add rough docs for unevaluatedItems

source/reference/array.rst

@@ -236,7 +236,193 @@ Unevaluated Items
 
 |draft2019-09|
 
-Documentation Coming Soon
+The ``unevaluatedItems`` keyword applies to any values not evaluated
+by an ``items``, ``prefixItems``, or ``contains`` keyword. Just as
+``unevaluatedProperties`` affects only **properties** in an object,
+``unevaluatedItems`` affects only **item**-related keywords.
+
+For this first example, let's assume you want to allow lists of items
+that begin with either ``1`` or ``"A"``, but anything after must be ``2``.
+
+.. schema_example::
+
+    {
+      "oneOf": [{"prefixItems": [{"const": 1}]}, {"prefixItems": [{"const": "a"}]}],
+      "items": {"const": 2}
+    }
+
+The logic here seems like it should be "one of either ``1`` or ``"A"``
+and then ``2``." Actually, it's "either ``1`` or ``"A"`` *and also* ``2``."
+And ``items`` expects a ``2`` here, so anything that's not a ``2``
+fails validation. That's because ``items`` doesn't "see inside" any
+instances of ``oneOf``, ``anyOf``, or ``allOf`` in the same subschema.
+
+But if you replace ``items`` with ``unevaluatedItems``, it passes.
+Anything that starts with ``1`` or ``A`` and then continues with a ``2``
+is valid in that case.
+
+Because booleans are valid schemas for any JSON Schema keyword, you can
+also prevent additional items by setting ``unevaluatedItems`` to
+``false.`` In the next example, let's use ``unevaluatedItems`` to make
+sure we allow no properties besides ``SKU`` and ``product``.
+
+.. note::
+    Watch out! The word "unevaluated" *does not* mean "not evaluated by
+    ``items``, ``prefixItems``, or ``contains``." "Unevaluated" means
+    "not successfully evaluated", or "doesn't evaluate to true".
+
+.. schema_example::
+
+    {
+      "allOf": [
+        {
+          "type": "array",
+          "items": {
+            "properties": {
+              "SKU": "number",
+              "product": "string"
+            }
+          },
+          "unevaluatedItems": false
+        }
+      ],
+
+      "items": {
+        "properties": {
+          "quantity": { "enum": ["1", "2", "3"] }
+        },
+        "required": ["quantity"]
+      }
+    }
+
+This schema will always fail validation because ``quantity`` is required
+but it's outside the ``allOf``, and ``unevaluatedItems``
+*does not see any items outside its own subschema*. Here,
+``unevaluatedItems`` considers anything outside of ``SKU`` and ``product``
+to be additional.
+
+Instead, keep all your ``unevaluatedItems`` in the same subschema:
+
+.. schema_example::
+
+    {
+      "items": {
+        "properties": {
+          "SKU": "number",
+          "product": "string",
+          "quantity": { "enum": ["1", "2", "3"] }
+        },
+        "required": ["quantity"]
+      },
+      "unevaluatedItems": false
+    }
+
+Similarly, ``unevaluatedItems`` can't see inside cousins (vertically
+adjacent properties inside a separate pair of {curly braces} with the
+same "parent"— ``anyOf``, ``if``, ``not``, or similar). For instance,
+in the example below, the ``unevaluatedItems`` doesn't "see inside"
+the ``prefixItems`` cousin before it. All instances fail vallidation
+because ``"prefixItems": [ true ]`` matches only length 1 arrays, and
+``{ "unevaluatedItems": false }`` matches only empty arrays.
+
+.. schema_example::
+
+    {
+      "allOf": [
+        { "prefixItems": [true] },
+        { "unevaluatedItems": false }
+      ]
+    }
+    --X
+    [1]
+
+You can also use ``unevaluatedItems`` when you're `structuring`.
+Let's make a "half-closed" schema: something useful when you want to
+keep the first two arguments, but also add more in certain situations.
+("Closed" to two arguments in some places, "open" to more arguments
+when you need it to be.)
+
+.. schema_example::
+
+    {
+      "$id": "https://example.com/my-tuple",
+
+      "type": "array",
+      "prefixItems": [
+        true,
+        { "type": "boolean" }
+      ],
+
+      "$defs": {
+        "closed": {
+          "$anchor": "closed",
+          "$ref": "#",
+          "unevaluatedItems": false
+        }
+      }
+    }
+
+Then we can extend the tuple with another value:
+
+.. schema_example::
+
+    {
+      "$id": "https://example.com/my-extended-tuple",
+
+      "$ref": "https://example.com/my-tuple",
+      "prefixItems": [
+        true,
+        true,
+        { "type": "boolean" }
+      ],
+      "unevaluatedItems": false
+    }
+
+With this, you can use ``$ref`` to reference the first two
+``prefixItems`` and keep the schema "closed" to two arguments when
+you need it, "open" to more arguments when you need it. A reference to
+``/my-tuple#closed`` would disallow more than two items.
+``unevaluatedItems`` only sees inside its own subschema, so if you
+want to add an item, add it inside that subschema.
+
+This means you can also put ``unevaluatedItems`` in a nested tuple.
+
+.. schema_example::
+
+    {
+      "prefixItems": [
+        { "type": "string" }
+      ],
+      "allOf": [
+        {
+          "prefixItems": [
+            true,
+            { "type": "number" }
+          ]
+        }
+      ],
+      "unevaluatedItems": false
+    }
+    --
+    ["foo", 42]
+    --X
+    ["foo", 42, null]
+
+In the first test, all the ``data`` values are evaluated, but in the
+second test, a third value exists. ``prefixItems`` contrains only two
+items, and ``unevaluatedItems`` applies only to those two.
+
+.. note::
+   For a tall list of more examples, read our `unevaluatedItems Test Suite <https://github.com/json-schema-org/JSON-Schema-Test-Suite/blob/main/tests/draft2020-12/unevaluatedItems.json>`_ on GitHub. We test a lot of use cases
+   there, including uncommon ones.
+
+Here are some of our examples in the suite:
+   * ``unevaluatedItems`` nested inside another ``unevaluatedItems``
+   * ``if/then/else`` statements interacting with ``unevaluatedItems``
+   * nested ``if/then/else`` statements interacting with ``unevaluatedItems``
+   * ``unevaluatedItems`` ignoring non-arrays
+   * ``unevaluatedItems`` interacting with the ``not`` keyword
+   * and more
 
 .. index::
    single: array; contains