DESIGN.txt


                                          ----------------------
                                          CLR/JavaScript Interop
                                          ----------------------

// TODO: Enum as string options
// TODO: Param array using JavaScript arguments convention
// TODO: Allow throw if import Keyed and has no key

Introduction
~~~~~~~~~~~~

CLR/JavaScript interop is used in two "modes":
 - CLR Mode:   Between code running in the CLR (which we'll call "managed CLR code") and native JavaScript (which
               we'll call "unmanaged JavaScript") running one or more JavaScript engines. There are a number of
               configurations:
                - The JavaScript engine is within a browser, and the CLR is hosted by the browser.
                - The JavaScript engine is hosted within the CLR.
                - The JavaScript engine and CLR are in separate processes.
               However the interop details are the same regardless of configuration.
 - IL2JS Mode: Between CLR code compiled to JavaScript by the IL2JS compiler (which we'll call "managed JavaScript"),
               and native JavaScript code (which we'll again call "unmanaged JavaScript"). In this case all code runs
               within the same JavaScript engine.

The interop mechanism allows:
 - Objects created by unmanaged JavaScript to be accessible to managed CLR and JavaScript code.
 - Objects created by managed CLR or JavaScript to be accessible to the unmanaged world.
 - Managed CLR or JavaScript to extend the objects of unmanaged JavaScript with additional fields.
 - Functions in unmanaged JavaScript to be callable as if they were managed methods.
 - Managed methods to be callabale from unmanaged JavaScript as if they were globally defined or implemented
   in a prototype.
To support this, we will "import" objects when they cross from unmanaged to managed so that they may peacefully
coexist with other managed objects in the runtime. Dually, we will "export" objects when they go the other
direction so that unmanaged JavaScript will see them as it expects.

There are four sources of unmanaged objects:
 - JavaScript proxies for COM-like entities in the host environment, such as XmlHttpRequest. These objects
   generally do not allow additional field bindings.
 - JavaScript objects created and manipulated by third-party JavaScript or the host environment itself.
   The layout of these objects is outside of our control. Fields may be bound within them, but at the risk
   of field name collision.
 - JavaScript objects created and manipulated by a combination of mananged and unmanaged code. In this
   case the developer may choose unmanaged field names to avoid collision with managed field names
   and the fields used by the managed runtime system.
 - JavaScript primitive objects, such as numbers, strings and functions. These objects have no fields by
   default, and generally do not allow any to be added. However they do allow fields to be bound within
   their constructor function prototypes.

When objects cross the mananged/unmanaged boundary we'd like as many of the following properties to hold as
feasible:

 - State 1
     If managed x crosses into unmanaged, clearly the unmanaged fields of x must be preserved.
     However, when it crosses back the managed fields of x must also be preserved:
         forall managed and unmanaged field names f . import(export(x)).f === x.f  (in managed)
         forall unmanaged field names f . export(x).f === x.f     (where lhs is in unmanaged and rhs in managed)

 - State 2
     An unmanaged object should not have its fields extended or altered by managed code:
         forall strings s . x[s] === export(import(x))[s]                          (in unmanaged)
         forall strings s . x[s] === import(export(x))[s]                          (in managed)

 - Reference equality 1
     Reference equality on managed objects should agree with referece equality on unmanaged objects:
         forall managed objects x, y : x === y ==> export(x) === export(y)
         forall unmanaged objects x, y : x === y ==> import(x) === import(y)

 - Reference equality 2
     Reference equality should be preserved over the boundary:
         forall managed objects x . import(export(x)) === x
         forall unmanaged objects x . export(import(x)) === x

 - Strong Subtyping
     Subtyping must be respected over the boundary:
        forall managed objects x .
          if x is instance of T and T is a subtype of U and U is non-normal then
          import(U)(export(T)(x)) === x

 - Weak Subtyping
     Subtyping must be respected over the boundary, but only between types which have the same interop style:
        forall managed objects x .
          if x is instance of T and T is a subtype of U and U and T are both style P (where P is not 'Normal') then
          import(U)(export(T)(x)) === x

Type Styles
~~~~~~~~~~~

When compiling to JavaScript, some CLR definitions are clearly unimplementable in JavaScript. We allow any
assembly, type, method, field, event or property may be marked as "not supported", which effectively makes those
definitions invisible to programs compiled to JavaScript. Not-supportedness is poisonous: if A
(statically) depends on B and B is not supported then A is not supported. In what follows we assume
not supported definitions have already been removed, and the resulting program has remained type correct.

