### Array

The elements in an array must have the same type.

`type Array a`
Representation of fast immutable arrays. You can create arrays of integers (Array Int) or strings (Array String) or any other type of value you can dream up.

`empty : Array a`
Return an empty array.

`repeat : Int -> a -> Array a`
Creates an array with a given length, filled with a default element.

```repeat 5 0     == fromList [0,0,0,0,0]
repeat 3 "cat" == fromList ["cat","cat","cat"]
```

Notice that repeat 3 x is the same as initialize 3 (always x).

`initialize : Int -> (Int -> a) -> Array a`
initialize n f creates an array of length n with the element at index i initialized to the result of (f i).

```initialize 4 identity    == fromList [0,1,2,3]
initialize 4 (\n -> n*n) == fromList [0,1,4,9]
initialize 4 (always 0)  == fromList [0,0,0,0]
```

`fromList : List a -> Array a`

`isEmpty : Array a -> Bool`

`length : Array a -> Int`

`push : a -> Array a -> Array a`
Push an element to the end of an array.

push 3 (fromList [1,2]) == fromList [1,2,3] `append : Array a -> Array a -> Array a`
Append two arrays to a new one.

append (repeat 2 42) (repeat 3 81) == fromList [42,42,81,81,81]

`get : Int -> Array a -> Maybe a`
Return Just the element at the index or Nothing if the index is out of range.

```get  0 (fromList [0,5,3]) == Just 0
get  2 (fromList [0,5,3]) == Just 3
get  5 (fromList [0,5,3]) == Nothing
get -1 (fromList [0,5,3]) == Nothing
```

`set : Int -> a -> Array a -> Array a`
Set the element at a particular index. Returns an updated array. If the index is out of range, the array is unaltered.

`slice : Int -> Int -> Array a -> Array a`
Get a sub-section of an array: (slice start end array). The start is a zero-based index where we will start our slice. The end is a zero-based index that indicates the end of the slice. The slice extracts up to but not including end.

```slice  0  3 (fromList [0,1,2,3,4]) == fromList [0,1,2]
slice  1  4 (fromList [0,1,2,3,4]) == fromList [1,2,3]
```

Both the start and end indexes can be negative, indicating an offset from the end of the array.

```slice  1 -1 (fromList [0,1,2,3,4]) == fromList [1,2,3]
slice -2  5 (fromList [0,1,2,3,4]) == fromList [3,4]
```

This makes it pretty easy to pop the last element off of an array: slice 0 -1 array

`toList : Array a -> List a`
Create a list of elements from an array.

toList (fromList [3,5,8]) == [3,5,8] `toIndexedList : Array a -> List (Int, a)`
Create an indexed list from an array. Each element of the array will be paired with its index.

toIndexedList (fromList ["cat","dog"]) == [(0,"cat"), (1,"dog")]

`map : (a -> b) -> Array a -> Array b`
Apply a function on every element in an array.

map sqrt (fromList [1,4,9]) == fromList [1,2,3]

`indexedMap : (Int -> a -> b) -> Array a -> Array b`
Apply a function on every element with its index as first argument.

indexedMap (*) (fromList [5,5,5]) == fromList [0,5,10]

`filter : (a -> Bool) -> Array a -> Array a`
Keep only elements that satisfy the predicate:

filter isEven (fromList [1,2,3,4,5,6]) == (fromList [2,4,6])

`foldl : (a -> b -> b) -> b -> Array a -> b`
Reduce an array from the left. Read foldl as “fold from the left”.

foldl (::) [] (fromList [1,2,3]) == [3,2,1]

`foldr : (a -> b -> b) -> b -> Array a -> b`
Reduce an array from the right. Read foldr as “fold from the right”.

foldr (+) 0 (repeat 3 5) == 15