Source code for pennylane.control_flow.while_loop

# Copyright 2025 Xanadu Quantum Technologies Inc.

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"""While loop."""
import functools
from collections.abc import Callable

from pennylane import capture
from pennylane.capture import FlatFn, determine_abstracted_axes, enabled
from pennylane.compiler.compiler import AvailableCompilers, active_compiler


[docs]def while_loop(cond_fn): """A :func:`~.qjit` compatible for-loop for PennyLane programs. When used without :func:`~.qjit`, this function will fall back to a standard Python for loop. This decorator provides a functional version of the traditional while loop, similar to `jax.lax.while_loop <https://jax.readthedocs.io/en/latest/_autosummary/jax.lax.while_loop.html>`__. That is, any variables that are modified across iterations need to be provided as inputs and outputs to the loop body function: - Input arguments contain the value of a variable at the start of an iteration - Output arguments contain the value at the end of the iteration. The outputs are then fed back as inputs to the next iteration. The final iteration values are also returned from the transformed function. The semantics of ``while_loop`` are given by the following Python pseudocode: .. code-block:: python def while_loop(cond_fn, body_fn, *args): while cond_fn(*args): args = body_fn(*args) return args Args: cond_fn (Callable): the condition function in the while loop Returns: Callable: A wrapper around the while-loop function. Raises: CompileError: if the compiler is not installed .. seealso:: :func:`~.for_loop`, :func:`~.qjit` **Example** .. code-block:: python dev = qml.device("lightning.qubit", wires=1) @qml.qnode(dev) def circuit(x: float): @qml.while_loop(lambda x: x < 2.0) def loop_rx(x): # perform some work and update (some of) the arguments qml.RX(x, wires=0) return x ** 2 # apply the while loop loop_rx(x) return qml.expval(qml.Z(0)) >>> circuit(1.6) -0.02919952 ``while_loop`` is also :func:`~.qjit` compatible; when used with the :func:`~.qjit` decorator, the while loop will not be unrolled, and instead will be captured as-is during compilation and executed during runtime: >>> qml.qjit(circuit)(1.6) Array(-0.02919952, dtype=float64) """ if active_jit := active_compiler(): compilers = AvailableCompilers.names_entrypoints ops_loader = compilers[active_jit]["ops"].load() return ops_loader.while_loop(cond_fn) # if there is no active compiler, simply interpret the while loop # via the Python interpretor. def _decorator(body_fn: Callable) -> Callable: """Transform that will call the input ``body_fn`` until the closure variable ``cond_fn`` is met. Args: body_fn (Callable): Closure Variables: cond_fn (Callable): Returns: Callable: a callable with the same signature as ``body_fn`` and ``cond_fn``. """ return WhileLoopCallable(cond_fn, body_fn) return _decorator
@functools.lru_cache def _get_while_loop_qfunc_prim(): """Get the while_loop primitive for quantum functions.""" # pylint: disable=import-outside-toplevel from pennylane.capture.custom_primitives import NonInterpPrimitive while_loop_prim = NonInterpPrimitive("while_loop") while_loop_prim.multiple_results = True while_loop_prim.prim_type = "higher_order" # pylint: disable=too-many-arguments @while_loop_prim.def_impl def _( *args, jaxpr_body_fn, jaxpr_cond_fn, body_slice, cond_slice, args_slice, abstract_shapes_slice, ): jaxpr_consts_body = args[body_slice] jaxpr_consts_cond = args[cond_slice] init_state = args[args_slice] abstract_shapes = args[abstract_shapes_slice] # If cond_fn(*init_state) is False, return the initial state fn_res = init_state while capture.eval_jaxpr(jaxpr_cond_fn, jaxpr_consts_cond, *abstract_shapes, *fn_res)[0]: fn_res = capture.eval_jaxpr(jaxpr_body_fn, jaxpr_consts_body, *abstract_shapes, *fn_res) return fn_res @while_loop_prim.def_abstract_eval def _(*args, args_slice, **__): return args[args_slice] return while_loop_prim class WhileLoopCallable: # pylint:disable=too-few-public-methods """Base class to represent a while loop. This class when called with an initial state will execute the while loop via the Python interpreter. Args: cond_fn (Callable): the condition function in the while loop body_fn (Callable): the function that is executed within the while loop """ def __init__(self, cond_fn, body_fn): self.cond_fn = cond_fn self.body_fn = body_fn def _call_capture_disabled(self, *init_state): args = init_state fn_res = args if len(args) > 1 else args[0] if len(args) == 1 else None while self.cond_fn(*args): fn_res = self.body_fn(*args) args = fn_res if len(args) > 1 else (fn_res,) if len(args) == 1 else () return fn_res def _call_capture_enabled(self, *init_state): import jax # pylint: disable=import-outside-toplevel while_loop_prim = _get_while_loop_qfunc_prim() abstracted_axes, abstract_shapes = determine_abstracted_axes(init_state) flat_body_fn = FlatFn(self.body_fn) jaxpr_body_fn = jax.make_jaxpr(flat_body_fn, abstracted_axes=abstracted_axes)(*init_state) jaxpr_cond_fn = jax.make_jaxpr(self.cond_fn, abstracted_axes=abstracted_axes)(*init_state) body_consts = slice(0, len(jaxpr_body_fn.consts)) cond_consts = slice(body_consts.stop, body_consts.stop + len(jaxpr_cond_fn.consts)) abstract_shapes_slice = slice(cond_consts.stop, cond_consts.stop + len(abstract_shapes)) args_slice = slice(abstract_shapes_slice.stop, None) flat_args, _ = jax.tree_util.tree_flatten(init_state) results = while_loop_prim.bind( *jaxpr_body_fn.consts, *jaxpr_cond_fn.consts, *abstract_shapes, *flat_args, jaxpr_body_fn=jaxpr_body_fn.jaxpr, jaxpr_cond_fn=jaxpr_cond_fn.jaxpr, body_slice=body_consts, cond_slice=cond_consts, args_slice=args_slice, abstract_shapes_slice=abstract_shapes_slice, ) assert flat_body_fn.out_tree is not None, "Should be set when constructing the jaxpr" return jax.tree_util.tree_unflatten(flat_body_fn.out_tree, results) def __call__(self, *init_state): if enabled(): return self._call_capture_enabled(*init_state) return self._call_capture_disabled(*init_state)