Add ADR for procedural device definition

docs/developer/explanations/decisions/0006-procedural-device-definitions.rst

@@ -0,0 +1,88 @@
+6. Procedural Device Definitions
+================================
+
+Date: 2023-09-11
+
+Status
+------
+
+Accepted
+
+Context
+-------
+
+Ophyd creates devices in a declarative way:
+
+.. code-block:: python
+
+    class Sensor(Device):
+        mode = Component(EpicsSignal, "Mode", kind="config")
+        value = Component(EpicsSignalRO, "Value", kind="hinted")
+
+This means when you make ``device = OldSensor(pv_prefix)`` then some metaclass
+magic will call ``EpicsSignal(pv_prefix + "Mode", kind="config")`` and make it
+available as ``device.mode``.
+
+ophyd-async could convert this approach to use type hints instead of metaclasses:
+
+.. code-block:: python
+
+    from typing import Annotated as A
+
+    class Sensor(EpicsDevice):
+        mode: A[SignalRW, CONFIG, pv_suffix("Mode")]
+        value: A[SignalR, READ, pv_suffix("Value")]
+
+The superclass init could then read all the type hints and instantiate them with
+the correct SignalBackends. 
+
+Alternatively it could use a procedural approach and be explicit about where the
+arguments are passed at the cost of greater verbosity:
+
+.. code-block:: python
+
+    class Sensor(StandardReadable):
+        def __init__(self, prefix: str, name="") -> None:
+            self.value = epics_signal_r(float, prefix + "Value")
+            self.mode = epics_signal_rw(EnergyMode, prefix + "Mode")
+            # Set name and signals for read() and read_configuration()
+            self.set_readable_signals(read=[self.value], config=[self.mode])
+            super().__init__(name=name)
+
+The procedural approach to creating child Devices is:
+
+.. code-block:: python
+
+    class SensorGroup(Device):
+        def __init__(self, prefix: str, num: int, name: Optional[str]=None):
+            self.sensors = DeviceVector(
+                {i: Sensor(f"{prefix}:CHAN{i}" for i in range(1, num+1))}
+            )
+            super().__init__(name=name)
+
+We have not been able to come up with a declarative approach that can describe
+the ``SensorGroup`` example in a succinct way.
+
+Decision
+--------
+
+Type safety and readability are regarded above velocity, and magic should be
+minimized. With this in mind we will stick with the procedural approach for now.
+We may find a less verbose way of doing ``set_readable_signals`` by using a
+context manager and overriding setattr in the future:
+
+.. code-block:: python
+
+    with self.signals_added_to(READ):
+        self.value = epics_signal_r(float, prefix + "Value")
+    with self.signals_added_to(CONFIG):
+        self.mode = epics_signal_rw(EnergyMode, prefix + "Mode")
+
+If someone comes up with a way to write ``SensorGroup`` in a declarative
+and readable way then we may revisit this.
+
+Consequences
+------------
+
+Ophyd and ophyd-async Devices will look less alike, but ophyd-async should be
+learnable for beginners.