Kernel.cs

using System;
using System.Collections.Generic;
namespace Niecza {
    // We like to reuse continuation objects for speed - every function only
    // creates one kind of continuation, but tweaks a field for exact return
    // point.  As such, call frames and continuations are in 1:1 correspondence
    // and are unified.  Functions take a current continuation and return a new
    // continuation; we tail recurse with trampolines.

    // Only call other functions in Continue, not in the CallableDelegate or
    // equivalent!
    public delegate Frame CallableDelegate(Frame caller,
            Variable[] pos, Dictionary<string, Variable> named);
    // Used by DynFrame to plug in code
    public delegate Frame DynBlockDelegate(Frame frame);

    public interface IP6 {
        Frame Invoke(Frame caller, Variable[] pos,
                Dictionary<string, Variable> named);
        // include the invocant in the positionals!  it will not usually be
        // this, rather a container of this
        Frame InvokeMethod(Frame caller, string name,
                Variable[] pos, Dictionary<string, Variable> named);
        Frame GetAttribute(Frame caller, string name);
        //public Frame WHERE(Frame caller);
        Frame HOW(Frame caller);
        // These exist as a concession to circularity - FETCH as a completely
        // ordinary method could not work under the current calling convention.
        Frame Fetch(Frame caller);
        Frame Store(Frame caller, IP6 thing);
    }

    // TODO: update this comment

    // Variables are things which can produce LValues, and can also bind
    // LValues.  They hold LValues and may or may not be bindable.  Variables
    // also tend to contextualize stuff put into them.
    //
    // Coercions are not used on binding unless necessary.
    //
    // Variables also have type constraints, that's how %foo and @foo differ...

    // A LValue is the meaning of function arguments, of any subexpression
    // except the targets of := and .VAR.
    //
    // They come in two flavors.  Scalary lvalues hold a container, which
    // can do FETCH and STORE.  Listy lvalues FETCH as the container itself,
    // and STORE as a method to the container.
    //
    // List->scalar context: create a simple container holding the list's
    // object, but !islist.  Read only.
    //
    // Scalar->list: bind islist, must be Iterable. Bind it same rwness.
    public class Variable {
        public bool bvalue;
        public bool rw;
        public bool islist;
        // If non-null, then this lv is, or at one time was, the result of
        // an autovivifying access, but has not yet committed to becoming
        // real or becoming undef.  We call these virtual containers; they
        // exist only in flight (resultSlots) and in parcel contexts.
        public IP6 whence;
        public IP6 container;

        public Variable(bool bvalue, bool rw, bool islist, IP6 whence,
                IP6 container) {
            this.bvalue = bvalue;
            this.whence = whence;
            this.container = container;
            this.rw = rw;
            this.islist = islist;
        }
    }

    // We need hashy frames available to properly handle BEGIN; for the time
    // being, all frames will be hashy for simplicity
    public class Frame: IP6 {
        public readonly Frame caller;
        public readonly Frame outer;
        public Frame proto;
        public object resultSlot = null;
        public int ip = 0;
        public readonly DynBlockDelegate code;
        public readonly Dictionary<string, object> lex
            = new Dictionary<string, object>();

        public Variable[] pos;
        public Dictionary<string, Variable> named;

        public Frame(Frame outer_) : this(null, outer_, null) {}

        public Frame(Frame caller_, Frame outer_,
                DynBlockDelegate code_) {
            caller = caller_;
            outer = outer_;
            code = code_;
        }

        public Frame Continue() {
            return code(this);
        }

        public Frame GetAttribute(Frame c, string name) {
            c.resultSlot = lex[name];
            return c;
        }

        public Frame Invoke(Frame c, Variable[] p,
                Dictionary<string, Variable> n) {
            return Kernel.Die(c, "Tried to invoke a Frame");
        }