We refer to the built-in value types of sbyte, int16, int32, int64, byte, uint16, uint32, uint64, single, double,
and the built-in reference type of string, as "primitive" types. These types are called out separately because:
 1. They are immutable.
 2. They are cheap to serialize/deserialize to/from JavaScript literals.
 3. The IL2JS runtime system represents them as JavaScript numbers or strings, which are also immutable.
 4. For strings, the JavaScript String prototype is assigned a type field. Thus a string created on the
    unmanaged side is already a legal managed string.

Some types must be handled specially:

 - Nullable<T>: Nullable is special because we identify the CLR 'no-value' Nullable with the JavaScript 'null' value.

 - ref T:       Pointer types are special because in IL2JS mode they are exported and imported using their IL2JS
                runtime representation. In particular, the 'this' parameter of a method on a value type is exported
                and imported as its underlying pointer. This is intended only for the runtime system. Pointer
                types cannot be imported or exported in CLR mode.

 - Delegates:   Delegates are special because we structurally coerce them between their managed and
                unmanaged versions. The "work" involved in coercion is proportional to the number of
                delegate arguments, and does not depend on the types of those arguments. Export builds a new
                delegate which imports arguments, invokes the original delegate, and exports the result (if any).
                Import does the converse. Imported/exported wrappers may be tied together to avoid redundant work,
                either by field or table, but this is not required. (Note that System.Delegate and
                System.MulticastDelegate are not delagates, in the same way System.ValueType is not a value type).

 - P[]:         (where P is a primitive type) Arrays of primitive values are special because we
                structurally coerce them between their managed and unmanaged versions. The "work" involved in
                coercion is proportional to the length of the array, but does not depend on the type of the
                array elements. (In the IL2JS runtime, CLR arrays are represented by JavaScript arrays with some
                additional fields, thus it is tempting to share the array object between managaged and unmanaged
                versions. However, we must still create a copy of the array on import and export to protect
                against mutation.)

 - Structs:     Structs which happen to be primitive types (eg int) are 'Primitive'. In IL2JS mode structs may
                also be annotated as 'Primitive', but not in CLR mode. All other structs cannot cross the
                interop boundary.

All other types fall into one of four 'styles':

 - Primitive:   All primitive types have 'Primitive' style. In IL2JS mode, additional types may be marked
                as 'Primitive'. Instances are objects with a merge of managed and unmanaged fields. If a
                constructor is imported then the fields of the unmanaged JavaScript object it returns are copied
                directly into the managed object, and the unmanaged object is subsequently ignored. Primitives objects
                are imported and exported as themselves. This style offers no protection against collision of managed
                and unmanaged fields within the same object, so is not intended for the end-user.

 - Normal:      Instances are ordinary managed objects. If they cross into the unmanaged world, they must be
                treated as abstract values which cannot be accessed or mutated, only passed back to the managed
                world via a delegate or exported method, or ignored. Instances may only be created on the managed
                side. Since unmanaged JavaScript cannot "reach into" these instances, they may be exported as
                any arbitrary JavaScript proxy, provided there exists a mechanism to recover the original managed
                object on import.

 - Keyed:       Instances consist of a managed wrapper around the unmanaged instance. The managed-to-unmanaged
                association may be implemented by a field or table. The unmanaged-to-managed association is
                via a designated "primary key" field whose value is unique over all unmanaged instances of the type
                which specified the 'Keyed' style (but not necessarily unique over all unmanaged instances in the
                runtime). Export recovers the unmanaged instance, while import attempts to recover any existing
                wrapper, or construct a new wrapper if none can be found. The programmer must call
                JSContext::Dispose to indicate no further interop is required on a given managed object.

 - Proxied:     Instances also consist of a managed wrapper around the unmanaged instance, with the
                managed-to-unmanaged association implemented by field or table. However, there is no
                unmanaged-to-managed association, and each import will create a fresh wrapper. Thus referential
                equality of managed instances must be decided by referential equality on their contained unmanaged
                instances. Managed instances cannot contain fields. The programmer must call
                JSContext::Dispose to indicate no further interop is required on a given managed object.


