Jefffrey commented on code in PR #24140:
URL: https://github.com/apache/datafusion/pull/24140#discussion_r3735036871


##########
datafusion/spark/README.md:
##########
@@ -27,23 +27,39 @@ This crate is a submodule of DataFusion that provides 
[Apache Spark] compatible
 [apache datafusion]: https://datafusion.apache.org/
 [apache spark]: https://spark.apache.org/
 
-## Testing Guide
+## Implementation Guidelines
 
-When testing functions by directly invoking them (e.g., 
`test_scalar_function!()`), input coercion (from the `signature`
-or `coerce_types`) is not applied.
+When implementing these functions, you can check if there are existing 
implementations
+in the [Sail] or [Comet] projects first. If you do port functionality from 
these
+sources, make sure to port over the corresponding tests too, to ensure 
correctness
+and compatibility.
 
-Therefore, direct invocation tests should only be used to verify that the 
function is correctly implemented.
+### `simplify()`

Review Comment:
   This is one of the cases where Rust unit tests would be needed; though 
perhaps it also suggests that maybe we should stick to `invoke_with_args` only, 
and avoid `simplify` altogether especially in cases where it is an 
unconditional rewrite 🤔 



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