A prior version of the proposal can be found here.

Introduction

Some functionalities are either not supported or too difficult to express using the DPath expression language. While it is possible to add desired functionality to the codebase, some may be applicable to only a small dataset and won't have widespread use. It also leaves the burden of implementing said functionalities on the Daffodil developers. We would like Daffodil to be able to register and execute external/user defined functions (UDFs) in DPath expressions.

See 

Use Cases/Examples

A trivial use case would be adding a replace function callable from a DPath expression. In the DFDL Schema, we might call  something like the below; Where the function will return "Hello_World" if the element resolves to "Hello World". 

...

@FunctionClassInfo(
	name = "convert_to_hae", 
	namespace = "http://extOther.UDFunction.ElevationConversions.com" 
)
public class MSLConversions {
	public Float evaluate(Integer elevation_msl_raw, Integer hae_adjustment_raw, Float scaling_factor) {
		//implementation..
	}
}

Requirements

1. The UDF will be defined in a stand-alone library outside of the Daffodil codebase
2. The UDF must be accessible to and callable from the Daffodil code
3. Daffodil must be able to process and pass the return value from the UDF back to the Schema
4. The support of UDFs in the DFDL Schema must be language agnostic and not Java, Scala or Daffodil specific

Proposed Solution

The Daffodil solution will use a combination of JAVA's ServiceLoader and Reflection APIs.

Daffodil Provided Jar

Daffodil will provide a UDFunctionProvider abstract class and an annotation class FunctionClassInfo.

...

The FunctionClassInfo annotation class must be applied and filled in for each function class. It provides name and namespace elements that must be filled out for the function class to be properly identified by the provider when queried by Daffodil .

UDF Implementation

The implementer will be expected to implement at least 2 classes: a provider class and at least one function class.

...

The function classes  will contain the functionality of the UDF embodied in an evaluate method. This class is what will be initialized and returned by the provider class. The function class will be expected to implement an evaluate method as well as apply the Daffodil provided functionClassInfo annotation to the class . Because the parameter types and the return types of the evaluate function are dependent on the functionality, and we really only care about the name, we will not provide an abstract class for it. Each function that the implementer wishes to expose must be defined in a class with an evaluate function, and must be available to the lookup function. See Proposal: User Defined Functions Use Cases/Examples for sample function class.

Daffodil Service Loader

Daffodil will use the ServiceLoader API to poll for UDF Provider classes and return the desired function class on request.

...

public Object lookupFunctionClass(String namespace, String name) {
	Object funcObj = null;

	try {
		UDFunctionProvider fp = this.functionProviderLookup.get(String.join("_", namespace, name));
		if (fp != null ) {
			funcObj = fp.lookupFunctionClass(namespace, name);
		}
	} catch (Exception e) {
		e.printStackTrace();
	}
	return funcObj;
}

Daffodil DPath Processing

Acquiring the UDFunctionClass

The Internal class referenced above will be instantiated, and its lookup function called with the namespace and name from functionQName object. If it returns null, we'll throw an error, otherwise we'll use Reflection API to query for the parameter type, return type and to find the evaluate function. The below implementation can also be distributed

val functionClass = functionClassObject.getClass

// infer argument types; this implementation supports overloading evaluate function
val pTypes = argumentsFromDpath.map { o => o.getClass() }
val fEvaluate = functionClass.getMethods.find { p =>
	p.getName == "evaluate" && p.getParameterTypes.sameElements(pTypes)
}

val dResultType = NodeInfo.fromObject(fEvaluate.getReturnType)
val dArgTypes = fEvaluate.getParameterType.map { NodeInfo.fromObject }

Calling the UDFunctionClass

Within the DPath processing code, Daffodil will define 2 case classes, a UDFunctionCallExpr and a UDFunctionCall. The dResultType and dArgTypes arguments for UDFArgsListExpr will be inferred from the functionClass as shown above. The Expression functionObject will be set to the below.

...

case class UDFunctionCallExpr(nameAsParsed: String, fnQName: RefQName, args: List[Expression],
resultType: NodeInfo.Kind, argsTypes: List[NodeInfo.Kind], constructor: List[CompiledDPath] => RecipeOp)
	extends FunctionCallBase(nameAsParsed, fnQName, args) {

	lazy val argsToArgType = (args zip argsTypes).toMap
	override lazy val inherentType = resultType

	override def targetTypeForSubexpression(childExpr: Expression): NodeInfo.Kind = {
		argsToArgType.get(childExpr) match {
			case Some(argType) => argType
			case None => {
				Assert.invariantFailed("subexpression %s is not an argument.".format(subexp))
			}
		}
	}
	override lazy val compiledDPath = {
		val recipes = args.map { _.compiledDPath }
		val res = new CompiledDPath(constructor(recipes) +: conversions)
		res
	}
}

case class UDFunctionCall(recipes: List[CompiledDPath], functionClassObject: Object, udfEvaluate: Method)
	extends FNArgsList(recipes) {

	override def computeValues(values: List[Any], dstate: DState) = {
		val res = udfEvaluate.invoke(functionClassObject, values: _*) //might need to explicitly cast values to type Object before hand
		res
	}
}

Diagnostics

We intend to supply the user will at least the following errors/warning

• Warning: No annotation present in function class(es) of \[Provider_ClassName]. Class will be ignored.
• Error: No User Defined function class with specified name/namespace found. (List all names/namespaces with associated providers)
• Error: No evaluate function found in function class object

Testing

Prototype

UDF Jars: HAEMSLConversions.jar and UDFunctionProviderImpl.jar. Both extend UDFunctionProvider.jar.

...