alamb commented on code in PR #13840:
URL: https://github.com/apache/datafusion/pull/13840#discussion_r1894012298
##########
datafusion/expr-common/src/signature.rs:
##########
@@ -123,24 +127,29 @@ pub enum TypeSignature {
/// One or more arguments belonging to the [`TypeSignatureClass`], in
order.
///
/// For example, `Coercible(vec![logical_float64()])` accepts
- /// arguments like `vec![DataType::Int32]` or `vec![DataType::Float32]`
+ /// arguments like `vec![Int32]` or `vec![Float32]`
/// since i32 and f32 can be cast to f64
///
/// For functions that take no arguments (e.g. `random()`) see
[`TypeSignature::Nullary`].
Coercible(Vec<TypeSignatureClass>),
- /// One or more arguments that can be "compared"
+ /// One or more arguments cast to single, comparable type.
+ ///
+ /// Each argument will be coerced to a single type using the
+ /// coercion rules described in [`comparison_coercion_numeric`].
+ ///
+ /// # Examples
+ ///
+ /// If the `nullif(1, 2)` function is called with `i32` and `i64` arguments
+ /// the types will both be coerced to `i64` before the function is invoked.
///
- /// Each argument will be coerced to a single type based on comparison
rules.
- /// For example a function called with `i32` and `i64` has coerced type
`Int64` so
- /// each argument will be coerced to `Int64` before the function is
invoked.
+ /// If the `nullif('1', 2)` function is called with `Utf8` and `i64`
arguments
+ /// the types will both be coerced to `Utf8` before the function is
invoked.
///
/// Note:
- /// - If compares with numeric and string, numeric is preferred for
numeric string cases. For example, `nullif('2', 1)` has coerced types `Int64`.
- /// - If the result is Null, it will be coerced to String (Utf8View).
- /// - See [`comparison_coercion`] for more details.
/// - For functions that take no arguments (e.g. `random()` see
[`TypeSignature::Nullary`]).
+ /// - If all arguments have type [`DataType::Null`], they are coerced to
`Utf8`
Review Comment:
In some magic ideal world where I had infinite time, I have dreamed about
refactor the coercion code into a set of structs that encoded the rules in a
more structured / composable manner. But I don't have time to pursue that dream
at this time 🏃
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