Document functional middleware

docs/execution/middleware.rst

@@ -3,7 +3,7 @@ Middleware
 
 You can use ``middleware`` to affect the evaluation of fields in your schema.
 
-A middleware is any object that responds to ``resolve(*args, next_middleware)``.
+A middleware is any object or function that responds to ``resolve(next_middleware, *args)``.
 
 Inside that method, it should either:
 
@@ -40,3 +40,32 @@ And then execute it with:
 .. code:: python
 
     result = schema.execute('THE QUERY', middleware=[AuthorizationMiddleware()])
+
+
+Functional example
+------------------
+
+Middleware can also be defined as a function. Here we define a middleware that
+logs the time it takes to resolve each field
+
+.. code:: python
+
+    from time import time as timer
+
+    def timing_middleware(next, root, info, **args):
+        start = timer()
+        return_value = next(root, info, **args)
+        duration = timer() - start
+        logger.debug("{parent_type}.{field_name}: {duration}".format(
+            parent_type=root._meta.name if root else '',
+            field_name=info.field_name,
+            duration=duration
+        ))
+        return return_value
+
+
+And then execute it with:
+
+.. code:: python
+
+    result = schema.execute('THE QUERY', middleware=[timing_middleware])