docs: broaden SQL lambda dialect coverage

Other dialects (ClickHouse, Snowflake, Databricks) also enable lambda
parsing via sqlparser-rs. Document the full set and recommend the
``lambda x: x`` keyword form, since DuckDB will drop the ``x -> x``
arrow form in v2.1. Parametrize the SQL test over the four dialects
using the keyword syntax.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/user-guide/common-operations/expressions.rst

@@ -186,10 +186,13 @@ If you need explicit control over parameter names, build the lambda with
 
     Lambda expressions cannot yet be serialized: calling
     :py:meth:`~datafusion.expr.Expr.to_bytes` or pickling an expression that
-    contains a lambda raises ``Lambda not implemented``. SQL lambda syntax
-    (``x -> x * 2``) is only parsed by dialects that support lambdas; set
-    ``datafusion.sql_parser.dialect`` to ``DuckDB`` to use it. The Python
-    expression builder shown above works regardless of dialect.
+    contains a lambda raises ``Lambda not implemented``. SQL lambda syntax is
+    only parsed by dialects that support lambdas; set
+    ``datafusion.sql_parser.dialect`` to one of ``DuckDB``, ``ClickHouse``,
+    ``Snowflake``, or ``Databricks``. Both arrow syntax (``x -> x * 2``) and
+    keyword syntax (``lambda x: x * 2``) parse. DuckDB will drop the arrow
+    form in v2.1, so prefer ``lambda x: x * 2`` for forward compatibility.
+    The Python expression builder shown above works regardless of dialect.
 
 
 Testing membership in a list

python/tests/test_lambda.py

@@ -125,19 +125,14 @@ def test_to_lambda_rejects_invalid_arg(arg, exc_type, match):
         f.array_transform(col("a"), arg)
 
 
-def test_sql_lambda_requires_duckdb_dialect():
-    # Lambda arrow syntax (``x -> ...``) is only parsed by dialects that
-    # support lambda functions. The default Generic dialect treats ``->`` as
-    # the JSON arrow operator, so ``x`` is read as a column reference.
-    ctx = SessionContext()
-    with pytest.raises(Exception, match="No field named x"):
-        ctx.sql("select array_transform([1, 2, 3], x -> x * 2) as d").collect()
-
-    duckdb_ctx = SessionContext(
-        SessionConfig().set("datafusion.sql_parser.dialect", "DuckDB")
-    )
-    result = duckdb_ctx.sql(
-        "select array_transform([1, 2, 3], x -> x * 2) as d"
+@pytest.mark.parametrize("dialect", ["DuckDB", "ClickHouse", "Snowflake", "Databricks"])
+def test_sql_lambda_keyword_syntax(dialect):
+    # ``lambda x: x * 2`` is the forward-compatible syntax. DuckDB will drop
+    # the arrow form (``x -> ...``) in v2.1; the keyword form is supported by
+    # every dialect in sqlparser-rs that enables lambda functions.
+    ctx = SessionContext(SessionConfig().set("datafusion.sql_parser.dialect", dialect))
+    result = ctx.sql(
+        "select array_transform([1, 2, 3], lambda x: x * 2) as d"
     ).collect_column("d")
     assert result.to_pylist() == [[2, 4, 6]]