Re: [PR] Add lambda support and array_transform udf [datafusion]

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

gstvg commented on code in PR #18921:
URL: https://github.com/apache/datafusion/pull/18921#discussion_r3022045503


##########
datafusion/expr/src/expr_schema.rs:
##########
@@ -199,6 +203,13 @@ impl ExprSchemable for Expr {
                 // Grouping sets do not really have a type and do not appear in projections
                 Ok(DataType::Null)
             }
+            Expr::HigherOrderFunction(_func) => {
+                Ok(self.to_field(schema)?.1.data_type().clone())
+            }
+            Expr::Lambda(Lambda { params: _, body }) => body.get_type(schema),

Review Comment:
   It's either this, `DataType::Null` or an error. I don't have a strong opinion on this, actually. Right now, the only place I call it is literally like this:
   
   
   ```rust
   .map(|arg: Expr| {
       let field = arg.to_field(schema)?.1;
           match arg {
               Expr::Lambda(_lambda) => Ok(ValueOrLambda::Lambda(field)),
               _ => Ok(ValueOrLambda::Value(field)),
           }
   })
   ```
   
   We can just change it to 
   
   ```rust
   .map(|arg: Expr| {
           match arg {
               Expr::Lambda(lambda) => Ok(ValueOrLambda::Lambda(lambda.body.to_field(schema)?.1)),
               _ => Ok(ValueOrLambda::Value(arg.to_field(schema)?.1)),
           }
   })
   ```
   
   During higher order function type coercion, arguments are ignored if they are a lambda and only value arguments are checked and coerced so the return type for a lamba is irrelevant.
   
   Some other expressions return `DataType::Null` like logical `Placeholder` without a specifed type, `Wildcard` and `GroupingSet` and physical `NoOp` and `UnknownColumn`. If you think that's better I'm happy to change it



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