[GitHub] [arrow] jorgecarleitao commented on a change in pull request #8024: ARROW-9809: [Rust][DataFusion] Fixed type coercion, supertypes and type checking.

[GitBox <gi...@apache.org>]

jorgecarleitao commented on a change in pull request #8024:
URL: https://github.com/apache/arrow/pull/8024#discussion_r475106741



##########
File path: rust/datafusion/src/execution/physical_plan/expressions.rs
##########
@@ -991,30 +991,236 @@ impl fmt::Display for BinaryExpr {
     }
 }
 
+// Returns a formatted error about being impossible to coerce types for the binary operator.
+fn coercion_error<T>(lhs_type: &DataType, op: &String, rhs_type: &DataType) -> Result<T> {
+    Err(ExecutionError::General(
+        format!(
+            "The binary operator '{}' can't evaluate with lhs = '{:?}' and rhs = '{:?}'",
+            op, lhs_type, rhs_type
+        )
+        .to_string(),
+    ))
+}
+
+// the type that both lhs and rhs can be casted to for the purpose of a string computation
+fn string_coercion(
+    lhs_type: &DataType,
+    op: &Operator,
+    rhs_type: &DataType,
+) -> Result<DataType> {
+    use arrow::datatypes::DataType::*;
+    match (lhs_type, rhs_type) {
+        (Utf8, Utf8) => Ok(Utf8),
+        (LargeUtf8, Utf8) => Ok(LargeUtf8),
+        (Utf8, LargeUtf8) => Ok(LargeUtf8),
+        (LargeUtf8, LargeUtf8) => Ok(LargeUtf8),
+        _ => coercion_error(lhs_type, &format!("{}", op), rhs_type),
+    }
+}
+
+/// coercion rule for numerical values
+pub fn numerical_coercion(
+    lhs_type: &DataType,
+    op: &String,

Review comment:
       It is only used for error messages atm. But yes, for consistency, we should probably receive an Operator. The operator will be needed in the future for the Operator::Modulus, that only handles integers AFAIK.




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