# :: [a] -> Int -> Maybe a

Safer version of !!, returns a Maybe. Get element from list using index value starting from `0`.
```>>> [] !!? 0
Nothing
```
```>>> ["a", "b", "c"] !!? 3
Nothing
```
```>>> [1, 2, 3] !!? (-1)
Nothing
```
```>>> ["a", "b", "c"] !!? 2
Just "c"
```
Lookup function (partially safe). O(min n index).
O(1) Safe indexing
A (!!) that fails using mzero
Assumed complexity: O(1) Yield the element at the given position in a monad. No range checks are performed. The monad allows us to be strict in the vector if we want. Suppose we had
```unsafeIndex :: v a -> Int -> a
```
instead. Now, if we wanted to copy a vector, we'd do something like
```copy mv v ... = ... unsafeWrite mv i (unsafeIndex v i) ...
```
For lazy vectors, the indexing would not be evaluated which means that we would retain a reference to the original vector in each element we write. This is not what we want! With basicUnsafeIndexM, we can do
```copy mv v ... = ... case basicUnsafeIndexM v i of
Box x -> unsafeWrite mv i x ...
```
which does not have this problem because indexing (but not the returned element!) is evaluated immediately.
O(1) Indexing in a monad. The monad allows operations to be strict in the vector when necessary. Suppose vector copying is implemented like this:
```copy mv v = ... write mv i (v ! i) ...
```
For lazy vectors, v ! i would not be evaluated which means that mv would unnecessarily retain a reference to v in each element written. With indexM, copying can be implemented like this instead:
```copy mv v = ... do
x <- indexM v i
write mv i x
```
Here, no references to v are retained because indexing (but not the elements) is evaluated eagerly.
O(1) Indexing in a monad without bounds checks. See indexM for an explanation of why this is useful.
!!? with its arguments flipped. Get element from list using index value starting from `0`.
```>>> maybeAt 0 []
Nothing
```
```>>> maybeAt 3 ["a", "b", "c"]
Nothing
```
```>>> maybeAt (-1) [1, 2, 3]
Nothing
```
```>>> maybeAt 2 ["a", "b", "c"]
Just "c"
```
Safe version of !!
Safer version of !! -
O(n) Convert the first n elements of a list to a vector
```fromListN n xs = fromList (take n xs)
```

#### Examples

```>>> import qualified Data.Vector as V

>>> V.fromListN 3 [1,2,3,4,5::Int]
[1,2,3]

>>> V.fromListN 3 [1::Int]

```
O(n) Convert the first n elements of a list to a vector
```fromListN n xs = fromList (take n xs)
```