Safe Serialization Under Mutual Suspicion/"Reversing" Evaluation

pragma.syntax("0.8")

def deSubgraphKit {
    to makeBuilder(scope) :near {

        # The index of the next temp variable

        var nextTemp := 0

        # The frame of temp variables
        def temps := [].diverge()

        # The type returned by "internal" productions and passed as arguments to represent

        # built subtrees.
        def Node := any

        # The type returned by the builder as a whole.
        def Root := any

        # DEBuilderOf is a parameterized type constructor.

        def deSubgraphBuilder implements DEBuilderOf(Node, Root) {
            to getNodeType() :near { Node }
            to getRootType() :near { Root }

            /** Called at the end with the reconstructed root to obtain the value to return. */
            to buildRoot(root :Node)        :Root  { root }

            /** A literal evaluates to its value. */
            to buildLiteral(value)          :Node  { value }

            /** A free variable's name is looked up in the scope. */
            to buildImport(varName :String) :Node  { scope[varName] }

            /** A temporary variable's index is looked up in the temps frame. */
            to buildIbid(tempIndex :int)    :Node  { temps[tempIndex] }

            /** Perform the  described call. */
            to buildCall(rec :Node, verb :String, args :Node[]) :Node {
                # E.call(..) is E's reflective invocation construct. For example, E.call(2, "add", [3])
                # performs the same call as 2.add(3).
                <u>E.call(rec, verb, args)</u>
            }

            /**
             * Called prior to building the right-hand side of a defexpr, to allocate and bind the
             * next two temp variables to a promise and its resolver.

             * 
             * @return the index of the temp holding the promise. The temp holding the
             *               resolver is understood to be this plus one.
             */
            to buildPromise() :int {
                def promIndex := nextTemp
                nextTemp += 2
                def [prom,res] := Ref.promise()
                temps[promIndex] := prom
                temps[promIndex+1] := res
                promIndex
            }

            /**
             * Called once the right-hand side of a defexpr is built, use the resolver to resolve
             * the value of the promise.
             * 
             * @return the value of the right-hand side.
             */

            to buildDefrec(resIndex :int, rValue :Node) :Node {
                temps[resIndex].resolve(rValue)
                rValue
            }

            # ... buildDefine is an optimization of buildDefrec for known non-cyclic cases.
        }
    }
    # ... other useful tools 

}