        // FIXME A horrible hack, code duplication and doesn't support FETCH etc
        public Frame InvokeMethod(Frame caller, string name,
                Variable[] pos, Dictionary<string, Variable> named) {
            IP6 m;
            foreach (DynMetaObject k in Kernel.CallFrameMO.mro) {
                if (k.local.TryGetValue(name, out m)) {
                    return m.Invoke(caller, pos, named);
                }
            }
            return Kernel.Die(caller, "Unable to resolve method " + name);
        }

        public Frame Fetch(Frame c) {
            return Kernel.Die(c, "Method FETCH not defined on Frame");
        }

        public Frame Store(Frame c, IP6 o) {
            return Kernel.Die(c, "Method STORE not defined on Frame");
        }

        public Frame HOW(Frame c) {
            c.resultSlot = Kernel.CallFrameMO.how;
            return c;
        }

        public int ExecutingLine() {
            object l;
            if (((proto != null) ? proto.lex : lex).
                    TryGetValue("?lines", out l)) {
                int[] rl = (int[])l;
                return ip >= rl.Length ? 0 : rl[ip];
            } else {
                return 0;
            }
        }

        public string ExecutingFile() {
            object l;
            if (((proto != null) ? proto.lex : lex).
                    TryGetValue("?files", out l)) {
                string[] rl = (string[])l;
                return ip >= rl.Length ? "" : rl[ip];
            } else {
                return "";
            }
        }

        public Variable LexicalFind(string name) {
            Frame csr = this;
            while (csr != null) {
                object o;
                if (csr.lex.TryGetValue(name, out o))
                    return (Variable)o;
                csr = csr.outer;
            }
            return Kernel.NewROScalar(Kernel.AnyP);
        }

        private static List<string> spacey = new List<string>();
        public string DepthMark() {
            Frame f = this;
            int ix = 0;
            while (f != null) { ix++; f = f.caller; }
            while (spacey.Count <= ix) { spacey.Add(new String(' ', spacey.Count * 2)); }
            return spacey[ix];
        }
    }

    // NOT IP6; these things should only be exposed through a ClassHOW-like
    // façade
    public class DynMetaObject {
        public IP6 how;
        public IP6 typeObject;
        public string name;

        public delegate Frame InvokeHandler(DynObject th, Frame c,
                Variable[] pos, Dictionary<string, Variable> named);
        public delegate Frame FetchHandler(DynObject th, Frame c);
        public delegate Frame StoreHandler(DynObject th, Frame c, IP6 n);

        public InvokeHandler OnInvoke;
        public FetchHandler OnFetch;
        public StoreHandler OnStore;

        public List<DynMetaObject> superclasses
            = new List<DynMetaObject>();
        public Dictionary<string, IP6> local
            = new Dictionary<string, IP6>();
        public Dictionary<string, IP6> local_attr
            = new Dictionary<string, IP6>();

        public List<DynMetaObject> mro;
        public HashSet<DynMetaObject> isa;

        public DynMetaObject(string name) {
            this.name = name;
            this.mro = new List<DynMetaObject>();
            mro.Add(this);

            isa = new HashSet<DynMetaObject>();
            isa.Add(this);
        }

        public bool HasMRO(DynMetaObject m) {
            return isa.Contains(m);
        }

        public void BuildC3MRO() {
            List<List<DynMetaObject>> toMerge = new List<List<DynMetaObject>>();
            mro = new List<DynMetaObject>();
            isa = new HashSet<DynMetaObject>();
            toMerge.Add(new List<DynMetaObject>());
            toMerge[0].Add(this);

            foreach (DynMetaObject dmo in superclasses) {
                toMerge[0].Add(dmo);
                toMerge.Add(new List<DynMetaObject>(dmo.mro));
            }

            while (true) {
top:
                foreach (List<DynMetaObject> h in toMerge) {
                    if (h.Count == 0) {
                        continue; // next CANDIDATE
                    }
                    DynMetaObject cand = h[0];
                    foreach (List<DynMetaObject> bs in toMerge) {
                        if (bs.Count == 0) {
                            continue; // next BLOCKER
                        }
                        if (bs[0] == cand) {
                            continue;
                        }
                        if (bs.Contains(cand)) {
                            goto blocked;
                        }
                    }
                    // no reason not to immediately put this, and by loop
                    // order the C3 condition is kept
                    mro.Add(cand);
                    isa.Add(cand);
                    foreach (List<DynMetaObject> l in toMerge) {
                        l.Remove(cand);
                    }
                    goto top;
blocked:
                    ;
                }
                foreach (List<DynMetaObject> l in toMerge) {
                    if (l.Count != 0) {
                        // should refactor this to use a real p6exception
                        throw new Exception("C3 MRO inconsistency detected");
                    }
                }
                return;
            }
        }
    }

