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 

}

def makeUnevaler(uncallerList, unscopeMap) :near {
    def unevaler {
        to recognize(root, builder) :(def Root := builder.getRootType()) {

            def Node := builder.getNodeType()

            def uncallers := uncallerList.snapshot()
            def unscope := unscopeMap.diverge()

            # forward declaration 

            def generate

            /** traverse an uncall portrayal */
            def genCall(rec, verb :String, args :any[]) :Node {
                def recExpr := generate(rec)
                var argExprs := []
                for arg in args {
                    argExprs := argExprs.with(generate(arg))
                }
                builder.buildCall(recExpr, verb, argExprs)
            }

            /** When we're past all the variable manipulation. */

            def genObject(obj) :Node {

                if (obj =~ i :int)     { return builder.buildLiteral(i) }
                if (obj =~ f :float64) { return builder.buildLiteral(f) }
                if (obj =~ c :char)    { return builder.buildLiteral(c) }
                if (obj =~ twine :Twine && twine.isBare()) {
                    return builder.buildLiteral(twine)
                }

                for uncaller in uncallers {
                    if (<u>uncaller.optUncall(obj)</u> =~ [rec, verb, args]) {
                        return genCall(rec, verb, args)
                    }
                }
                throw(`Can't uneval ${E.toQuote(obj)}`)
            }

            /** Build a use-occurrence of a variable. */

            def genVarUse(varID :(String | int)) :Node {
                if (varID =~ varName :String) {
                    builder.buildImport(varName)
                } else {
                    builder.buildIbid(varID)
                }
            }

            /**
             * The internal recursive routine that will traverse the
             * subgraph and build a Data-E Node while manipulating the
             * above state.
             */

            # "bind" resolves a forward declaration
            bind generate(obj) :Node {

                if (unscope.fetch(obj, thunk{}) =~ varID :notNull) {
                    return genVarUse(varID)
                }
                def promIndex := builder.buildPromise()
                unscope[obj] := promIndex
                def rValue := genObject(obj)
                builder.buildDefrec(promIndex+1, rValue)
            }

            builder.buildRoot(generate(root))
        }
    }
}