PennyLaneAI · josh146 · Jun 3, 2021 · May 14, 2021 · May 14, 2021 · May 14, 2021
diff --git a/.github/CHANGELOG.md b/.github/CHANGELOG.md
@@ -2,6 +2,66 @@
 
 <h3>New features since last release</h3>
 
+* Adds a decorator `@qml.qfunc_transform` to easily create a transformation
+  that modifies the behaviour of a quantum function.
+  [(#1315)](https://github.com/PennyLaneAI/pennylane/pull/1315)
+
+  For example, consider the following transform, which replaces all
+  `CRX` gates with a sequence of `RX`, `RY`, and `CZ` gates:
+
+  ```python
+  @qml.qfunc_transform
+  def my_transform(tape, x, y):
+      for op in tape.operations + tape.measurements:
+          if op.name == "CRX":
+              wires = op.wires
+              param = op.parameters[0]
+              qml.RX(x * param, wires=wires[1])
+              qml.RY(y * qml.math.sqrt(param), wires=wires[1])
+              qml.CZ(wires=[wires[1], wires[0]])
+          else:
+              op.queue()
+  ```
+
+  We can now apply this transform to any quantum function:
+
+  ```python
+  dev = qml.device("default.qubit", wires=2)
+
+  def ansatz(x):
+      qml.Hadamard(wires=0)
+      qml.CRX(x, wires=[0, 1])
+
+  @qml.qnode(dev)
+  def circuit(param, transform_weights):
+      qml.RX(0.1, wires=0)
+
+      # apply the transform to the ansatz
+      my_transform(*transform_weights)(ansatz)(param)
+
+      return qml.expval(qml.PauliZ(1))
+  ```
+
+  We can print this QNode to show that the qfunc transform is taking place:
+
+  ```pycon
+  >>> x = np.array(0.5, requires_grad=True)
+  >>> y = np.array([0.1, 0.2], requires_grad=True)
+  >>> print(qml.draw(circuit)(x, y))
+   0: ──RX(0.1)───H──────────╭Z──┤
+   1: ──RX(0.05)──RY(0.141)──╰C──┤ ⟨Z⟩  
+  ```
+
+  Evaluating the QNode, as well as the derivative, with respect to the gate
+  parameter *and* the transform weights:
+
+  ```pycon
+  >>> circuit(x, y)
+  0.9887793925354269
+  >>> qml.grad(circuit)(x, y)
+  (array(-0.02485651), array([-0.02474011, -0.09954244]))
+  ```
+
 * Added validation for noise channel parameters. Invalid noise parameters now
   raise a `ValueError`. [(#1357)](https://github.com/PennyLaneAI/pennylane/pull/1357)
 

diff --git a/pennylane/__init__.py b/pennylane/__init__.py
@@ -49,6 +49,8 @@
     ctrl,
     measurement_grouping,
     metric_tensor,
+    qfunc_transform,
+    single_tape_transform,
 )
 from pennylane.utils import inv
 from pennylane.vqe import ExpvalCost, Hamiltonian, VQECost

diff --git a/pennylane/transforms/__init__.py b/pennylane/transforms/__init__.py
@@ -1,66 +1,80 @@
-# Copyright 2018-2021 Xanadu Quantum Technologies Inc.
-
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-
-#     http://www.apache.org/licenses/LICENSE-2.0
-
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-"""
-This subpackage contains QNode, quantum function, device, and tape transforms.
-
-
-.. currentmodule:: pennylane
-
-QNode transforms
-----------------
-
-The following transforms act on QNodes. They return new transformed functions
-that compute the desired quantity.
-
-.. autosummary::
-    :toctree: api
-
-    ~transforms.classical_jacobian
-    ~draw
-    ~metric_tensor
-
-Quantum function transforms
----------------------------
-
-The following transforms act on quantum functions (Python functions
-containing quantum operations) that are used *inside* QNodes.
-
-.. autosummary::
-    :toctree: api
-
-    ~adjoint
-    ~ctrl
-    ~transforms.invisible
-
-Tape transforms
----------------
-
-The following transforms act on quantum tapes, and return one or
-more tapes as well as a classical processing function.
-
-.. autosummary::
-    :toctree: api
-
-    ~transforms.measurement_grouping
-    ~transforms.metric_tensor_tape
-    ~transforms.hamiltonian_expand
-"""
-from .adjoint import adjoint
-from .classical_jacobian import classical_jacobian
-from .control import ControlledOperation, ctrl
-from .draw import draw
-from .invisible import invisible
-from .measurement_grouping import measurement_grouping
-from .metric_tensor import metric_tensor, metric_tensor_tape
-from .hamiltonian_expand import hamiltonian_expand
+# Copyright 2018-2021 Xanadu Quantum Technologies Inc.
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""
+This subpackage contains QNode, quantum function, device, and tape transforms.
+
+
+.. currentmodule:: pennylane
+
+QNode transforms
+----------------
+
+The following transforms act on QNodes. They return new transformed functions
+that compute the desired quantity.
+
+.. autosummary::
+    :toctree: api
+
+    ~transforms.classical_jacobian
+    ~draw
+    ~metric_tensor
+
+Quantum function transforms
+---------------------------
-Quantum function transforms
---------------------------
-Quantum function transforms
---------------------------
+
+The following transforms act on quantum functions (Python functions
+containing quantum operations) that are used *inside* QNodes.
+
+.. autosummary::
+    :toctree: api
+
+    ~adjoint
+    ~ctrl
+    ~transforms.invisible
+
+Tape transforms
+---------------
+
+The following transforms act on quantum tapes, and return one or
+more tapes as well as a classical processing function.
+
+.. autosummary::
+    :toctree: api
+
+    ~transforms.measurement_grouping
+    ~transforms.metric_tensor_tape
+    ~transforms.hamiltonian_expand
+
+Decorators and utility functions
+--------------------------------
+
+The following decorators and convenience functions are provided
+to help build custom QNode, quantum function, and tape transforms:
+
+.. autosummary::
+    :toctree: api
+
+    ~single_tape_transform
+    ~qfunc_transform
+    ~transforms.make_tape
+"""
+from .adjoint import adjoint
+from .classical_jacobian import classical_jacobian
+from .control import ControlledOperation, ctrl
+from .draw import draw
+from .hamiltonian_expand import hamiltonian_expand
+from .invisible import invisible
+from .measurement_grouping import measurement_grouping
+from .metric_tensor import metric_tensor, metric_tensor_tape
+from .qfunc_transforms import make_tape, single_tape_transform, qfunc_transform