findepi commented on code in PR #14440:
URL: https://github.com/apache/datafusion/pull/14440#discussion_r1959483130
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datafusion/expr-common/src/signature.rs:
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@@ -466,6 +551,186 @@ fn get_data_types(native_type: &NativeType) ->
Vec<DataType> {
}
}
+/// Represents type coercion rules for function arguments, specifying both the
desired type
+/// and optional implicit coercion rules for source types.
+///
+/// # Examples
+///
+/// ```
+/// use datafusion_expr_common::signature::{Coercion, TypeSignatureClass};
+/// use datafusion_common::types::{NativeType, logical_binary, logical_string};
+///
+/// // Exact coercion that only accepts timestamp types
+/// let exact = Coercion::new_exact(TypeSignatureClass::Timestamp);
+///
+/// // Implicit coercion that accepts string types but can coerce from binary
types
+/// let implicit = Coercion::new_implicit(
+/// TypeSignatureClass::Native(logical_string()),
+/// vec![TypeSignatureClass::Native(logical_binary())],
+/// NativeType::String
+/// );
+/// ```
+///
+/// There are two variants:
+///
+/// * `Exact` - Only accepts arguments that exactly match the desired type
+/// * `Implicit` - Accepts the desired type and can coerce from specified
source types
+#[derive(Debug, Clone, Eq, PartialOrd)]
+pub enum Coercion {
+ /// Coercion that only accepts arguments exactly matching the desired type.
+ Exact {
+ /// The required type for the argument
+ desired_type: TypeSignatureClass,
+ },
+
+ /// Coercion that accepts the desired type and can implicitly coerce from
other types.
+ Implicit {
+ /// The primary desired type for the argument
+ desired_type: TypeSignatureClass,
+ /// Rules for implicit coercion from other types
+ implicit_coercion: ImplicitCoercion,
+ },
Review Comment:
An implementor of a function `foo(s) wants to say:
- `foo(s)` accepts s being "a string" (eg via logical type
`NativeType::String` or old school `DataType::Utf8`)
so the query `SELECT foo(s)` should succeed when s is a string or s is
coercible to string.
Neither Coercion option gives that.
In theory Coercion::Implicit gives that, but alas, it requires the function
implementor to know the coercion rules of the engine, to enumerate the "allowed
source types".
How is it supposed to be used?
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datafusion/functions/src/datetime/date_part.rs:
##########
@@ -95,24 +96,29 @@ impl DatePartFunc {
signature: Signature::one_of(
vec