Add Using When Clauses In A With Construct as an elixir til

elixir/using-when-clauses-in-a-with-construct.md

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+# Using When Clauses In A With Construct
+
+Because Elixir's `with` construct supports the full power of the language's
+pattern matching, we can use `when` clauses to further narrow down our
+matches.
+
+For instance, if we want to match against the response to an API request,
+but only for response status codes in the 2xx range, we can do something
+like the following:
+
+```elixir
+with %{status_code: code, body: body}
+       when code >= 200 && code < 300 <- HTTPoison.get!(url),
+     {:ok, decoded_body} <- Poison.decode(body) do
+  {:ok, decoded_body}
+end
+```
+
+See the [docs for
+`with`](https://hexdocs.pm/elixir/Kernel.SpecialForms.html#with/1) for more
+details.