ScuDF doc updates

docs/api.rst

@@ -37,13 +37,16 @@ Backends
     akimbo.pandas.PandasAwkwardAccessor
     akimbo.dask.DaskAwkwardAccessor
     akimbo.polars.PolarsAwkwardAccessor
+    akimbo.cudf.CudfAwkwardAccessor
 
 .. autoclass:: akimbo.pandas.PandasAwkwardAccessor
 
 .. autoclass:: akimbo.dask.DaskAwkwardAccessor
 
 .. autoclass:: akimbo.polars.PolarsAwkwardAccessor
 
+.. autoclass:: akimbo.polars.CudfAwkwardAccessor
+
 
 Extensions
 ~~~~~~~~~~
@@ -63,3 +66,12 @@ to see the operations available.
 
     <script data-goatcounter="https://akimbo.goatcounter.com/count"
             async src="//gc.zgo.at/count.js"></script>
+
+The cuDF backend also has these implemented with GPU-specific variants,
+``akimbo.cudf.CudfStringAccessor`` and ``akimbo.cudf.CudfDatetimeAccessor``.
+
+Adding Extensions (advanced)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The patterns used in the two builtin extensions above can be used to add
+type-specific functionality to ``akimbo``. One 

src/akimbo/apply_tree.py

@@ -1,16 +1,15 @@
+from __future__ import annotations
+
 import functools
 import inspect
-from typing import Sequence
+from typing import Callable, Literal, Sequence
 
 import awkward as ak
 import pyarrow as pa
 
 
-class NoDtype:
-    kind = ""
-
-
 def leaf(*layout, **_):
+    """True for the lowest elements of any akwward layout tree"""
     return layout[0].is_leaf
 
 
@@ -42,8 +41,20 @@ def func(layout, **kwargs):
     return ak.transform(func, arr, *others)
 
 
-def dec(func, match=leaf, outtype=None, inmode="arrow"):
-    """Make a nested/ragged version of an operation to apply throughout a tree"""
+def dec(func: callable, match: Callable[[ak.contents.Content], bool] = leaf,
+        outtype: Callable[[ak.contents.Content], ak.contents.Content] | None = None,
+        inmode: Literal["arrow", "numpy", "ak"] = "arrow"):
+
+    """Make a nested/ragged version of an operation to apply throughout a tree
+
+    Parameters
+    ----------
+    func: which we want to apply to (parts of) inputted data
+    match: function to determine if a part of the data structure matches the type we want to
+        operate on
+    outtype: postprocessing function after transform
+    inmode: how ``func`` expects its inputs: as awkward arrays (ak), numpy or arrow
+    """
 
     @functools.wraps(func)
     def f(self, *args, where=None, match_kwargs=None, **kwargs):