[GitHub] [arrow] thisisnic commented on a diff in pull request #34050: GH-31910: [C++] Add support for Substrait cast expression

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

thisisnic commented on code in PR #34050:
URL: https://github.com/apache/arrow/pull/34050#discussion_r1099942179


##########
cpp/src/arrow/engine/substrait/expression_internal.cc:
##########
@@ -320,6 +320,30 @@ Result<compute::Expression> FromProto(const substrait::Expression& expr,
       return function_converter(substrait_call);
     }
 
+    case substrait::Expression::kCast: {
+      const auto& cast_exp = expr.cast();
+      ARROW_ASSIGN_OR_RAISE(auto input,
+                            FromProto(cast_exp.input(), ext_set, conversion_options));
+
+      // need to convert from the Substrait type spec to the relevant Arrow thing
+      // do we need a switch/case or can we just get code similar to the below FromProto?
+      // the type should come from cast_expr.type()
+      
+      // this will be a Result<std::pair<std::shared_ptr<DataType>
+      ARROW_ASSIGN_OR_RAISE(auto type,
+                            FromProto(cast_exp.type(), ext_set, conversion_options));
+
+      if(cast_exp.failure_behavior() == substrait::Expression_Cast_FailureBehavior::Expression_Cast_FailureBehavior_FAILURE_BEHAVIOR_THROW_EXCEPTION){
+        return compute::call("cast", std::move(input), compute::CastOptions::Safe(type));
+      } else if(cast_exp.failure_behavior() == substrait::Expression_Cast_FailureBehavior::Expression_Cast_FailureBehavior_FAILURE_BEHAVIOR_RETURN_NULL){
+
+      // e.g. if unspecified  
+      } else {
+        return compute::call("cast", std::move(input), compute::CastOptions::Unsafe(type));

Review Comment:
   Thanks for highlighting this, I can see that these are different things, so I'll update this.



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