Review Partial Application before proceeding.

In Haskell, a function that can take other functions as arguments or return functions is called a *higher-order function*.

The following are all *higher-order functions*:

```
map :: (a -> b) -> [a] -> [b]
filter :: (a -> Bool) -> [a] -> [a]
takeWhile :: (a -> Bool) -> [a] -> [a]
dropWhile :: (a -> Bool) -> [a] -> [a]
iterate :: (a -> a) -> a -> [a]
zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
scanr :: (a -> b -> b) -> b -> [a] -> [b]
scanl :: (b -> a -> b) -> b -> [a] -> [b]
```

These are particularly useful in that they allow us to create new functions on top of the ones we already have, by passing functions as arguments to other functions. Hence the name, *higher-order functions*.

Consider:

```
Prelude> :t (map (+3))
(map (+3)) :: Num b => [b] -> [b]
Prelude> :t (map (=='c'))
(map (=='c')) :: [Char] -> [Bool]
Prelude> :t (map zipWith)
(map zipWith) :: [a -> b -> c] -> [[a] -> [b] -> [c]]
```

This ability to easily create functions (like e.g. by partial application as used here) is one of the features that makes functional programming particularly powerful and allows us to derive short, elegant solutions that would otherwise take dozens of lines in other languages. For example, the following function gives us the number of aligned elements in two lists.

```
aligned :: [a] -> [a] -> Int
aligned xs ys = length (filter id (zipWith (==) xs ys))
```

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