checkify.py

# Copyright 2021 The JAX Authors.
#
# 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
#
#     https://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.

import dataclasses
import functools
import itertools as it
import types
from typing import Union, Optional, Callable, Dict, Tuple, TypeVar, FrozenSet, Iterable, Type, Set, List

import jax
from jax import core
from jax import lax
from jax import linear_util as lu
from jax._src import prng
from jax._src import source_info_util
from jax._src import traceback_util
from jax._src.config import config
from jax._src.lax import control_flow as cf
from jax._src.sharding import OpShardingSharding
from jax._src.typing import Array
from jax._src.util import (as_hashable_function, unzip2, split_list, safe_map,
                           safe_zip)
from jax.api_util import flatten_fun
from jax.api_util import flatten_fun_nokwargs
from jax.experimental import maps
from jax.experimental import pjit
from jax.interpreters import ad
from jax.interpreters import batching
from jax.interpreters import mlir
from jax.interpreters import partial_eval as pe
from jax.tree_util import tree_flatten
from jax.tree_util import tree_map
from jax.tree_util import tree_unflatten
import jax.numpy as jnp
import jax.tree_util as jtu
import numpy as np

source_info_util.register_exclusion(__file__)
traceback_util.register_exclusion(__file__)

map, unsafe_map = safe_map, map
zip, unsafe_zip = safe_zip, zip

Bool = Union[bool, Array]
Int = Union[int, Array]
ErrorCategory = Type['JaxException']
Payload = List[Union[np.ndarray, Array]]
PyTreeDef = jtu.PyTreeDef

## Utils

def popattr(obj, attrname):
  val = getattr(obj, attrname)
  delattr(obj, attrname)
  return val

def setnewattr(obj, name, val):
  sentinel = object()
  assert getattr(obj, name, sentinel) is sentinel
  setattr(obj, name, val)

# Concrete errors

class JaxException(Exception):
  """Python exception which can contain an error message with JAX run-time info."""

  def __init__(self, traceback_info):
    self.traceback_info = traceback_info
    # TODO(lenamartens): re-enable tracebacks when they don't leak tracers.
    # self.with_traceback(self.traceback_info)

  def __init_subclass__(cls):
    jtu.register_pytree_node_class(cls)

  def tree_flatten(self):
    return ([], self.traceback_info)

  @classmethod
  def tree_unflatten(cls, metadata, payload):
    del payload
    return cls(metadata)

  def get_effect_type(self) -> core.Effect:
    pass


@functools.total_ordering
@dataclasses.dataclass(eq=True, frozen=True)
class ErrorEffect:
  error_type: Type[JaxException]
  shape_dtypes: Tuple[jax.ShapeDtypeStruct, ...]

  def __post_init__(self):
    cf.allowed_effects.add(self)
    mlir.lowerable_effects.add(self)

  def __lt__(self, other: 'ErrorEffect'):
    shape_dtypes = lambda x: tuple((sd.shape, str(sd.dtype))  # dtype is not comparable
                                   for sd in x.shape_dtypes)
    unpack = lambda x: (str(x.error_type), shape_dtypes(x))
    return (unpack(self) < unpack(other))


class DivisionByZeroError(JaxException):

  def __str__(self):
    return f'division by zero at {self.traceback_info}'

  def get_effect_type(self):
    return ErrorEffect(DivisionByZeroError, ())

class NaNError(JaxException):

  def __init__(self, traceback_info, primitive_name):
    super().__init__(traceback_info)
    self.prim = primitive_name

  def tree_flatten(self):
    return ([], (self.traceback_info, self.prim))

  @classmethod
  def tree_unflatten(cls, metadata, _):
    return cls(*metadata)

  def get_effect_type(self):
    return ErrorEffect(NaNError, ())

  def __str__(self):
    return f'nan generated by primitive: {self.prim} at {self.traceback_info}'

class OOBError(JaxException):

  def __init__(self, traceback_info, primitive_name, operand_shape, payload):
    super().__init__(traceback_info)
    self.prim = primitive_name
    self.operand_shape = operand_shape
    self._payload = payload

  def tree_flatten(self):
    return ([self._payload], (self.traceback_info, self.prim, self.operand_shape))

  @classmethod
  def tree_unflatten(cls, metadata, payload):
    return cls(*metadata, payload[0])

  def __str__(self):
    return (f'out-of-bounds indexing for array of '
            f'shape {self.operand_shape}: '
            f'index {self._payload[0]} is out of bounds for axis '
            f'{self._payload[1]} with size {self._payload[2]}. '
            f'Failed at {self.traceback_info}')

  def get_effect_type(self):
    return ErrorEffect(OOBError, (jax.ShapeDtypeStruct((3,), jnp.int32),))

class FailedCheckError(JaxException):

  def __init__(self, traceback_info, fmt_string, *a, **k):
    super().__init__(traceback_info)
    self.fmt_string = fmt_string
    self.args = a
    self.kwargs = k

  def tree_flatten(self):
    return ((jnp.array([], jnp.int32), self.args, self.kwargs),
            (self.traceback_info, self.fmt_string))

  @classmethod
  def tree_unflatten(cls, metadata, payload):
    _, args, kwargs = payload
    return cls(*metadata, *args, **kwargs)

  def __str__(self):
    return (self.fmt_string.format(*self.args, **self.kwargs)
            + f' (check failed at {self.traceback_info})')

  def get_effect_type(self):
    vals = jtu.tree_leaves((self.args, self.kwargs))
    return ErrorEffect(
        FailedCheckError,
        # Need a 0-size array here for data-dependence.
        (jax.ShapeDtypeStruct((0,), jnp.int32),
         *tuple(jax.ShapeDtypeStruct(x.shape, x.dtype) for x in vals)))

@dataclasses.dataclass
class BatchedError(JaxException):
  error_mapping: Dict[Tuple[int, ...], JaxException]

  def __post_init__(self):
    traceback_info = list(self.error_mapping.values())[0].traceback_info
    super().__init__(traceback_info)


  def __str__(self):
    return '\n'.join(f'at mapped index {", ".join(map(str, idx))}: {e}'
                     for idx, e in self.error_mapping.items())


# Error Value

@jtu.register_pytree_node_class
@dataclasses.dataclass(frozen=True)
class Error:
  _pred: Dict[ErrorEffect, Bool]
  _code: Dict[ErrorEffect, Int]
  _metadata: Dict[Int, PyTreeDef]  # mapping of code to JaxException treedef.
  _payload: Dict[ErrorEffect, Payload]

  def get(self) -> Optional[str]:
    """Returns error message if error happened, None if no error happened."""
    exp = self.get_exception()
    if exp is not None:
      return str(exp)
    return None

  def get_exception(self) -> Optional[JaxException]:
    """Returns Python exception if error happened, None if no error happened."""
    if any(map(np.shape, self._pred.values())):
      return self._get_batched_exception()
    else:
      min_code = None
      cur_effect = None
      for error_effect, code in self._code.items():
        if self._pred[error_effect]:
          if min_code is None or code < min_code:
            min_code = code
            cur_effect = error_effect

      if cur_effect is not None:
        return tree_unflatten(self._metadata[int(min_code)],  # type: ignore
                              self._payload[cur_effect])
    return None

  def throw(self):
    check_error(self)

  def __str__(self):
    return f'Error({self.get()})'

  # Internal helpers

  def _get_batched_exception(self):
    shape = np.shape(list(self._pred.values())[0])
    error_mapping = {}
    for idx in np.ndindex(*shape):
      min_code = None
      cur_effect = None
      for error_effect, code in self._code.items():
        if self._pred[error_effect][idx]:   # type: ignore
          if min_code is None or code[idx] < min_code:
            min_code = code[idx]   # type: ignore
            cur_effect = error_effect

      if cur_effect is not None:
        payload = tree_map(lambda x, i=idx: x[i], self._payload[cur_effect])
        jax_error = tree_unflatten(self._metadata[int(min_code)], payload)  # type: ignore
        error_mapping[idx] = jax_error
    return BatchedError(error_mapping)

  def _update(self, effect_type: ErrorEffect, pred, code, metadata, payload):
    new_errs = {**self._pred, **{effect_type: pred}}  # type: ignore
    new_codes = {**self._code, **{effect_type: code}}  # type: ignore
    new_payload = {**self._payload, **{effect_type: payload}}  # type: ignore
    new_metadata = {**self._metadata, **metadata}
    return Error(new_errs, new_codes, new_metadata, new_payload)

  def _add_placeholder_effects(self, effects: Set[ErrorEffect]):
    """Fill out Error with `effects` and np.ones arrays of their payloads."""
    new_err = self._pred.copy()
    new_code = self._code.copy()
    new_payload = self._payload.copy()
    for effect in effects:
      if effect not in self._pred.keys():
        new_err[effect] = False
        new_payload[effect] = list(
            tree_map(lambda a: jnp.ones(a.shape, a.dtype), effect.shape_dtypes))
        # The error value associated with this effect will never become True, so
        # we don't need to set a meaningful code.
        new_code[effect] = -1
    return Error(new_err, new_code, self._metadata, new_payload)

  def _replace(self, *args, **kwargs):
    return dataclasses.replace(self, *args, **kwargs)

  # PyTree methods

  def tree_flatten(self):
    return ((self._pred, self._code, self._payload), (self._metadata))

  @classmethod
  def tree_unflatten(cls, metadata, data):
    pred, code, payload = data
    return cls(pred, code, metadata, payload)

init_error = Error({}, {}, {}, {})  # value used as initial (empty) error.
next_code = it.count(1).__next__  # globally unique ids, could be uuid4

def assert_func(error: Error, pred: Bool, new_error: JaxException) -> Error:
  code = next_code()
  effect_type = new_error.get_effect_type()
  new_payload, new_metadata = tree_flatten(new_error)
  return update_error(error, pred, code, {code: new_metadata}, new_payload, effect_type)

def update_error(error, pred, code, metadata, payload, effect_type):
  err_of_type = error._pred.get(effect_type, False)
  out_err = err_of_type | pred
  out_code = lax.select(err_of_type, error._code.get(effect_type, -1), code)
  cur_payload = error._payload.get(effect_type, None)
  if cur_payload is not None:
    out_payload = tree_map(functools.partial(lax.select, err_of_type), cur_payload, payload)
  else:
    out_payload = payload
  return error._update(effect_type, out_err, out_code, metadata, out_payload)


## Checkify transformation for plumbing functional error values.

class CheckifyTracer(core.Tracer):
  def __init__(self, trace, val):
    self._trace = trace
    self.val = val
  aval = property(lambda self: core.get_aval(self.val))
  full_lower = lambda self: self

class CheckifyTrace(core.Trace):
  pure = lift = lambda self, val: CheckifyTracer(self, val)

  def __init__(self, main: core.MainTrace, sublevel: core.Sublevel,
               enabled_errors: FrozenSet['ErrorCategory']) -> None:
    self.main = main
    self.level = main.level
    self.sublevel = sublevel
    self.main.enabled_errors = enabled_errors

  def sublift(self, tracer):
    return CheckifyTracer(self, tracer.val)

  def process_primitive(self, primitive, tracers, params):
    in_vals = [t.val for t in tracers]
    rule = error_checks.get(primitive)
    if rule:
      out, self.main.error = rule(self.main.error, self.main.enabled_errors,  # type: ignore
                                  *in_vals, **params)
    else:
      out = primitive.bind(*in_vals, **params)
    if primitive.multiple_results:
      return [CheckifyTracer(self, x) for x in out]
    else:
      return CheckifyTracer(self, out)

  def process_call(self, primitive, f, tracers, params):
    in_vals = [t.val for t in tracers]
    e = popattr(self.main, 'error')
    flat_vals, in_tree = tree_flatten((e, *in_vals))
    f = checkify_subtrace(f, self.main)
    f, out_tree = flatten_fun_nokwargs(f, in_tree)
    if 'donated_invars' in params:
      params = dict(params, donated_invars=(*[False]*len(jtu.tree_leaves(e)),
                                            *params['donated_invars']))
    all_vals = primitive.bind(f, *flat_vals, **params)
    error, *out_vals = tree_unflatten(out_tree(), all_vals)
    setnewattr(self.main, 'error', error)
    return [CheckifyTracer(self, x) for x in out_vals]

  def process_map(self, primitive, f, tracers, params):
    in_vals = [t.val for t in tracers]
    e = popattr(self.main, 'error')
    flat_vals, in_tree = tree_flatten((e, *in_vals))
    num_error_vals = len(jtu.tree_leaves(e))
    f = checkify_subtrace(f, self.main)
    f, out_tree = flatten_fun_nokwargs(f, in_tree)

    @as_hashable_function(closure=params['out_axes_thunk'])
    def new_out_axes_thunk():
      out_val_axes = params['out_axes_thunk']()
      out_err_num = out_tree().num_leaves - len(out_val_axes)
      return (*(0,)*out_err_num, *out_val_axes)

    params_ = dict(params, in_axes=(*(None,)*num_error_vals, *params['in_axes']),
                   out_axes_thunk=new_out_axes_thunk,
                   donated_invars=(*(False,)*num_error_vals, *params['donated_invars']))
    all_vals = primitive.bind(f, *flat_vals, **params_)
    error, *out_vals = tree_unflatten(out_tree(), all_vals)
    error = _reduce_any_error(error)
    setnewattr(self.main, 'error', error)
    return [CheckifyTracer(self, x) for x in out_vals]

  def post_process_call(self, primitive, tracers, params):
    vals = [t.val for t in tracers]
    main = self.main
    e = popattr(main, 'error')
    err_leaves, err_tree = tree_flatten(e)
    setnewattr(main, 'err_tree', err_tree)
    def todo(vals):
      err_tree = popattr(main, 'err_tree')
      err_vals, vals = split_list(vals, [err_tree.num_leaves])
      setnewattr(main, 'error', tree_unflatten(err_tree, err_vals))
      trace = main.with_cur_sublevel()
      return [CheckifyTracer(trace, x) for x in vals]
    return (*err_leaves, *vals), todo

  def post_process_map(self, primitive, tracers, params):
    vals = [t.val for t in tracers]
    main = self.main
    e = popattr(main, 'error')
    err_leaves, err_tree = tree_flatten(e)
    num_err_leaves = len(err_leaves)
    setnewattr(main, 'err_tree', err_tree)
    def todo(vals):
      err_tree = popattr(main, 'err_tree')
      err_vals, vals = split_list(vals, [err_tree.num_leaves])
      error = tree_unflatten(err_tree, err_vals)
      error = _reduce_any_error(error)
      setnewattr(main, 'error', error)
      trace = main.with_cur_sublevel()
      return [CheckifyTracer(trace, x) for x in vals]
    def out_axes_transform(out_axes):
      return (*(0,)*num_err_leaves, *out_axes)
    return (*err_leaves, *vals), (todo, out_axes_transform)

  def process_custom_jvp_call(self, prim, f, jvp, tracers):
    in_vals = [t.val for t in tracers]
    e = popattr(self.main, 'error')
    err_vals, err_tree = tree_flatten(e)
    flat_vals, in_tree = tree_flatten((e, *in_vals))
    num_error_vals = len(err_vals)
    f = checkify_subtrace(f, self.main)
    f, f_out_tree = flatten_fun_nokwargs(f, in_tree)
    jvp, jvp_err_tree = checkify_custom_jvp_subtrace(jvp, self.main,
                                                     num_error_vals, err_tree)
    all_outs = prim.bind(f, jvp, *flat_vals)
    fst, out_tree = lu.merge_linear_aux(f_out_tree, jvp_err_tree)
    if fst:
      out_err, *out_vals = tree_unflatten(out_tree, all_outs)
    else:
      err_vals, out_vals = split_list(all_outs, [num_error_vals])
      # forward input error values to output
      out_err = tree_unflatten(out_tree, err_vals)
    setattr(self.main, 'error', out_err)
    return [CheckifyTracer(self, x) for x in out_vals]

  def post_process_custom_jvp_call(self, tracers, jvp_was_run):
    if jvp_was_run:
      msg = ('support for custom_jvp rules which close over checkify values is '
             'not implemented. If you see this, open an issue at '
             'https://github.com/google/jax/issues!')
      raise NotImplementedError(msg)
    vals = [t.val for t in tracers]
    main = self.main
    e = popattr(main, 'error')
    err_leaves, err_tree = tree_flatten(e)
    def todo(vals):
      err_vals, vals = split_list(vals, [len(err_leaves)])
      setnewattr(main, 'error', tree_unflatten(err_tree, err_vals))
      trace = main.with_cur_sublevel()
      return [CheckifyTracer(trace, x) for x in vals]
    return (*err_leaves, *vals), todo

  def process_custom_vjp_call(self, prim, fun, fwd, bwd, tracers, out_trees):
    in_vals = [t.val for t in tracers]
    e = popattr(self.main, 'error')
    err_vals, err_tree = tree_flatten(e)
    flat_vals, in_tree = tree_flatten((e, *in_vals))
    num_error_vals = len(err_vals)

    fun = checkify_subtrace(fun, self.main)
    fun, fun_out_tree = flatten_fun_nokwargs(fun, in_tree)
    fwd, fwd_err_tree = checkify_custom_vjp_subtrace(fwd, self.main,
                                                     err_tree, num_error_vals)

    all_out_vals = prim.bind(fun, fwd, bwd, *flat_vals, out_trees=out_trees)
    fst, out_tree = lu.merge_linear_aux(fun_out_tree, fwd_err_tree)
    if fst:
      error, *out = tree_unflatten(out_tree, all_out_vals)
    else:
      _, out = split_list(all_out_vals, [num_error_vals])
      # forward input error values to output
      error = tree_unflatten(err_tree, err_vals)
    setattr(self.main, 'error', error)
    return [CheckifyTracer(self, x) for x in out]

def _reduce_any_error(error: Error):
  out_error = init_error
  for error_effect in error._pred.keys():
    errs, codes, payloads = (error._pred[error_effect],
                             error._code[error_effect],
                             error._payload[error_effect])
    reduced_idx = jnp.argsort(errs)[-1]
    pred, code, payload = tree_map(lambda x, idx=reduced_idx: x[idx],
                                   (errs, codes, payloads))
    out_error = out_error._update(error_effect, pred, code, {}, payload)

  out_error = out_error._replace(_metadata=error._metadata)
  return out_error

ErrorCheckRule = Callable  # (Error, FrozenSet[ErrorCategory], *in_vals, **params) -> (Any, Error)
error_checks: Dict[core.Primitive, ErrorCheckRule] = {}

def checkify_flat(fun: lu.WrappedFun, enabled_errors: FrozenSet['ErrorCategory'],
                  *args):
  fun = checkify_subtrace(fun)
  fun = checkify_traceable(fun, enabled_errors)
  error, *outvals = fun.call_wrapped(init_error, *args)
  return error, outvals

@lu.transformation
def checkify_traceable(enabled_errors, error, *args):
  with core.new_main(CheckifyTrace, enabled_errors=enabled_errors) as main:
    outs = yield (main, error, *args), {}
    del main
  yield outs

@lu.transformation
def checkify_subtrace(main, error, *args):
  setnewattr(main, 'error', error)
  trace = main.with_cur_sublevel()
  in_tracers = [CheckifyTracer(trace, x) for x in args]
  out = yield in_tracers, {}
  out_tracers = map(trace.full_raise, out)
  out_vals = [t.val for t in out_tracers]
  error = main.error
  del main.error
  yield (error, *out_vals)

@lu.transformation_with_aux
def checkify_custom_jvp_subtrace(main, num_error_vals, out_tree, *args):
  # Like checkify_subtrace, but used specifically on the custom JVP rules
  # associated with a custom_jvp. This code is called in the context of a
  # jvp-of-checkify-of-custom_jvp. It takes both primal and tangent inputs,
  # flattened into a single args tuple, and similarly must produce flattened
  # primal and tangent outputs. Both primals and tangents include error values,
  # but the tangent error values are trivially zero.
  # The types to have in mind are:
  #   jvp : (a -> b) -> (a, T a) -> (b, T b)
  #   checkify : (a -> b) -> a -> Err b
  #   jvp-of-checkify : (a -> b) -> (a, T a) -> (Err b, T (Err b))
  # where because Err is a pytree, we necessarily have T (Err b) = Err' (T b)
  # where the other Err' components are trivial (of float0 dtype).
  # Semantically, we don't add checks to the JVP rule. To check the result of a
  # JVP rule, one must instead use checkify-of-jvp. Thus this implementation
  # just forwards the input error and code (and trivial tangents) to the output.
  del main
  n, ragged = divmod(len(args), 2)
  assert not ragged
  err_primals, primals = split_list(args[:n], [num_error_vals])
  err_tangents, tangents = split_list(args[n:], [num_error_vals])
  outs = yield (*primals, *tangents), {}
  m, ragged = divmod(len(outs), 2)
  assert not ragged
  out_primals, out_tangents = outs[:m], outs[m:]
  yield (*err_primals, *out_primals, *err_tangents, *out_tangents), out_tree

@lu.transformation_with_aux
def checkify_custom_vjp_subtrace(main, err_tree, num_error_vals, *args):
  del main
  # We don't add any checks; just drop input error values.
  _, args = split_list(args, [num_error_vals])
  outs = yield args, {}
  yield outs, err_tree

@lu.transformation_with_aux
def query_error_effects(*args):
  (error, *outs) = yield args, {}
  yield (error, *outs), set(error._pred.keys())

def checkify_jaxpr(jaxpr, error,
                   enabled_errors) -> Tuple[core.ClosedJaxpr,
                                            Tuple[PyTreeDef,
                                                  FrozenSet[ErrorEffect]]]:
  f = lu.wrap_init(core.jaxpr_as_fun(jaxpr))
  return checkify_fun_to_jaxpr(f, error, enabled_errors, jaxpr.in_avals)

def checkify_fun_to_jaxpr(
    f, error, enabled_errors,
    in_avals) -> Tuple[core.ClosedJaxpr, Tuple[PyTreeDef, FrozenSet[ErrorEffect]]]:
  flat_error_vals, in_tree = tree_flatten(error)
  f = checkify_subtrace(f)
  f = checkify_traceable(f, enabled_errors)
  f, error_effect = query_error_effects(f)
  in_tree = jtu.tree_structure((error, *in_avals))
  f, out_tree = flatten_fun_nokwargs(f, in_tree)
  err_vals = map(lambda x: core.raise_to_shaped(core.get_aval(x)),
                 flat_error_vals)
  avals_in = [*err_vals, *in_avals]
  jaxpr_out, _, literals_out = pe.trace_to_jaxpr_dynamic(f, avals_in)
  return (core.ClosedJaxpr(jaxpr_out, literals_out), (out_tree(), error_effect()))



def check(pred: Bool, msg: str, *fmt_args, **fmt_kwargs) -> None:
  """Check a predicate, add an error with msg if predicate is False.

  This is an effectful operation, and can't be staged (jitted/scanned/...).
  Before staging a function with checks, :func:`~checkify` it!

  Args:
    pred: if False, an error is added.
    msg: error message if error is added. Can be a format string.
    fmt_args, fmt_kwargs: Positional and keyword formatting arguments for
      `msg`, eg.:
      ``check(.., "check failed on values {} and {named_arg}", x, named_arg=y)``
      Note that these arguments can be traced values allowing you to add
      run-time values to the error message.
      Note that tracking these run-time arrays will increase your memory usage,
      even if no error happens.

  For example:

    >>> import jax
    >>> import jax.numpy as jnp
    >>> from jax.experimental import checkify
    >>> def f(x):
    ...   checkify.check(x>0, "{x} needs to be positive!", x=x)
    ...   return 1/x
    >>> checked_f = checkify.checkify(f)
    >>> err, out = jax.jit(checked_f)(-3.)
    >>> err.throw()  # doctest: +IGNORE_EXCEPTION_DETAIL
    Traceback (most recent call last):
      ...
    jax._src.checkify.JaxRuntimeError: -3. needs to be positive!

  """
  if not is_scalar_pred(pred):
    raise TypeError(f'check takes a scalar pred as argument, got {pred}')
  new_error = FailedCheckError(summary(), msg, *fmt_args, **fmt_kwargs)
  error = assert_func(init_error, jnp.logical_not(pred), new_error)
  return check_error(error)


def is_scalar_pred(pred) -> bool:
  return (isinstance(pred, bool) or
          isinstance(pred, jnp.ndarray) and pred.shape == () and
          pred.dtype == jnp.dtype('bool'))

def check_error(error: Error) -> None:
  """Raise an Exception if ``error`` represents a failure. Functionalized by :func:`~checkify`.

  The semantics of this function are equivalent to:

  >>> def check_error(err: Error) -> None:
  ...   err.throw()  # can raise ValueError

  But unlike that implementation, ``check_error`` can be functionalized using
  the :func:`~checkify` transformation.

  This function is similar to :func:`~check` but with a different signature: whereas
  :func:`~check` takes as arguments a boolean predicate and a new error message
  string, this function takes an ``Error`` value as argument. Both :func:`~check`
  and this function raise a Python Exception on failure (a side-effect), and
  thus cannot be staged out by :func:`~jax.jit`, :func:`~jax.pmap`,
  :func:`~jax.lax.scan`, etc. Both also can
  be functionalized by using :func:`~checkify`.

  But unlike :func:`~check`, this function is like a direct inverse of
  :func:`~checkify`:
  whereas :func:`~checkify` takes as input a function which
  can raise a Python
  Exception and produces a new function without that effect but which produces
  an ``Error`` value as output, this ``check_error`` function can accept an
  ``Error`` value as input and can produce the side-effect of raising an
  Exception. That is, while :func:`~checkify` goes from
  functionalizable Exception
  effect to error value, this ``check_error`` goes from error value to
  functionalizable Exception effect.

  ``check_error`` is useful when you want to turn checks represented by an
  ``Error`` value (produced by functionalizing ``checks`` via
  :func:`~checkify`) back into Python Exceptions.

  Args:
    error: Error to check.

  For example, you might want to functionalize part of your program through
  checkify, stage out your functionalized code through :func:`~jax.jit`, then
  re-inject your error value outside of the :func:`~jax.jit`:

  >>> import jax
  >>> from jax.experimental import checkify
  >>> def f(x):
  ...   checkify.check(x>0, "must be positive!")
  ...   return x
  >>> def with_inner_jit(x):
  ...   checked_f = checkify.checkify(f)
  ...   # a checkified function can be jitted
  ...   error, out = jax.jit(checked_f)(x)
  ...   checkify.check_error(error)
  ...   return out
  >>> _ = with_inner_jit(1)  # no failed check
  >>> with_inner_jit(-1)  # doctest: +IGNORE_EXCEPTION_DETAIL
  Traceback (most recent call last):
    ...
  jax._src.JaxRuntimeError: must be positive!
  >>> # can re-checkify
  >>> error, _ = checkify.checkify(with_inner_jit)(-1)
  """
  if not isinstance(error, Error):
    raise ValueError('check_error takes an Error as argument, '
                     f'got type {type(error)} instead.')

  error = tree_map(core.raise_as_much_as_possible, error)
  if any(map(np.shape, error._pred.values())):
    error = _reduce_any_error(error)
  err_args, tree_def = tree_flatten(error)

  return check_p.bind(*err_args, err_tree=tree_def)

## check primitive

check_p = core.Primitive('check')
check_p.multiple_results = True  # zero results

# TODO(lenamartens): inherit from Exception instead of ValueError.
class JaxRuntimeError(ValueError):
  pass

@check_p.def_impl
def check_impl(*args, err_tree):
  error = tree_unflatten(err_tree, args)
  exc = error.get_exception()
  if exc:
    raise JaxRuntimeError(str(exc)) from exc
  return []

@check_p.def_effectful_abstract_eval
def check_abstract_eval(*args, err_tree):
  return [], set(tree_unflatten(err_tree, args)._pred.keys())

# TODO(lenamartens) add in-depth error explanation to link to in module docs.
functionalization_error = ValueError(
    'Cannot abstractly evaluate a checkify.check which was not'
    ' functionalized. This probably means you tried to stage'
    ' (jit/scan/pmap/...) a `check` without functionalizing it'
    ' through `checkify.checkify`.'
    )

def check_lowering_rule(ctx, *args, err_tree):
  if not config.jax_experimental_unsafe_xla_runtime_errors:
    raise functionalization_error

  out_op, _, keep_alive = mlir.emit_python_callback(
      ctx, callback=functools.partial(python_err, err_tree),
      token=None,
      operands=args,
      operand_avals=list(ctx.avals_in),
      result_avals=list(ctx.avals_out),
      has_side_effect=True)
  ctx.module_context.add_keepalive(keep_alive)
  return out_op

def check_lowering_rule_unsupported(*a, **k):
  raise functionalization_error

def python_err(err_tree, *args):
  error = tree_unflatten(err_tree, args)
  check_error(error)
  return []

mlir.register_lowering(check_p, check_lowering_rule_unsupported,
                       platform='tpu')
mlir.register_lowering(check_p, check_lowering_rule,
                       platform='cpu')
mlir.register_lowering(check_p, check_lowering_rule,
                       platform='gpu')

def check_batching_rule(batched_args, batch_dims, *, err_tree):
  size = next(x.shape[dim] for x, dim in zip(batched_args, batch_dims)
              if dim is not batching.not_mapped)
  batched_args = (batching.bdim_at_front(a, d, size)
                  for a, d in zip(batched_args, batch_dims))
  err = tree_unflatten(err_tree, batched_args)
  check_error(err)
  return [], []

batching.primitive_batchers[check_p] = check_batching_rule

def check_jvp_rule(primals, _, *, err_tree):
  # Check primals, discard tangents.
  check_p.bind(*primals, err_tree=err_tree)
  return [], []

ad.primitive_jvps[check_p] = check_jvp_rule

## checkify rules

def _get_current_traceback(skip_frames = 0) -> Optional[types.TracebackType]:
  # TODO(lenamartens): use c++ version from XLA?
  tb = None
  import inspect
  for frame_info in inspect.stack():
    frame = frame_info.frame
    if skip_frames:
      skip_frames -= 1
    elif not traceback_util.include_frame(frame):
      continue
    else:
      tb = types.TracebackType(tb, frame, frame.f_lasti, frame.f_lineno)
  return tb

def summary() -> str:
  return str(source_info_util.summarize(source_info_util.current()))

def nan_error_check(prim, error, enabled_errors, *in_vals, **params):
  out = prim.bind(*in_vals, **params)
  err = check_nans(prim, error, enabled_errors, out)
  return out, err

def check_nans(prim, error, enabled_errors, out):
  if NaNError not in enabled_errors:
    return error

  def isnan(x):
    if isinstance(x, prng.PRNGKeyArray):
      return False
    return jnp.any(jnp.isnan(x))

  any_nans = (jnp.any(jnp.array([isnan(x) for x in out]))
              if prim.multiple_results else isnan(out))
  return assert_func(error, any_nans, NaNError(summary(), prim.name))


# All primitives which can generate a NaN.
nan_primitives = [lax.acos_p, lax.acosh_p, lax.add_p, lax.asin_p, lax.asinh_p,
                  lax.atan2_p, lax.atan_p, lax.atanh_p, lax.bessel_i0e_p,
                  lax.bessel_i1e_p, lax.cbrt_p, lax.conv_general_dilated_p,
                  lax.cos_p, lax.cosh_p, lax.cumlogsumexp_p, lax.cummax_p,
                  lax.cummin_p, lax.cumprod_p, lax.cumsum_p, lax.digamma_p,
                  lax.dot_general_p, lax.erf_inv_p, lax.erf_p, lax.erfc_p,
                  lax.exp_p, lax.expm1_p, lax.fft_p, lax.igamma_grad_a_p,
                  lax.igamma_p, lax.igammac_p, lax.integer_pow_p, lax.lgamma_p,
                  lax.linear_solve_p, lax.log1p_p, lax.log_p, lax.logistic_p,
                  lax.mul_p, lax.pad_p, lax.pow_p, lax.psum_p,
                  lax.random_gamma_grad_p, lax.reduce_p, lax.reduce_prod_p,
                  lax.reduce_sum_p, lax.reduce_window_p,
                  lax.reduce_window_sum_p, lax.regularized_incomplete_beta_p,
                  lax.rem_p, lax.rng_uniform_p, lax.rsqrt_p, lax.sin_p,
                  lax.sinh_p, lax.sqrt_p, lax.sub_p, lax.tan_p, lax.tanh_p]

for prim in nan_primitives:
  error_checks[prim] = functools.partial(nan_error_check, prim)


def gather_error_check(error, enabled_errors, operand, start_indices, *,
                       dimension_numbers, slice_sizes, unique_indices,
                       indices_are_sorted, mode, fill_value):
  out = lax.gather_p.bind(
      operand, start_indices, dimension_numbers=dimension_numbers,
      slice_sizes=slice_sizes, unique_indices=unique_indices,
      indices_are_sorted=indices_are_sorted, mode=mode, fill_value=fill_value)

  if OOBError not in enabled_errors:
    return out, error

  # compare to OOB masking logic in lax._gather_translation_rule
  dnums = dimension_numbers
  operand_dims = np.array(operand.shape)
  num_batch_dims = len(start_indices.shape) - 1

  upper_bound = operand_dims[np.array(dnums.start_index_map)]
  upper_bound -= np.array(slice_sizes)[np.array(dnums.start_index_map)]
  upper_bound = jnp.expand_dims(upper_bound, axis=tuple(range(num_batch_dims)))
  oob_mask = (start_indices < 0) | (start_indices > upper_bound.astype(start_indices.dtype))

  payload = oob_payload(oob_mask, start_indices, dnums.start_index_map, operand.shape)
  return out, assert_func(error, jnp.any(oob_mask), OOBError(summary(), "gather", operand.shape, payload))
error_checks[lax.gather_p] = gather_error_check

def div_error_check(error, enabled_errors, x, y):
  """Checks for division by zero and NaN."""
  if DivisionByZeroError in enabled_errors:
    any_zero = jnp.any(jnp.equal(y, 0))
    error = assert_func(error, any_zero, DivisionByZeroError(summary()))
  return nan_error_check(lax.div_p, error, enabled_errors, x, y)
error_checks[lax.div_p] = div_error_check

def oob_payload(oob_mask, indices, dims_map, operand_shape):
  # Get first OOB index, axis and axis size so it can be added to the error msg.
  flat_idx = jnp.argmin(jnp.logical_not(oob_mask))
  multi_idx = jnp.unravel_index(flat_idx, indices.shape)
  oob_axis = jnp.array(dims_map)[multi_idx[-1]]
  oob_axis_size = jnp.array(operand_shape)[oob_axis]
  oob_index = jnp.ravel(indices)[flat_idx]
  payload = jnp.array([oob_index, oob_axis, oob_axis_size], dtype=jnp.int32)
  return payload

def scatter_oob(operand, indices, updates, dnums):
  # Ref: see clamping code used in scatter_translation_rule
  slice_sizes = []
  pos = 0
  for i in range(len(operand.shape)):
    if i in dnums.inserted_window_dims:
      slice_sizes.append(1)
    else:
      slice_sizes.append(updates.shape[dnums.update_window_dims[pos]])
      pos += 1

  upper_bound = np.array([operand.shape[i] - slice_sizes[i]
                          for i in dnums.scatter_dims_to_operand_dims],
                         np.int64)
  upper_bound = np.minimum(upper_bound, np.iinfo(indices.dtype).max)
  upper_bound = lax.broadcast_in_dim(upper_bound, indices.shape,
                                     (len(indices.shape) - 1,))

  lower_oob = jnp.less(indices, 0)
  upper_oob = jnp.greater(indices, upper_bound.astype(indices.dtype))
  oob_mask = jnp.logical_or(lower_oob, upper_oob)
  payload = oob_payload(oob_mask, indices,
                        dnums.scatter_dims_to_operand_dims, operand.shape)
  return jnp.any(oob_mask), payload

def scatter_error_check(prim, error, enabled_errors, operand, indices, updates,
                        *, update_jaxpr, update_consts, dimension_numbers,
                        indices_are_sorted, unique_indices, mode):
  """Checks if indices are within bounds and update does not generate NaN."""
  out = prim.bind(
      operand, indices, updates, update_jaxpr=update_jaxpr,
      update_consts=update_consts, dimension_numbers=dimension_numbers,
      indices_are_sorted=indices_are_sorted, unique_indices=unique_indices,
      mode=mode)

  if OOBError not in enabled_errors:
    return out, error

  out_of_bounds, payload = scatter_oob(operand, indices, updates, dimension_numbers)
  oob_error = OOBError(summary(), prim.name, operand.shape, payload)
  error = assert_func(error, out_of_bounds, oob_error)
  return out, check_nans(prim, error, enabled_errors, out)
error_checks[lax.scatter_p] = functools.partial(scatter_error_check, lax.scatter_p)
error_checks[lax.scatter_add_p] = functools.partial(scatter_error_check,
                                                    lax.scatter_add_p)
error_checks[lax.scatter_mul_p] = functools.partial(scatter_error_check,
                                                    lax.scatter_mul_p)
error_checks[lax.scatter_min_p] = functools.partial(scatter_error_check,
                                                    lax.scatter_min_p)
error_checks[lax.scatter_max_p] = functools.partial(scatter_error_check,
                                                    lax.scatter_max_p)

def cond_error_check(error, enabled_errors, index, *ops, branches, linear):
  _, out_trees_and_effects = unzip2(checkify_jaxpr(jxpr, error,
                                                   enabled_errors)
                                    for jxpr in branches)
  _, effects = unzip2(out_trees_and_effects)

  merged_error = error._add_placeholder_effects(set().union(*effects))
  new_branches, out_trees_and_effects = unzip2(checkify_jaxpr(jxpr, merged_error,
                                                              enabled_errors)
                                               for jxpr in branches)
  out_trees, _ = unzip2(out_trees_and_effects)

  flat_error, _ = tree_flatten(merged_error)
  new_linear = (*[False] * len(flat_error), *linear)
  err_and_outs = lax.cond_p.bind(
      index, *flat_error, *ops,
      branches=tuple(new_branches), linear=new_linear)

  # we need to merge metadata across out_trees (a tuple)
  # maybe there's a better way to do this, but we can use the outs
  # to unflatten all trees.
  err0, *out = tree_unflatten(out_trees[0], err_and_outs)
  merged_metadata = err0._metadata
  for tr in out_trees[1:]:
    err, *_ = tree_unflatten(tr, err_and_outs)
    merged_metadata = {**merged_metadata, **err._metadata}
  return out, err0._replace(_metadata=merged_metadata)
error_checks[lax.cond_p] = cond_error_check

def scan_error_check(error, enabled_errors, *in_flat, reverse, length, jaxpr,
                     num_consts, num_carry, linear, unroll):
  consts, carry, xs = split_list(in_flat, [num_consts, num_carry])
  _, (_, effects) = checkify_jaxpr(jaxpr, error, enabled_errors)
  merged_error = error._add_placeholder_effects(effects)
  checked_jaxpr_, (out_tree, _) = checkify_jaxpr(jaxpr, merged_error, enabled_errors)

  flat_error_vals, _ = tree_flatten(merged_error)
  tomove = [False] * len(flat_error_vals) + [True] * len(consts) + [False] * (len(carry) + len(xs))
  checked_jaxpr = pe.move_binders_to_front(checked_jaxpr_, tomove)
  new_linear = (*[False] * len(flat_error_vals), *linear)
  new_in_flat = [*consts, *flat_error_vals, *carry, *xs]
  err_and_out = lax.scan_p.bind(
      *new_in_flat, reverse=reverse, length=length, jaxpr=checked_jaxpr,
      num_consts=len(consts), num_carry=len(carry)+len(flat_error_vals),
      linear=new_linear, unroll=unroll)
  err, *out = tree_unflatten(out_tree, err_and_out)
  return out, err

error_checks[lax.scan_p] = scan_error_check

def checkify_while_body_jaxpr(cond_jaxpr, body_jaxpr, error, enabled_errors, c_consts):
  cond_f = core.jaxpr_as_fun(cond_jaxpr)
  body_f = core.jaxpr_as_fun(body_jaxpr)
  def new_body_f(*vals):
    out = body_f(*vals)
    # This checks if the next cond application will error
    _ = cond_f(*c_consts, *out)
    return out
  return checkify_fun_to_jaxpr(lu.wrap_init(new_body_f), error, enabled_errors,
                               body_jaxpr.in_avals)

def ignore_error_output_jaxpr(jaxpr, num_error_vals):
  """Constructs a checked jaxpr which does not output its error value."""
  consts = jaxpr.consts
  jaxpr = jaxpr.jaxpr
  new_jaxpr = jaxpr.replace(outvars=jaxpr.outvars[num_error_vals:])
  return core.ClosedJaxpr(new_jaxpr, consts)

def while_loop_error_check(error, enabled_errors, *in_flat, cond_nconsts,
                           cond_jaxpr, body_nconsts, body_jaxpr):
  if cond_jaxpr.out_avals[0].shape:
    # TODO(lenamartens, sharadmv): support batched while.
    raise ValueError('Checkify does not support batched while-loops '
                     '(checkify-of-vmap-of-while). \nHint: if possible, move '
                     'the vmap to the outer level to get '
                     'vmap-of-checkify-of-while.')

  err_vals, _ = tree_flatten(error)
  c_consts, b_consts, carry = split_list(in_flat, [cond_nconsts, body_nconsts])

  # Check if the first cond application will error.
  checked_cond_jaxpr, (cond_out_tree, _) = checkify_jaxpr(
      cond_jaxpr, error, enabled_errors)
  outs = core.jaxpr_as_fun(checked_cond_jaxpr)(*err_vals, *c_consts, *carry)
  error, _ = tree_unflatten(cond_out_tree, outs)

  checked_body_jaxpr_, (_, error_effects) = checkify_while_body_jaxpr(
      cond_jaxpr, body_jaxpr, error, enabled_errors, c_consts)
  # merged error!
  error = error._add_placeholder_effects(error_effects)
  checked_body_jaxpr_, (body_out_tree, _) = checkify_while_body_jaxpr(
      cond_jaxpr, body_jaxpr, error, enabled_errors, c_consts)
  err_vals = jtu.tree_leaves(error)
  num_error_vals = len(err_vals)
  to_move = [False] * num_error_vals + [True] * body_nconsts + [False] * len(carry)
  checked_body_jaxpr = pe.move_binders_to_front(checked_body_jaxpr_, to_move)

  checked_cond_jaxpr, _ = checkify_jaxpr(cond_jaxpr, error, enabled_errors)
  compat_cond_jaxpr_ = ignore_error_output_jaxpr(checked_cond_jaxpr, num_error_vals)
  to_move = [False] * num_error_vals + [True] * cond_nconsts + [False] * len(carry)
  compat_cond_jaxpr = pe.move_binders_to_front(compat_cond_jaxpr_, to_move)
  new_in_flat = [*c_consts, *b_consts, *err_vals, *carry]

  all_out_vals = lax.while_p.bind(
      *new_in_flat, cond_nconsts=cond_nconsts, cond_jaxpr=compat_cond_jaxpr,
      body_nconsts=body_nconsts, body_jaxpr=checked_body_jaxpr)
  # body_out_tree will have all the metadata of cond because it executes a cond!
  # only need to merge metadata on the input error.
  error, *out = tree_unflatten(body_out_tree, all_out_vals)
  return out, error
error_checks[lax.while_p] = while_loop_error_check


def pjit_error_check(error, enabled_errors, *vals_in, jaxpr,
                     in_shardings, out_shardings, resource_env,
                     donated_invars, name,
                     in_positional_semantics, out_positional_semantics):
  checked_jaxpr, (out_tree, effects) = checkify_jaxpr(jaxpr, error,
                                                      enabled_errors)
  out_error = error._add_placeholder_effects(effects)

  flat_error_vals = jtu.tree_leaves(error)
  num_error_vals = len(flat_error_vals)
  new_vals_in = [*flat_error_vals, *vals_in]

  sharding = OpShardingSharding.get_replicated(
      list(resource_env.physical_mesh.devices.flat))
  new_in_shardings = (*[sharding] * num_error_vals, *in_shardings)
  new_out_shardings = (*[sharding] * len(jtu.tree_leaves(out_error)),
                       *out_shardings)

  if config.jax_array:
    pos_sem = maps._PositionalSemantics.GLOBAL
  else:
    pos_sem = maps._positional_semantics.val

  if not isinstance(in_positional_semantics, Iterable):
    in_positional_semantics = (in_positional_semantics,)
  if not isinstance(out_positional_semantics, Iterable):
    out_positional_semantics = (out_positional_semantics,)
  new_positional_sems_in = (*[pos_sem] * num_error_vals,
                            *in_positional_semantics)
  new_positional_sems_out = (*[pos_sem] * num_error_vals,
                             *out_positional_semantics)
  new_donated_invars = (*[False] * num_error_vals, *donated_invars)

  err_and_out = pjit.pjit_p.bind(
      *new_vals_in,
      jaxpr=checked_jaxpr,
      in_shardings=new_in_shardings,
      out_shardings=new_out_shardings,
      resource_env=resource_env,
      donated_invars=new_donated_invars,
      name=name,
      in_positional_semantics=new_positional_sems_in,
      out_positional_semantics=new_positional_sems_out)
  err, *out = tree_unflatten(out_tree, err_and_out)
  return out, err
error_checks[pjit.pjit_p] = pjit_error_check


def check_discharge_rule(error, enabled_errors, *args, err_tree):
  new_error = tree_unflatten(err_tree, args)
  # Split up new_error into error to be functionalized if it's included in
  # enabled_errors (=discharged_error) and an error to be defunctionalized if
  # it's not included (=recharged_error)
  discharged_error = error
  recharged_error = init_error
  for error_effect in new_error._pred.keys():
    pred = new_error._pred[error_effect]
    code = new_error._code[error_effect]
    payload = new_error._payload[error_effect]
    if error_effect.error_type in enabled_errors:
      discharged_error = update_error(discharged_error, pred, code, {}, payload,
                                      error_effect)
    else:
      recharged_error = update_error(recharged_error, pred, code, {}, payload,
                                     error_effect)

  discharged_error = discharged_error._replace(
      _metadata={**new_error._metadata, **discharged_error._metadata})
  recharged_error = recharged_error._replace(_metadata=new_error._metadata)
  # TODO(lenamartens): we actually need to recharge, but this would be a
  # breaking API change so leaving for a follow-up.
  # check_error(recharged_error)
  return [], discharged_error
error_checks[check_p] = check_discharge_rule


## checkify api

user_checks = frozenset({FailedCheckError})
nan_checks = frozenset({NaNError})
index_checks = frozenset({OOBError})
div_checks = frozenset({DivisionByZeroError})
float_checks = nan_checks | div_checks
automatic_checks = float_checks | index_checks
all_checks = automatic_checks | user_checks

Out = TypeVar('Out')


def checkify(fun: Callable[..., Out],
             errors: FrozenSet[ErrorCategory] = user_checks
             ) -> Callable[..., Tuple[Error, Out]]:
  """Functionalize `check` calls in `fun`, and optionally add run-time error checks.

  Run-time errors are either user-added :func:`~check` assertions, or
  automatically added checks like NaN checks, depending on the ``errors``
  argument.

  The returned function will return an Error object `err` along with the output
  of the original function. ``err.get()`` will either return ``None`` (if no
  error occurred) or a string containing an error message. This error message
  will correspond to the first error which occurred. ``err.throw()`` will raise
  a ValueError with the error message if an error occurred.

  By default only user-added :func:`~check` assertions are enabled. You can
  enable automatic checks through the ``errors`` argument.

  The automatic check sets which can be enabled, and when an error is generated:
    - ``user_checks``: a :func:`~check` evaluated to False.
    - ``nan_checks``: a floating-point operation generated a NaN value
      as output.
    - ``div_checks``: a division by zero.
    - ``index_checks``: an index was out-of-bounds.

  Multiple categories can be enabled together by passing in an error `Set` (eg.
  ``errors=nan_checks``). Multiple sets can be re-combined (eg.
  ``errors=float_checks|user_checks``)

  Args:
    fun: Callable which can contain user checks (see :func:`~check`).
    errors: A set of ErrorCategory values which defines the set of enabled
      checks. By default only explicit ``checks`` are enabled
      (``user_checks``). You can also for example enable NAN and
      DIV errors by passing the ``float_checks`` set, or for
      example combine multiple sets through set operations
      (``float_checks | user_checks``)
  Returns:
    A function which accepts the same arguments as ``fun`` and returns as output
    a pair where the first element is an ``Error`` value, representing the first
    failed :func:`~check`, and the second element is the original output of
    ``fun``.

  For example:

    >>> import jax
    >>> import jax.numpy as jnp
    >>> from jax.experimental import checkify
    >>>
    >>> @jax.jit
    ... def f(x):
    ...   y = jnp.sin(x)
    ...   return x+y
    >>> err, out = checkify.checkify(f, errors=checkify.float_checks)(jnp.inf)
    >>> err.throw()  # doctest: +IGNORE_EXCEPTION_DETAIL
    Traceback (most recent call last):
      ...
    jax._src.checkify.JaxRuntimeError: nan generated by primitive: sin
  """
  @traceback_util.api_boundary
  def checked_fun(*args, **kwargs):
    args_flat, in_tree = tree_flatten((args, kwargs))
    f, out_tree = flatten_fun(lu.wrap_init(fun), in_tree)
    error, out_flat = checkify_flat(f, errors, *args_flat)
    out = tree_unflatten(out_tree(), out_flat)
    return error, out
  return checked_fun