Add Assert An Exception Is Raised as an elixir til

elixir/assert-an-exception-is-raised.md

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+# Assert An Exception Is Raised
+
+Elixir's [`ExUnit`](http://elixir-lang.org/docs.html) comes with a number of
+different ways to make assertions in your tests. One of those functions is
+[`assert_raise`](http://elixir-lang.org/docs/stable/ex_unit/ExUnit.Assertions.html#assert_raise/2)
+which allows you to test that a particular exception is raised when the
+given function is invoked.
+
+Using `assert_raise/2` looks something like this:
+
+```elixir
+assert_raise FunctionClauseError, fn ->
+  Enum.chunk([1,2,3], 0) == []
+end
+```
+
+The `assert_raise/3` form is also available which allows you to test both
+the type of exception and the resulting message.
+
+```elixir
+assert_raise FunctionClauseError, ~r/^no function clause matching/, fn ->
+  Enum.chunk([1,2,3], 0) == []
+end
+```
+
+Using the regex sigil for the second argument is generally a good way to go
+to keep tests from getting too brittle.