Add Check List Membership as an elixir til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_781 TILs and counting..._
+_782 TILs and counting..._
 
 ---
 
@@ -123,6 +123,7 @@ _781 TILs and counting..._
 - [Assert An Exception Is Raised](elixir/assert-an-exception-is-raised.md)
 - [Binary Representation Of A String](elixir/binary-representation-of-a-string.md)
 - [Check For A Substring Match](elixir/check-for-a-substring-match.md)
+- [Check List Membership](elixir/check-list-membership.md)
 - [Comparing DateTime Structs](elixir/comparing-datetime-structs.md)
 - [Compute md5 Digest Of A String](elixir/compute-md5-digest-of-a-string.md)
 - [Counting Records With Ecto](elixir/counting-records-with-ecto.md)

elixir/check-list-membership.md

@@ -0,0 +1,21 @@
+# Check List Membership
+
+You can use the [`in` operator](https://hexdocs.pm/elixir/operators.html) to
+check if something appears in a list. This is a handy way of checking if a
+variable is one of a few acceptable or expected values.
+
+For instance, a common DateTime comparison pattern relies on this to check
+if a DateTime is `>=` or `<=` to another DateTime.
+
+```elixir
+{:ok, datetime} = DateTime.from_naive(~N[2016-05-24 13:26:08.003], "Etc/UTC")
+
+DateTime.compare(datetime, DateTime.utc_now()) in [:lt, :eq]
+```
+
+Alternatively, you can check that something does not have membership in a
+list by also including the `not` operator.
+
+```elixir
+DateTime.compare(datetime, DateTime.utc_now()) not in [:lt, :eq]
+```