We may determine the style of a type using these classification rules:
 - Static types are 'Normal'.
 - Nullable<_>, pointers, delegates and arrays are special, as mentioned above.
 - If U has style S, S is not 'Normal' or 'Primitive', and T derives from U, then T has style S.
 - If T is not covered by above and is annotated as style S (where S cannot be 'Normal') then T is S.
 - If C<_> has style S then C<T> has style S for all types T.
 - If T has any instance (including constructor) imports, or any instance (but not constructor) exports, it is 'Keyed'
 - All other types (including System.Object and interface types) are 'Normal'.

Constructors
~~~~~~~~~~~~

We have two distinct paths for constructing 'Keyed' or 'Proxied' instances:
 - We may invoke an imported constructor, which assumes the underlying JavaScript function returns the initialized
   unmanaged instance.
 - We may receive a new unmanaged JavaScript instance from the result of an imported method, or the argument of
   an exported method or delegate.

In both cases, we need a way to control the initialization of the corresponding managed object. In particular,
we may need to reach-in to the unmanaged object in order to initialize the managed object.

('Normal' types are constructed as per the CLR constructor rules. 'Primitive' types are constructed either by
a normal CLR constructor, by an imported constructor, or by unmanaged JavaScript, and it is up to the programmer
to keep these three methods coherent.)

To this end we introduce a special type:

  namespace Microsoft.LiveLabs.JavaScript {
      class JSContext {
          private JSContext();
          public static void Dispose(object obj);
          public bool HasField(string name);
          public T GetField<T>(string name);
          public void SetField<T>(string name, T val);
      }
  }

We allow any 'Keyed' or 'Proxied' type T to implement a family of special "importing constructor" with signatures:

  T(JSContext ctxt, <zero or more arguments>)

It is the programmer's responsibility:
 - to avoid infinite regress by ensuring importing constructors do not implicitly or explicitly chain to
   an imported constructor.
 - to avoid creating managed instances without corresponding unmanaged instances by ensuring the importing
   constructors are never invoked by 'new'.

If T does not contain a constructor with signature T(JSContext ctxt, ...), then one is created according to
the following rules:

 - If T derives from U and U has U(JSContext ctxt) (either defined by user or created), then

     T(JSContext ctxt) : base(ctxt) {}

 - Otherwise, if T derives from U and U is 'Normal' and U has constructor U(), then

     T(JSContext ctxt) : base() {}

 - Otherwise, T is illegal.

An importing constructor may not itself be imported. An importing constructor is invoked from an imported
constructor under the following circumstances:

 - If T has an imported constructor

     T(<zero or more arguments)

   and T has constructor

     T(JSContext ctxt, <matching arguments>)

   then the imported constuctor invokes the above constructor with a context object and the arguments of
   the original constructor. The context object allows the constructor to "temporarily" access the
   unmanaged object via HasField, GetField and SetField

 - Otherwise, if T has an imported constructor

     T(<zero or more arguments)

   and T has constructor (either defined by user or created)

     T(JSContext ctxt)

   then the importing constuctor invokes the above constructor with a context object only.

An importing constructor with signature

     T(JSContext ctxt)

is invoked when an unmanaged object without associated managed object is imported.

Type Classifiers
~~~~~~~~~~~~~~~~

Both 'Proxied' and 'Keyed' root types may supply a type classifier function. Given the assembly name
of the assembly containing the root type and an instance, the type classifier function may return
either a qualified type name or null. The former indicates the instance should be regarded as belonging
to the named type on the managed side. The latter indicates the instance should be regarded as the type
under which it is imported (which may be derived from the root type containing the classifier function.)

Note that objects are imported and exported based on their static type and not their dynamic type. For example,
an object whose dynamic type has style 'Keyed' may be exported as type object, in which case the managed wrapper
is passed unchanged to the unmanaged world. In this case, the object must be imported back as type object also.
However, as mentioned above, types may have type classifiers which may influence the dynamic type of an imported
instance.

