[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #6415: Improve error message for wrong built-in scalar function signatures.

["alamb (via GitHub)" <gi...@apache.org>]

alamb commented on code in PR #6415:
URL: https://github.com/apache/arrow-datafusion/pull/6415#discussion_r1201009613


##########
datafusion/expr/src/function_err.rs:
##########
@@ -0,0 +1,91 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! Function_err module enhances frontend error messages for unresolved functions due to incorrect parameters,
+//! by providing the correct function signatures.
+//!
+//! For example, a query like `select round(3.14, 1.1);` would yield:
+//! ```text
+//! Error during planning: No function matches 'round(Float64, Float64)'. You might need to add explicit type casts.
+//!     Candidate functions:
+//!     round(Float64, Int64)
+//!     round(Float32, Int64)
+//!     round(Float64)
+//!     round(Float32)
+//! ```
+
+use crate::function::signature;
+use crate::{BuiltinScalarFunction, TypeSignature};
+use arrow::datatypes::DataType;
+
+impl TypeSignature {
+    fn to_string_repr(&self) -> Vec<String> {
+        match self {
+            TypeSignature::Variadic(types) => {
+                vec![format!("{}, ..", join_types(types, "/"))]
+            }
+            TypeSignature::Uniform(arg_count, valid_types) => {
+                vec![std::iter::repeat(join_types(valid_types, "/"))
+                    .take(*arg_count)
+                    .collect::<Vec<String>>()
+                    .join(", ")]
+            }
+            TypeSignature::Exact(types) => {
+                vec![join_types(types, ", ")]
+            }
+            TypeSignature::Any(arg_count) => {
+                vec![std::iter::repeat("Any")
+                    .take(*arg_count)
+                    .collect::<Vec<&str>>()
+                    .join(", ")]
+            }
+            TypeSignature::VariadicEqual => vec!["T, .., T".to_string()],
+            TypeSignature::VariadicAny => vec!["Any, .., Any".to_string()],
+            TypeSignature::OneOf(sigs) => {
+                sigs.iter().flat_map(|s| s.to_string_repr()).collect()
+            }
+        }
+    }
+}
+
+/// Helper function to join types with specified delimiter.
+fn join_types<T: std::fmt::Debug>(types: &[T], delimiter: &str) -> String {
+    types
+        .iter()
+        .map(|t| format!("{:?}", t))

Review Comment:
   I suggest we use the [`Display` impl ](https://docs.rs/arrow/latest/arrow/datatypes/enum.DataType.html#impl-Display-for-DataType) here to better follow the Rust convention of `Display` for users and `Debug` for developers. 
   
   ```suggestion
           .map(|t| t.to_string())
   ```
   
   However, under the covers that simply calls the `Debug` impl 🤦 so it won't make any practical difference
   
   https://docs.rs/arrow-schema/40.0.0/src/arrow_schema/datatype.rs.html#307



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