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Friday, April 6, 2018

The FHIRWalker Interface

The DOM has the TreeWalker interface, I would suggest that FHIR would benefit from a FhirWalker interface which would allow a resource, datatype or component to be walked in a traversal and processed in certain ways.  Definition of such an interface would benefit many.  Already such interfaces about in implementation code, e.g., in HAPI there are multiple parsers and readers that do the very thing that my FhirWalker interface would support.

Two such walkers would be valuable, one which walked a structure definition for a prospective resource, and the other which walked an actual resource.

Such walkers could be used for reporting, query implementation, et cetera.

I've personally written three, four, (I've now lost count) of these for various purposes.  I got fed up with writing one more and wrote a Walker class for myself.  It looks something like this (though I must admit to some artistic license in the names of things.

interface FhirWalker() {

   // Step describes the interface of the methods that can be
   // performed with the Victim of the FhirWalk.  It's not really
   // used in any way by walk or walkBackwards, but might find 
   // some use in implementations.
   interface Step implements BiFunction<String, Any, Boolean>;

   // Walk defines the interface for a traversal. Like 
   interface Walk implements BiConsumer<Any, Victim>;

   interface Victim {
       // tremble is called before walking on any coal.
       // if tremble returns true, the processor chickened out.
       boolean tremble(String path, Any coal) throws FhirWalkerTrippedException;

       // stumble is called on anything that caused a problem.
       // think of it an an exception handler.  If it returns
       // true, the victim recovered.  Otherwise, the victim 
       // fell down and help needs to be called (an exception thrown)
       // immediately.
       boolean stumble(String path, Any coal) throws FhirWalkerTrippedException;

       // burned is called after walking on a coal.
       boolean burned(String path, Any coalthrows FhirWalkerTrippedException;
   }

   public class FhirWalkerTrippedException {
       public FhirWalkerTrippedException(
           String exclamation, Any coalStumbledUpon, Throwable excuse
       );
   }

   // Walk the FHIR tree in order
   public void walk(Any startingCoal, Victim victim) 
      throws FhirWalkerTrippedException;

   // Walk the FHIR tree in reverse order
   public void walkBackwards(Any startingCoal, Victim victim)
      throws FhirWalkerTrippedException;

   public void walkCanonically(Any startingCoal, Victim victim)
      throws FhirWalkerTrippedException;
}


The walk() method implements a NLRN traversal, calling tremble before processing a node, then processing the children left to right, then calling burned after processing the children.  The walkBackwards() method implements an NRLN traversal, calling tremble first, and burned last. 

Tremble and burned have the same signature even though the return on burned is ignored (once burned it twice shy?)  The reason for that is so that a function used for burned in one place can be
used for tremble in a different traversal.

For what it is worth, something implementing the FhirWalker interface need not traverse an entire tree of a FHIR Resource.  It could implement a traversal of selected subnodes.  For example, a DataTypeWalker might only call tremble and burned on FHIR Data types found, a PrimitiveWalker only on primitives.  A whole host of walkers could be created with great benefit.

Somewhere in the back of my head is a BlindfoldedWalker but I don't know where that's going.

Anyway, enough amusement for the day.  I have my own walker now, time to go for a stroll.

   Keith


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