Validity of Type Styles
~~~~~~~~~~~~~~~~~~~~~~~

We define the 'root type' of a type T to be the unique type U s.t.:
 - T is U or is a derived type of U
 - U has the same style as T
 - U's immediate base type has style 'Normal' or 'Primitive'
Thus the root type represents the base of a heirachy of types which share the same style.

We have the following validity rules:
 - No class may be declared as 'Normal'. Classes are declared 'Keyed', 'Proxied' or 'Primitive' using custom
   attributes described in the next section.
 - Fields may not be imported or exported (however properties, since they are implemented as methods,
   may be imported and exported).
 - Any type may appear in the argument and result types of imported and exported methods, constructors,
   properties and events.
 - A 'Normal' type must not contain:
    - an imported constructor
    - an imported instance property or event
    - an exported instance property, event or method
 - A value type cannot be 'Proxied' or 'Keyed', but may be 'Primitive' or 'Normal'.
 - A 'Proxied' type must not contain fields or any exported instance properties, events or methods.
 - A 'Keyed' type must contain exactly one 'Key' property (from supertype or type itself)
 - A class declared as style S may derive from a type of style S, style 'Normal', or style 'Primitive'.

We assume the programmer obeys these subtyping rules:
 - If an object is exported under a type with style S and later re-imported, it's imported type also has style S.
   For example, the programmer should not export using a 'Proxied' type but import as object. However, the
   type of the import need not be the same, or even related via subtyping, to the type of the export.
 - If an object is exported as a value type and later re-imported, it's imported type must be the
   same value type. For example, the programmer should not export as a struct but import as object.
In other words, the interop mechanism only respects subtyping between types A and B if they agree on both their
style and boxity. In particular, we don't allow boxed values to be exported as values, nor values to be imported
as boxed values:
 - We can't box on import since the JavaScript value doesn't carry enough type information to place in the box.
 - We can't unbox on export in the IL2JS runtime since it wreaks havoc with code which expects to export a 'this'
   pointer (recall pointers and boxes have the same representation)

Exceptions
~~~~~~~~~~

If an exception is throw by an imported method, it could either be:
 - A managed exception thrown by an exported method or delegate which the imported method called. In this case
   we wish to throw the same exception to the managed side.
 - An arbitrary JavaScript object throw by the unmanaged JavaScript method itself. In this case we wish
   to wrap the exception as a JSException. The original exception object is available in the UnderlyingException
   field, of type JSObject.

Dually, if an exception is throw by an exported method or delegate, it could either be:
 - A wrapped JSException caught from an imported method call. In this case we wish to throw the UnderlyingException.
 - A managed exception, in which case we wish to throw it as a 'Normal' object.

This special import/export behaviour is captured below:

    (* XREF1493 *)
    function ImportException(x) {
        if (x == null || x.T == null)
            return root.JSException(x);
        if (root.IsAssignableTo(x.T, root.ExceptionType))
            return x; // ie import as 'Normal'
        return root.JSException(x);
    }

    (* XREF1499 *)
    function ExportException(x) {
        if (x == null)
            return null;
        if (root.IsAssignableTo(x.T, root.JSExceptionType))
            return root.GetUnderlyingException(x);
        return x; // ie export as 'Normal'
    }

Note that all exception types, including JSException, are 'Normal', and that the import and export for these
types are defined as for 'Normal' reference types. It is only when an exception is actally thrown across the
interop boundary that the above special import/export behaviour comes into play.

