Add array.at function (#48)

* Add array.at function

* Fix wording

Co-authored-by: Glenn Slotte <glenn@slotte.net>

* Update changelog

---------

Co-authored-by: Glenn Slotte <glenn@slotte.net>

Author: dkirchhof

CHANGELOG.md

@@ -8,6 +8,7 @@
 - Change `Set.add` to not return self, to indicate that it's mutable. https://github.com/rescript-association/rescript-core/pull/35
 - Change `Iterator` bindings to have the same shape as `AsyncIterator` for consistency. https://github.com/rescript-association/rescript-core/pull/34
 - Add `Iterator.toArray` binding for turning an iterator into an array. https://github.com/rescript-association/rescript-core/pull/34
+- Add `Array.at` binding for returning an array item by its index. https://github.com/rescript-association/rescript-core/pull/48
 
 ### Documentation
 

src/Core__Array.res

@@ -196,3 +196,5 @@ let filterMap = (a, f) => filterMapU(a, (. a) => f(a))
 
 // TODO: Change this implementation?
 let flatMap = (a, f) => []->concatMany(map(a, f))
+
+@send external at: (array<'a>, int) => option<'a> = "at"

src/Core__Array.resi

@@ -103,3 +103,21 @@ let filterMap: (array<'a>, 'a => option<'b>) => array<'b>
 let shuffle: array<'a> => array<'a>
 let shuffleInPlace: array<'a> => unit
 let flatMap: (array<'a>, 'a => array<'b>) => array<'b>
+
+
+/** 
+  `at(array, index)`
+
+  Get an element by its index. Negative indices count backwards from the last item.
+
+  ## Examples
+  ```rescript
+  ["a", "b", "c"]->Array.at(0) // Some("a")
+  ["a", "b", "c"]->Array.at(2) // Some("c")
+  ["a", "b", "c"]->Array.at(3) // None
+  ["a", "b", "c"]->Array.at(-1) // Some("c")
+  ["a", "b", "c"]->Array.at(-3) // Some("a")
+  ["a", "b", "c"]->Array.at(-4) // None
+  ```
+*/
+@send external at: (array<'a>, int) => option<'a> = "at"