Add Append To A Keyword List as an elixir til

Add a new Elixir section too

Author: jbranchaud

README.md

@@ -7,14 +7,15 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
-_329 TILs and counting..._
+_330 TILs and counting..._
 
 ---
 
 ### Categories
 
 * [Clojure](#clojure)
 * [Devops](#devops)
+* [Elixir](#elixir)
 * [Git](#git)
 * [Go](#go)
 * [JavaScript](#javascript)
@@ -62,6 +63,10 @@ _329 TILs and counting..._
 - [Running Out Of inode Space](devops/running-out-of-inode-space.md)
 - [Wipe A Heroku Postgres Database](devops/wipe-a-heroku-postgres-database.md)
 
+### Elixir
+
+- [Append To A Keyword List](elixir/append-to-a-keyword-list.md)
+
 ### Git
 
 - [Accessing a Lost Commit](git/accessing-a-lost-commit.md)

elixir/append-to-a-keyword-list.md

@@ -0,0 +1,42 @@
+# Append To A Keyword List
+
+If you have two keyword lists, you can append them like so:
+
+```elixir
+> a = [a: 1]
+[a: 1]
+> b = [b: 2]
+[b: 2]
+> a ++ b
+[a: 1, b: 2]
+```
+
+But what if something a bit more programmatic is happening and you are
+building up the additions to the keyword list based on variables?
+
+```elixir
+> x = :x
+:x
+> c = a ++ [x 5]
+** (CompileError) iex:5: undefined function x/1
+    (stdlib) lists.erl:1353: :lists.mapfoldl/3
+    (stdlib) lists.erl:1354: :lists.mapfoldl/3
+```
+
+That makes elixir think `x` is some function when in fact it is just a
+variable containing the keyword `:x`.
+
+Simply adding a comma doesn't quite do it either.
+
+```elixir
+> c = a ++ [x, 5]
+[{:a, 1}, :x, 5]
+```
+
+We need to wrap the internal part with curly braces to create the tuple that
+can then be appended to `a`.
+
+```elixir
+> c = a ++ [{x, 5}]
+[a: 1, x: 5]
+```