Import and Export
~~~~~~~~~~~~~~~~~

We now describe the 'Import' and 'Export' operations for all types. We describe them as they appear in the
IL2JS runtime system. The corresponding implementations in the CLR are similar, except where explicitly noted.

  - SPECIAL CASE: For Nullable<T>:

    (* XREF1223 *)
    function Import(x) {
        var t = <type structure of T>;
        if (x == null)
            // A null from the unmanaged side denotes the "no-value" nullable, which the
            // managed side reperesents as null
            return null;
        if (!t.IsValidJavaScriptType(typeof x))
            throw root.InvalidCastException();
        // Clone value
        return t.C(x);
    }

    (* XREF1229 *)
    function Export(x) {
        var t = <type structure of T>;
        if (x == null)
            return null;
        // Clone value
        return t.C(x);
    }

 - SPECIAL CASE: ref T:

    function Import(x) {
        if (x == null || x.T == null || x.R == null || x.W == null)
            throw root.InvalidCastException();
        return x;
    }

    function Export(x) {
        return x;
    }

 - SPECIAL CASE: For delegate R D(A a): (w.l.o.g. considering one argument delegate with return value)

    (* XREF1039 *)
    function Import(f) {
        var t = <type structure of A>;
        var u = <type structure of R>;
        var dt = <type structure of D>;
        if (f == null)
            return null;
        // Must be a function
        if (typeof f != "function")
            throw root.InvalidCastException();
        if (f.Managed == null)
            f.Managed = root.NewDelegate(null, function(x) { return u.Import(f(t.Export(x))) }, dt);
        f.Managed.Unmanaged = f;
        return f.Managed;
    }

    (* XREF1049 *)
    function Export(d) {
        var t = <type structure of A>;
        var u = <type structure of R>;
        if (d == null)
            return null;
        if (d.Unmanaged == null)
            d.Unmanaged = function(x) { return u.Export(d(t.Import(x))) };
        d.Unmanaged.Managed = f;
        return d.Unmanaged;
    }

 - SPECIAL CASE: For Array<P> where P is a primitive type:

    (* XREF1451 *)
    function Import(x) {
        if (x == null)
            return null;
        // Must be an array
        if (x.length === undefined)
            throw root.InvalidCastException();
        var t = <type structure of P>;
        // Clone array, but no need to clone array elements since they are primitive
        var y = root.NewArray(t, x.length);
        for (var i = 0; i < x.length; i++) {
            if (x[i] == null)
                y[i] = t.D();
            else {
                // Element must be correctly typed
                if (!t.IsValidJavaScriptType(typeof x[i]))
                    throw root.InvalidCastException();
                y[i] = x[i];
            }
        }
        return y;
    }

    (* XREF1453 *)
    function Export(x) {
        if (x == null)
            return null;
        // Clone array
        var y = new Array(x.length);
        for (var i = 0; i < x.length; i++)
          y[i] = x[i];
        return y;
    }

 - For 'Normal' or 'Primitive' value type T:

    (* XREF1319 *)
    function Import(x) {
        var t = <type structure of T>;
        if (x == null)
            // Convert JavaScript null to the default value for type
            return t.D();
        if (x.T == null || x.T.Id != t.Id)
            throw root.InvalidCastException();
        // Clone value
        return t.C(x);
    }

    (* XREF1327 *)
    function Export(x) {
        var t = <type structure of T>;
        // Clone value
        return t.C(x);
    }

 - For 'Normal' or 'Primitive' reference type T:

    (* XREF1009 *)
    function Import(x) {
        var t = <type structure of T>;
        if (x == null)
            return null;
        // Must be appropriately typed
        if (x.T == null || !root.IsAssignableTo(x.T, t))
            throw root.InvalidCastException();
        return x;
    }

    (* XREF1013 *)
    function Export(x) {
        return x;
    }

 - For 'Keyed' type T:

    (* XREF1093 *)
    function Import(x) {
        var t = <type structure of T>;
        var u = <type structure of root of T>;
        var key = <T.Key>;
        if (x == null)
            return null;
        var y;
        if (x[key] == null) {
            // Give object a unique key, pair it with a new managed object
            var v = t;
            if (u.TypeClassifier != null) {
                var n = u.TypeClassifier(u.Assembly.Name, x);
                if (n != null)
                    v = root.ResolveQualifiedType(n);
            }
            y = v.I(v);
            // Invoke default "importing constructor"
            v.IC(y, x);
            y.Unmanaged = x;
            x[key] = y.Id;
            var k = root.QualifiedTypeName(u.Assembly, u.Name) + "::" + x[key];
            root.KeyToObject[k] = y;
        }
        else {
            // Try to retrieve paired object
            var k = root.QualifiedTypeName(u.Assembly, u.Name) + "::" + x[key];
            y = root.KeyToObject[k];
            if (y == null)
            {
                // Pair with new managed object
                var v = t;
                if (u.TypeClassifier != null) {
                    var n = u.TypeClassifier(u.Assembly.Name, x);
                    if (n != null)
                        v = root.ResolveQualifiedType();
                }
                y = v.I(v);
                // Invoke default "importing constructor"
                v.IC(y, x);
                y.Unmanaged = x;
                root.KeyToObject[k] = y;
            }
        }
        // Managed object must be appropriately typed
        if (!root.IsAssignableTo(y.T, t))
            throw root.InvalidCastException();
        return y;
    }

    (* XREF1097 *)
    function Export(x) {
        if (x == null)
            return null;
        // Object must have been previously imported
        if (x.Unmanaged == null)
            throw root.InvalidOperationException();
        return x.Unmanaged;
    }

 - For 'Proxied' type T:

    (* XREF1051 *)
    function Import(x) {
        var t = <type structure of T>;
        var u = <type structure of root of T>;
        if (x === null)
            return null;
        // JSObject needs to be able to represent 'undefined' as a valid unmanaged object value. We encode it as null.
        if (x === undefined)
            x = null;
        // Determine type
        var v = t;
        if (x != null && u.TypeClassifier != null) {
            var n = u.TypeClassifier(u.Assembly.Name, x);
            if (n != null)
                v = root.ResolveQualifiedType(n);
        }
        var y = v.I(v);
        // Invoke default "importing constructor"
        v.IC(y, x);
        y.Unmanaged = x;
        return y;
    }

    (* XREF1061 *)
    function Export(x) {
        if (x == null)
            return null;
        var y = x.Unmanaged;
        if (y === undefined)
            // Object must have been previously imported
            throw root.InvalidOperationException();
        if (y === null)
            // Undo encoding for 'undefined' introduced by Import above
            y = undefined;
        return y;
    }

Given the above we can revisit the desired properties:

                             Normal      Keyed (1)   Proxied
                             ----------  ----------  ----------
  State 1                    holds       holds       holds
  State 2                    holds       holds       holds
  Reference equality 1       holds       holds       fails (2)
  Reference equality 2       holds       holds       fails (2)
  Strong subtyping                         (3)
  Weak subtyping             holds       holds       holds

  Notes: (1) However, suffers from potential memory leak.
         (2) Import always creates a fresh wrapper.
         (3) Always fails

Methods (static or instance) and constructors (which resemble static methods) are imported and
exported as if they were delegates of the appropriate type. Given this, we can see we reject
imported constructors on normal types because the Import call (at least for reference types) will
always throw.

Interop Attributes
~~~~~~~~~~~~~~~~~~

