All functions can take a middleware function as the first parameter.
This allow to use convenient syntax for modifying the behaviour in runtime.
Consider the following case: GitHub API can be accessed using OAuth token authorization.
We can't use `plug Tesla.Middleware.Headers, %{"Authorization" => "token here"}` since this would be compiled only once and there is no way to insert dynamic user token.
Instead, we can use `Tesla.build_client` to create a dynamic middleware function:
```elixir
defmodule GitHub do
# same as above with a slightly change to `user_repos/1`
There is no distinction between request and response middleware, it's all about executing `Tesla.run/2` function at the correct time.
For example, z request logger middleware could be implemented like this:
```elixir
defmodule Tesla.Middleware.RequestLogger do
def call(env, next, _) do
IO.inspect env # print request env
Tesla.run(env, next)
end
end
```
and response logger middleware like this:
```elixir
defmodule Tesla.Middleware.ResponseLogger do
def call(env, next, _) do
res = Tesla.run(env, next)
IO.inspect res # print response env
res
end
end
```
See [`core.ex`](https://github.com/teamon/tesla/blob/master/lib/tesla/middleware/core.ex) and [`json.ex`](https://github.com/teamon/tesla/blob/master/lib/tesla/middleware/json.ex) for more examples.
## Streaming body
If adapter supports it, you can pass a [Stream](http://elixir-lang.org/docs/stable/elixir/Stream.html) as body, e.g.: