Merge pull request #257 from patrick-kidger/better-rk

Better rk

.flake8

@@ -1,4 +1,4 @@
 [flake8]
 max-line-length = 88
-ignore = W291,W503,W504,E121,E123,E126,E203,E402,E701,E702,E731
+ignore = W291,W503,W504,E121,E123,E126,E203,E402,E701,E702,E731,F722
 per-file-ignores = __init__.py: F401

.github/workflows/build_docs.yml

@@ -9,7 +9,7 @@ jobs:
   build:
     strategy:
       matrix:
-        python-version: [ 3.8 ]
+        python-version: [ 3.11 ]
         os: [ ubuntu-latest ]
     runs-on: ${{ matrix.os }}
     steps:

.github/workflows/release.yml

@@ -12,7 +12,7 @@ jobs:
       - name: Release
         uses: patrick-kidger/action_update_python_project@v1
         with:
-            python-version: "3.8"
+            python-version: "3.11"
             test-script: |
                 python -m pip install pytest psutil jax jaxlib equinox scipy optax
                 cp -r ${{ github.workspace }}/test ./test

.github/workflows/run_tests.yml

@@ -7,7 +7,7 @@ jobs:
   run-tests:
     strategy:
       matrix:
-        python-version: [ 3.8, 3.9 ]
+        python-version: [ 3.9, 3.11 ]
         os: [ ubuntu-latest ]
       fail-fast: false
     runs-on: ${{ matrix.os }}

README.md

@@ -21,7 +21,7 @@ _From a technical point of view, the internal structure of the library is pretty
 pip install diffrax
 ```
 
-Requires Python 3.8+, JAX 0.4.3+, and [Equinox](https://github.com/patrick-kidger/equinox) 0.10.0+.
+Requires Python 3.9+, JAX 0.4.4+, and [Equinox](https://github.com/patrick-kidger/equinox) 0.10.4+.
 
 ## Documentation
 

diffrax/__init__.py

@@ -31,6 +31,7 @@
 from .misc import adjoint_rms_seminorm
 from .nonlinear_solver import (
     AbstractNonlinearSolver,
+    AffineNonlinearSolver,
     NewtonNonlinearSolver,
     NonlinearSolution,
 )
@@ -60,14 +61,19 @@
     Heun,
     ImplicitEuler,
     ItoMilstein,
+    KenCarp3,
+    KenCarp4,
+    KenCarp5,
     Kvaerno3,
     Kvaerno4,
     Kvaerno5,
     LeapfrogMidpoint,
     Midpoint,
+    MultiButcherTableau,
     Ralston,
     ReversibleHeun,
     SemiImplicitEuler,
+    Sil3,
     StratonovichMilstein,
     Tsit5,
 )

diffrax/adjoint.py

@@ -14,7 +14,7 @@
 from .ad import implicit_jvp
 from .heuristics import is_sde, is_unsafe_sde
 from .saveat import save_y, SaveAt, SubSaveAt
-from .solver import AbstractItoSolver, AbstractStratonovichSolver
+from .solver import AbstractItoSolver, AbstractRungeKutta, AbstractStratonovichSolver
 from .term import AbstractTerm, AdjointTerm
 
 
@@ -332,6 +332,7 @@ class DirectAdjoint(AbstractAdjoint):
     def loop(
         self,
         *,
+        solver,
         max_steps,
         terms,
         throw,
@@ -362,10 +363,15 @@ def loop(
         else:
             kind = "bounded"
             msg = None
+        # Support forward-mode autodiff.
+        # TODO: remove this hack once we can JVP through custom_vjps.
+        if isinstance(solver, AbstractRungeKutta) and solver.scan_kind is None:
+            solver = eqx.tree_at(lambda s: s.scan_kind, solver, "bounded")
         inner_while_loop = ft.partial(_inner_loop, kind=kind)
         outer_while_loop = ft.partial(_outer_loop, kind=kind)
         final_state = self._loop(
             **kwargs,
+            solver=solver,
             max_steps=max_steps,
             terms=terms,
             inner_while_loop=inner_while_loop,
@@ -535,6 +541,8 @@ def _loop_backsolve_bwd(
     zeros_like_diff_args = jtu.tree_map(jnp.zeros_like, diff_args)
     zeros_like_diff_terms = jtu.tree_map(jnp.zeros_like, diff_terms)
     del diff_args, diff_terms
+    # TODO: have this look inside MultiTerms? Need to think about the math. i.e.:
+    # is_leaf=lambda x: isinstance(x, AbstractTerm) and not isinstance(x, MultiTerm)
     adjoint_terms = jtu.tree_map(
         AdjointTerm, terms, is_leaf=lambda x: isinstance(x, AbstractTerm)
     )
@@ -762,6 +770,11 @@ def loop(
                     "`BacksolveAdjoint` will only produce the correct solution for "
                     "Stratonovich SDEs."
                 )
+        if jtu.tree_structure(solver.term_structure) != jtu.tree_structure(0):
+            raise NotImplementedError(
+                "`diffrax.BacksolveAdjoint` is only compatible with solvers that take "
+                "a single term."
+            )
 
         y = init_state.y
         init_state = eqx.tree_at(lambda s: s.y, init_state, object())

diffrax/custom_types.py

@@ -1,7 +1,8 @@
 import inspect
 import typing
-from typing import Dict, Generic, Tuple, TypeVar, Union
+from typing import Any, Dict, Generic, Tuple, TypeVar, Union
 
+import equinox.internal as eqxi
 import jax.tree_util as jtu
 
 
@@ -129,3 +130,4 @@ def __class_getitem__(cls, item):
 
 DenseInfo = Dict[str, PyTree[Array]]
 DenseInfos = Dict[str, PyTree[Array["times", ...]]]  # noqa: F821
+sentinel: Any = eqxi.doc_repr(object(), "sentinel")