feat(extensions): add subscript_operator and index_of functions

extensions/functions_list.yaml

@@ -130,3 +130,48 @@ scalar_functions:
             value: func<any1 -> boolean?>
         nullability: DECLARED_OUTPUT
         return: boolean?
+
+  - name: "element_at"
+    description: >-
+      Returns the element at the specified index from the list.
+
+      Index is 1-based (i.e., the first element is at index 1). Zero is not
+      a valid index. Handling of negative indices depends on the
+      negative_index_semantics argument.
+
+      If the input list is null, the result is null. If the index is null,
+      the result is null. If the element at the specified index is null,
+      the result is null.
+    impls:
+      - args:
+          - name: input
+            value: list<any1>
+          - name: index
+            value: i64
+          - name: negative_index_semantics
+            description: >-
+              How to interpret negative index values.
+
+              * WRAP_AROUND: -1 refers to the last element, -2 to second-to-last, etc.
+              Negative indices whose absolute value exceeds the list length are still
+              out of bounds (e.g., -6 on a 5-element list is out of bounds, not wrapped).
+
+              * OUT_OF_BOUNDS: All negative indices are treated as out of bounds.
+            options: [WRAP_AROUND, OUT_OF_BOUNDS]
+        options:
+          on_out_of_bounds:
+            description: >-
+              Behavior when the index is out of bounds (index is 0, positive index > list length,
+              or for WRAP_AROUND negative index whose absolute value > list length,
+              or for OUT_OF_BOUNDS any negative index).
+
+              * RETURN_NULL: Return null for out-of-bounds indices.
+
+              * ERROR: Raise an error for out-of-bounds indices.
+
+              * ERROR_ON_ZERO_ONLY: Raise an error if the index is 0, but return null for
+              other out-of-bounds indices. This matches Trino's behavior which treats index 0
+              as a programmer error (wrong indexing convention) rather than a simple out-of-bounds.
+            values: [RETURN_NULL, ERROR, ERROR_ON_ZERO_ONLY]
+        nullability: DECLARED_OUTPUT
+        return: any1?

tests/cases/list/element_at.test

@@ -0,0 +1,52 @@
+### SUBSTRAIT_SCALAR_TEST: v1.0
+### SUBSTRAIT_INCLUDE: '/extensions/functions_list.yaml'
+
+# basic: Basic positive index (1-based)
+element_at([10, 20, 30]::list<i32>, 1::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = 10::i32?
+element_at([10, 20, 30]::list<i32>, 2::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = 20::i32?
+element_at([10, 20, 30]::list<i32>, 3::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = 30::i32?
+element_at(['a', 'b', 'c']::list<string>, 2::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = 'b'::string?
+
+# wrap_around: Negative indices count from end with WRAP_AROUND
+element_at([10, 20, 30]::list<i32>, -1::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = 30::i32?
+element_at([10, 20, 30]::list<i32>, -2::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = 20::i32?
+element_at([10, 20, 30]::list<i32>, -3::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = 10::i32?
+
+# out_of_bounds_negative: Negative indices are out of bounds with OUT_OF_BOUNDS
+element_at([10, 20, 30]::list<i32>, -1::i64, OUT_OF_BOUNDS::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+element_at([10, 20, 30]::list<i32>, -3::i64, OUT_OF_BOUNDS::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+
+# out_of_bounds_positive: Positive index exceeds list length
+element_at([10, 20, 30]::list<i32>, 5::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+element_at([10, 20, 30]::list<i32>, 100::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+
+# out_of_bounds_wrap_around: Negative index exceeds list length with WRAP_AROUND
+element_at([10, 20, 30]::list<i32>, -5::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+
+# zero_out_of_bounds: Zero index is out of bounds
+element_at([10, 20, 30]::list<i32>, 0::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+element_at([10, 20, 30]::list<i32>, 0::i64, OUT_OF_BOUNDS::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+
+# null_list: Null list returns null
+element_at(null::list?<i32>, 1::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+
+# null_index: Null index returns null
+element_at([10, 20, 30]::list<i32>, null::i64?, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+
+# null_element: Retrieving a null element returns null
+element_at([1, null, 3]::list<i32?>, 2::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+
+# empty_list: Out of bounds on empty list
+element_at([]::list<i32>, 1::i64, WRAP_AROUND::enum) [on_out_of_bounds:RETURN_NULL] = null::i32?
+
+# error_on_out_of_bounds: ERROR option raises error for out-of-bounds indices
+element_at([10, 20, 30]::list<i32>, 0::i64, WRAP_AROUND::enum) [on_out_of_bounds:ERROR] = <!ERROR>
+element_at([10, 20, 30]::list<i32>, 5::i64, WRAP_AROUND::enum) [on_out_of_bounds:ERROR] = <!ERROR>
+element_at([10, 20, 30]::list<i32>, -1::i64, OUT_OF_BOUNDS::enum) [on_out_of_bounds:ERROR] = <!ERROR>
+element_at([]::list<i32>, 1::i64, WRAP_AROUND::enum) [on_out_of_bounds:ERROR] = <!ERROR>
+
+# error_on_zero_only: ERROR_ON_ZERO_ONLY errors on index 0, returns null otherwise (Trino behavior)
+element_at([10, 20, 30]::list<i32>, 0::i64, WRAP_AROUND::enum) [on_out_of_bounds:ERROR_ON_ZERO_ONLY] = <!ERROR>
+element_at([10, 20, 30]::list<i32>, 5::i64, WRAP_AROUND::enum) [on_out_of_bounds:ERROR_ON_ZERO_ONLY] = null::i32?
+element_at([10, 20, 30]::list<i32>, -5::i64, WRAP_AROUND::enum) [on_out_of_bounds:ERROR_ON_ZERO_ONLY] = null::i32?
+element_at([10, 20, 30]::list<i32>, 2::i64, WRAP_AROUND::enum) [on_out_of_bounds:ERROR_ON_ZERO_ONLY] = 20::i32?