmptyNode) 3 EmptyNode) 4 EmptyNode) 5 EmptyNode) 6 EmptyNode) 7 EmptyNode) 8 EmptyNode) 9 EmptyNode) 10 EmptyNode
+λ> insertIntoT 30
+Node (Node (Node (Node (Node (Node (Node (Node (Node (Node EmptyNode 1 EmptyNode) 2 EmptyNode) 3 EmptyNode) 4 EmptyNode) 5 EmptyNode) 6 EmptyNode) 7 EmptyNode) 8 EmptyNode) 9 EmptyNode) 10 (Node EmptyNode 30 EmptyNode)
+λ> insertIntoT (-30)
+Node (Node (Node (Node (Node (Node (Node (Node (Node (Node (Node EmptyNode (-30) EmptyNode) 1 EmptyNode) 2 EmptyNode) 3 EmptyNode) 4 EmptyNode) 5 EmptyNode) 6 EmptyNode) 7 EmptyNode) 8 EmptyNode) 9 EmptyNode) 10 EmptyNode
+λ> :t insertIntoT
+insertIntoT :: Int -> BST
+λ> :t uncurry
+uncurry :: (a -> b -> c) -> (a, b) -> c
+λ> :t uncurry insert
+uncurry insert :: (BST, Int) -> BST
+λ> :t curry . uncurry $ insert
+curry . uncurry $ insert :: BST -> Int -> BST
+λ> (find 99 (insert 4 (fromList [1..10])))
+False
+λ> find 99 . insert 4 . fromList $ [1..10]
+False
+λ> find 99 $ insert 4 $ fromList $ [1..10]
+False
+λ> let f = find 99 . insert 4 . fromList
+λ> f [1..10]
+False
+λ> :t flip
+flip :: (a -> b -> c) -> b -> a -> c
+λ> :t find
+find :: Int -> BST -> Bool
+λ> :t flip find
+flip find :: BST -> Int -> Bool
+λ> :m +Data.Function
+λ> :t (&)
+(&) :: a -> (a -> b) -> b
+λ> [1..10] & fromList & insert 4 & find 99
+False
+λ> let f |> g = \x -> g . f $ x
+λ> let f = fromList |> insert 4 |> find 99
+λ> :t (|>)
+(|>) :: (r -> b) -> (b -> c) -> r -> c
+λ>  [1..1] & fromList |> insert 4 |> find 99
+False
+λ> fromList |> insert 4 |> find 99 $ [1..10]
+False
+λ> fromList |> (find 99 . insert 4) $ [1..10]
+False
+```
+  
+Homework: write these functions
+- map :: (a -> b) -> [a] -> [b]
+- filter :: (a -> Bool) -> [a] -> [a]
+- fold :: (r -> a -> r) -> r -> [a] -> r