    // This is quite similar to DynFrame and I wonder if I can unify them.
    // These are always hashy for the same reason as Frame above
    public class DynObject: IP6 {
        // the slots have to support non-containerized values, because
        // containers are objects now
        public Dictionary<string, object> slots
            = new Dictionary<string, object>();
        public DynMetaObject klass;

        public DynObject(DynMetaObject klass) {
            this.klass = klass;
        }

        private Frame Fail(Frame caller, string msg) {
            return Kernel.Die(caller, msg + " in class " + klass.name);
        }

        public Frame InvokeMethod(Frame caller, string name,
                Variable[] pos, Dictionary<string, Variable> named) {
            IP6 m;
            foreach (DynMetaObject k in klass.mro) {
                if (k.local.TryGetValue(name, out m)) {
                    return m.Invoke(caller, pos, named);
                }
            }
            return Fail(caller, "Unable to resolve method " + name);
        }

        public Frame GetAttribute(Frame caller, string name) {
            if (slots == null) {
                return Fail(caller, "Attempted to access slot " + name +
                        " via a protoobject");
            }
            caller.resultSlot = slots[name];
            return caller;
        }

        public Frame HOW(Frame caller) {
            caller.resultSlot = klass.how;
            return caller;
        }

        public Frame Invoke(Frame c, Variable[] p,
                Dictionary<string, Variable> n) {
            if (klass.OnInvoke != null) {
                return klass.OnInvoke(this, c, p, n);
            } else {
                Variable[] np = new Variable[p.Length + 1];
                Array.Copy(p, 0, np, 1, p.Length);
                np[0] = Kernel.NewROScalar(this);
                return InvokeMethod(c, "INVOKE", np, n);
            }
        }

        public Frame Fetch(Frame c) {
            if (klass.OnFetch != null) {
                return klass.OnFetch(this, c);
            } else {
                return InvokeMethod(c, "FETCH", new Variable[1] {
                        Kernel.NewROScalar(this) }, null);
            }
        }

        public Frame Store(Frame c, IP6 o) {
            if (klass.OnStore != null) {
                return klass.OnStore(this, c, o);
            } else {
                return InvokeMethod(c, "STORE", new Variable[2] {
                        Kernel.NewROScalar(this), Kernel.NewROScalar(o) },
                        null);
            }
        }
    }

    // Allows native CLR objects to be treated as Perl 6 data.  They don't
    // currently support any operations; you'll need to use CLR code to work
    // with them.
    public class CLRImportObject : IP6 {
        public readonly object val;

        public CLRImportObject(object val_) { val = val_; }

        public Frame GetAttribute(Frame c, string nm) {
            return Kernel.Die(c, "Attribute " + nm +
                    " not available on CLRImportObject");
        }

        public Frame Invoke(Frame c, Variable[] p,
                Dictionary<string, Variable> n) {
            return Kernel.Die(c, "Tried to invoke a CLRImportObject");
        }

        public Frame InvokeMethod(Frame c, string nm, Variable[] p,
                Dictionary<string, Variable> n) {
            return Kernel.Die(c, "Method " + nm +
                    " not defined on CLRImportObject");
        }

        public Frame Fetch(Frame c) {
            return Kernel.Die(c, "Method FETCH not defined on CLRImportObject");
        }

        public Frame Store(Frame c, IP6 o) {
            return Kernel.Die(c, "Method STORE not defined on CLRImportObject");
        }

        public Frame HOW(Frame c) {
            //TODO
            return Kernel.Die(c, "No metaobject available for CLRImportObject");
        }
    }

    // A bunch of stuff which raises big circularity issues if done in the
    // setting itself.
    public class Kernel {
        public static DynBlockDelegate MainlineContinuation;

        private static object UnboxDO(DynObject o) {
            return o.slots["value"];
        }

        private static Frame SCFetch(DynObject th, Frame caller) {
            caller.resultSlot = UnboxDO(th);
            return caller;
        }

        private static Frame SCStore(DynObject th, Frame caller, IP6 nv) {
            th.slots["value"] = nv;
            return caller;
        }

        private static Frame SubCloneC(Frame th) {
            DynObject a, b;
            Frame c;
            switch (th.ip) {
                case 0:
                    th.ip = 1;
                    return th.pos[0].container.Fetch(th);
                case 1:
                    th.lex["s0"] = th.resultSlot;
                    th.ip = 2;
                    return th.pos[1].container.Fetch(th);
                case 2:
                    a = (DynObject) th.lex["s0"];
                    c = (Frame) th.resultSlot;
                    b = new DynObject(a.klass);
                    b.slots = new Dictionary<string,object>(a.slots);
                    b.slots["outer"] = c;
                    th.caller.resultSlot = NewROScalar(b);
                    return th.caller;
                default:
                    return Kernel.Die(th, "invalid IP");
            }
        }

        public static Frame TakeReturnFrame;

        public static Frame Take(Frame th, Variable payload) {
            Frame r = TakeReturnFrame;
            TakeReturnFrame = null;
            r.lex["$nextframe"] = NewROScalar(th);
            r.resultSlot = payload;
            th.resultSlot = payload;
            return r;
        }

        public static Frame CoTake(Frame th, Frame from) {
            TakeReturnFrame = th;
            return from;
        }

        public static Frame GatherHelper(Frame th, IP6 sub) {
            DynObject dyo = (DynObject) sub;
            Frame n = new Frame(th,
                                (Frame) dyo.slots["outer"],
                                (DynBlockDelegate) dyo.slots["code"]);
            n.proto = (Frame) dyo.slots["proto"];
            th.resultSlot = n;
            return th;
        }

        private static Frame SubInvoke(DynObject th, Frame caller,
                Variable[] pos, Dictionary<string,Variable> named) {
            Frame proto = (Frame) th.slots["proto"];
            Frame outer = (Frame) th.slots["outer"];
            DynBlockDelegate code = (DynBlockDelegate) th.slots["code"];

            Frame n = new Frame(caller, outer, code);
            n.proto = proto;
            n.pos = pos;
            n.named = named;

            return n;
        }
        private static Frame SubInvokeSubC(Frame th) {
            Variable[] post;
            switch (th.ip) {
                case 0:
                    th.ip = 1;
                    return th.pos[0].container.Fetch(th);
                default:
                    post = new Variable[th.pos.Length - 1];
                    Array.Copy(th.pos, 1, post, 0, th.pos.Length - 1);
                    return SubInvoke((DynObject)th.resultSlot, th.caller,
                            post, th.named);
            }
        }

        public static Frame Die(Frame caller, string msg) {
            // TODO: Unbreak p6exceptions
            throw new Exception(msg);
            //Frame f = new Frame(caller, null, new DynBlockDelegate(ThrowC));
            //f.pos = new LValue[1] { new LValue(true, false, new CLRImportObject(msg)) };
            //f.named = null;
            //return f;
        }

        public static readonly DynMetaObject SubMO;
        public static readonly DynMetaObject ScalarMO;

        public static bool TraceCont;

        public static IP6 MakeSub(DynBlockDelegate code, Frame proto,
                Frame outer) {
            DynObject n = new DynObject(SubMO);
            n.slots["outer"] = outer;
            n.slots["code"] = code;
            n.slots["proto"] = proto;
            return n;
        }

        public static Variable BoxAny(object v, IP6 proto) {
            DynObject n = new DynObject(((DynObject)proto).klass);
            n.slots["value"] = v;
            return NewROScalar(n);
        }

        public static object UnboxAny(IP6 o) {
            // TODO: Check for compatibility?
            return UnboxDO((DynObject)o);
        }

        public static IP6 MakeSC(IP6 inside) {
            DynObject n = new DynObject(ScalarMO);
            n.slots["value"] = inside;
            return n;
        }

        // check whence before calling
        public static Frame Vivify(Frame th, Variable v) {
            IP6 w = v.whence;
            v.whence = null;
            return w.Invoke(th, new Variable[1] { v }, null);
        }

        private static Frame BindC(Frame th) {
            switch (th.ip) {
                case 0:
                    // autovivify rhs if needed
                    if (th.pos[1].whence == null || ((bool)th.lex["ro"]))
                        goto case 1;
                    th.ip = 1;
                    return Vivify(th, th.pos[1]);
                case 1:
                    if (th.pos[0].whence == null)
                        goto case 2;
                    th.ip = 2;
                    return Vivify(th, th.pos[0]);
                case 2:
                    if (!th.pos[0].islist || th.pos[1].islist)
                        goto case 4;
                    th.ip = 3;
                    return th.pos[1].container.Fetch(th);
                case 3:
                    // having to fetch because of a $ -> @ conversion
                    th.pos[0].container = (IP6) th.resultSlot;
                    th.pos[0].rw = true;
                    return th.caller;
                case 4:
                    th.pos[0].container = th.pos[1].container;
                    th.pos[0].rw = th.pos[1].rw && !((bool)th.lex["ro"]);
                    if (th.pos[1].islist && !th.pos[0].islist) {
                        th.pos[0].rw = false;
                        th.pos[0].container = MakeSC(th.pos[0].container);
                    }
                    return th.caller;
                default:
                    throw new Exception("IP invalid");
            }
        }

        public static Frame Bind(Frame th, Variable lhs, Variable rhs,
                bool ro, bool forcerw) {
            // TODO: need exceptions for forcerw to be used
            Frame n;
            // fast path
            if (lhs.islist == rhs.islist &&
                    (ro || rhs.whence == null) &&
                    (lhs.whence == null)) {
                lhs.container = rhs.container;
                lhs.rw = !ro && rhs.rw;
                return th;
            }

            n = new Frame(th, null, new DynBlockDelegate(BindC));
            n.pos = new Variable[2] { lhs, rhs };
            n.lex["ro"] = ro;
            return n;
        }

        // This isn't just a fetch and a store...
        private static Frame AssignC(Frame th) {
            switch (th.ip) {
                case 0:
                    if (th.pos[0].whence == null)
                        goto case 1;
                    th.ip = 1;
                    return Vivify(th, th.pos[0]);
                case 1:
                    if (!th.pos[0].rw) {
                        throw new Exception("assigning to readonly value");
                    }
                    if (th.pos[0].islist) {
                        return th.pos[0].container.InvokeMethod(th.caller,
                                "LISTSTORE", th.pos, null);
                    } else {
                        if (th.pos[1].islist) {
                            return th.pos[0].container.Store(th.caller,
                                    th.pos[1].container);
                        } else {
                            th.ip = 2;
                            return th.pos[1].container.Fetch(th);
                        }
                    }
                case 2:
                    return th.pos[0].container.Store(th.caller,
                            (IP6)th.resultSlot);
                default:
                    throw new Exception("invalid IP");
            }
        }

        public static Frame Assign(Frame th, Variable lhs, Variable rhs) {
            Frame n = new Frame(th, null,
                    new DynBlockDelegate(AssignC));
            n.pos = new Variable[2] { lhs, rhs };
            return n;
        }

        public static Frame Fetch(Frame th, Variable vr) {
            if (vr.islist) {
                th.resultSlot = vr.container;
                return th;
            } else {
                return vr.container.Fetch(th);
            }
        }

        // ro, not rebindable
        public static Variable NewROScalar(IP6 obj) {
            return new Variable(false, false, false, null, MakeSC(obj));
        }

        public static Variable NewRWScalar(IP6 obj) {
            return new Variable(true, true, false, null, MakeSC(obj));
        }

        public static Variable NewRWListVar(IP6 container) {
            return new Variable(true, true, true, null, container);
        }

        public static List<Variable> SlurpyHelper(Frame th, int from) {
            List<Variable> lv = new List<Variable>();
            for (int i = from; i < th.pos.Length; i++) {
                lv.Add(th.pos[i]);
            }
            return lv;
        }

        public static Variable ContextHelper(Frame th, string name) {
            object rt;
            while (th != null) {
                if (th.lex.TryGetValue(name, out rt)) {
                    return (Variable)rt;
                }
                th = th.caller;
            }
            name = name.Remove(1,1);
            Dictionary<string,Variable> gstash = (Dictionary<string,Variable>)
                (((CLRImportObject)GlobalO).val);
            Variable v;

            if (gstash.TryGetValue(name, out v)) {
                return v;
            } else {
                return PackageLookup(ProcessO, name);
            }
        }

        public static Variable DefaultNew(IP6 proto) {
            DynObject n = new DynObject(((DynObject)proto).klass);
            List<DynMetaObject> mro = n.klass.mro;

            for (int i = mro.Count - 1; i >= 0; i--) {
                foreach (string s in mro[i].local_attr.Keys) {
                    n.slots[s] = NewRWScalar(AnyP);
                }
            }

            return NewROScalar(n);
        }

        public static IP6 AnyP;
        public static IP6 ArrayP;
        public static IP6 StrP = new DynObject(null);
        public static DynMetaObject CallFrameMO;

        public static Variable PackageLookup(IP6 parent, string name) {
            Dictionary<string,Variable> stash = (Dictionary<string,Variable>)
                (((CLRImportObject)parent).val);
            Variable v;

            if (stash.TryGetValue(name, out v)) {
                return v;
            } else if (name.EndsWith("::")) {
                Dictionary<string,Variable> newstash =
                    new Dictionary<string,Variable>();
                newstash["PARENT::"] = NewROScalar(parent);
                return (stash[name] = NewROScalar(
                            new CLRImportObject(newstash)));
            } else {
                // TODO: @foo, %foo
                return (stash[name] = NewRWScalar(AnyP));
            }
        }

        public static void RunLoop(DynBlockDelegate boot) {
            Kernel.TraceCont = (Environment.GetEnvironmentVariable("NIECZA_TRACE") != null);
            Frame root_f = new Frame(null, null, boot);
            Frame current = root_f;
            while (current != null) {
                try {
                    current = current.Continue();
                } catch (Exception ex) {
                    ExceptionPacket ep = new FatalException(
                            new CLRImportObject(ex));
                    current = ep.SearchForHandler(current);
                }
            }
        }

        // XXX should be per-unit
        public static Variable Global;
        public static IP6 GlobalO;
        public static Variable Process;
        public static IP6 ProcessO;

        static Kernel() {
            SubMO = new DynMetaObject("Sub");
            SubMO.OnInvoke = new DynMetaObject.InvokeHandler(SubInvoke);
            SubMO.local["clone"] = MakeSub(new DynBlockDelegate(SubCloneC),
                    null, null);
            SubMO.local["INVOKE"] = MakeSub(new DynBlockDelegate(SubInvokeSubC),
                    null, null);

            ScalarMO = new DynMetaObject("Scalar");
            ScalarMO.OnFetch = new DynMetaObject.FetchHandler(SCFetch);
            ScalarMO.OnStore = new DynMetaObject.StoreHandler(SCStore);

            GlobalO = new CLRImportObject(new Dictionary<string,Variable>());
            Global = NewROScalar(GlobalO);
            ProcessO = new CLRImportObject(new Dictionary<string,Variable>());
            Process = NewROScalar(ProcessO);
        }
    }

    public abstract class ExceptionPacket {
        public Frame bot;
        public Frame top;

        public abstract bool Filter(Frame cand);
        // for setting $!
        public abstract IP6 Payload();
        // CATCH vs CONTROL
        public abstract bool IsFatal();
        public abstract Frame Process(Frame target);

        // stage 2!
        public Frame Unwind(Frame caller) {
            while (top != bot) {
                Frame pop = bot;
                bot = bot.caller;
                object o;

                if (pop.proto.lex.TryGetValue("!unwind", out o)) {
                    Frame n = new Frame(bot, bot, (DynBlockDelegate) o);
                    n.proto = (Frame) pop.proto.lex["!unwindp"];
                    n.lex["!reunwind"] = this;
                    return n;
                }
            }

            return bot;
        }

        public Frame SearchForHandler(Frame caller) {
            if (bot == null) { // first call
                bot = top = caller;
            }
            string key = IsFatal() ? "!ehcatch" : "!ehcontrol";
            while (top != null) {
                Frame test = top;
                if (test.proto != null) { test = test.proto; }

                object o;
                if (test.lex.TryGetValue(key, out o)) {
                    Frame fn = new Frame(bot, top, (DynBlockDelegate) o);
                    fn.proto = (Frame) test.lex[key + "p"];
                    fn.lex["!rethrow"] = this;
                    return fn;
                }

                if (top.lex.ContainsKey("!rethrow")) {
                    // this is an active exception handling frame!  skip
                    // the corresponding handler
                    top = top.outer.caller;
                } else {
                    if (Filter(top)) {
                        return Process(top);
                    }
                    top = top.caller;
                }
            }

            Unhandled();
            return null;
        }

        // This *probably* ought to be handled in Perl 6 code, using .Str,
        // but we're now at a very low bootstrap level;
        private void Unhandled() {
            IP6 p = Payload();
            CLRImportObject cp = p as CLRImportObject;
            DynObject dp = p as DynObject;
            Exception e = (cp != null) ? (cp.val as Exception) : null;
            string s = (dp != null && dp.slots.ContainsKey("value")) ?
                (dp.slots["value"] as string) : null;

            if (e != null) {
                // Wrapped CLR exception; has a message, and inner frames
                s = e.ToString();
            } else if (s == null) {
                s = "(no message available)";
            }
            Console.Error.WriteLine("Unhandled exception: " + s);
            Frame c;
            for (c = bot; c != null; c = c.caller) {
                Console.Error.WriteLine("  at {0} line {1}",
                        c.ExecutingFile(), c.ExecutingLine());
            }
            Environment.Exit(1);
        }
    }

    // no payload; you should have already poked the right value into a
    // result slot somewhere.  ip = 0 means immediate return (leave)
    public class LexoticControlException : ExceptionPacket {
        public Frame target;
        public int ip;

        public LexoticControlException(Frame target, int ip) {
            this.target = target;
            this.ip = ip;
        }

        public override bool Filter(Frame cand) { return cand == target; }
        public override IP6 Payload() { return Kernel.AnyP; } // XXX
        public override bool IsFatal() { return false; }
        public override Frame Process(Frame t) {
            if (ip != 0) {
                t.ip = ip;
            }
            return t;
        }
    }

    public class FatalException : ExceptionPacket {
        public IP6 payload;

        public FatalException(IP6 payload) {
            this.payload = payload;
        }

        // can only be explicitly caught
        public override bool Filter(Frame cand) { return false; }
        public override bool IsFatal() { return true; }
        public override IP6 Payload() { return payload; }
        public override Frame Process(Frame t) { return t; }
    }
}

// The root setting
public class NULL {
    public static Niecza.Frame Environment = null;
    public static Niecza.IP6 Installer = Niecza.Kernel.MakeSub(
            new Niecza.DynBlockDelegate(MAIN), null, null);

    private static Niecza.Frame MAIN(Niecza.Frame th) {
        switch (th.ip) {
            case 0:
                th.ip = 1;
                return Niecza.Kernel.Fetch(th, th.pos[0]);
            default:
                return ((Niecza.IP6)th.resultSlot).Invoke(th.caller,
                        new Niecza.Variable[0] {}, null);
        }
    }
    public static void Initialize() {}
}