Custom attributes on declarations influence how the managed and unmanaged JavaScript worlds may interoperate.

The following fragment of JavaScript is accepted as legal scripts in imports:

    FunctionExpression    ::= <as per JavaScript grammar>
    Identifier            ::= <as per JavaScript grammar>
    StringLiteral         ::= <as per JavaScript grammar>
    DottedExpression      ::= PropertyName
                            | DottedExpression '.' Identifier
                            | DottedExpression '[' NumericLiteral ']'
                            | DottedExpression '[' StringLiteral ']'
    Script                ::= <empty>
                            | DottedExpression
                            | FunctionExpression

The custom attributes understood by the compiler are as follows:

    InteropAttributes     ::= ('[' InteropAttribute ']')*
    InteropAttribute      ::= TypeAttribute
                            | ImportAttribute
                            | ExportAttribute
                            | NotSupportedAttribute
                            | NotExportedAttribute

    TypeAttribute         ::= ('Proxy' | 'Keyed') '(' (TypeProperty (',' TypeProperty)*)? ')'
    NamingAttribute       ::= 'Naming' '(' (NamingProperty (',' NamingProperty)*)? ')'
    ImportAttribute       ::= ('Import' | 'ImportGet' | 'ImportSet' | 'ImportAdd' | 'ImportRemove' | 'ImportKey')
                              '(' (ImportProperty (',' ImportProperty)*)? ')'
    ExportAttribute       ::= ('Export' | 'ExportGet' | 'ExportSet' | 'ExportAdd' | 'ExportRemove')
                              '(' (ExportProperty (',' ExportProperty)*)? ')'
    NotSupportedAttribute ::= 'NotSupported' '(' (NotSupportedProperty, (',' NotSupportedProperty)*)? ')'
    NotExportedAttribute  ::= 'NotExported'

    TypeProperty          ::= 'Script' '=' Script
    NamingProperty        ::= 'Qualification' '=' Qualification
                            | 'NamespaceCasing' '=' Casing
                            | 'TypeNameCasing' '=' Casing
                            | 'PrefixNameCasing' '=' Casing
                            | 'MemberNameCasing' '=' Casing
                            | 'RemoveAccessorPrefix' '=' bool
                            | 'RemoveAccessorUnderscore' '=' bool
                            | 'DefaultKey' '=' DottedIdentifier
    ImportProperty        ::= 'Script' '=' Script
                            | 'PassRootAsArgument' '=' bool
                            | 'PassInstanceAsArgument' '='  bool
                            | 'Creation' '=' Creation
                            | 'SimulateMulticastEvents' '=' bool
                            | NamingProperty
    NotSupportedProperty  ::= 'Reason' '=' string
    ExportProperty        ::= 'Script' '=' Script
                            | 'PassRootAsArgument' '=' bool
                            | NamingProperty

    (* Determines the body of an imported constructor when it does not specify a Script *)
    Creation              ::= 'Constructor'
                            | 'Object'
                            | 'Array'
    (* Indicates how the first letter of the various components of CLR-derived names should be capitalized. *)
    Casing                ::= 'Exact'
                            | 'Camel'     (* lower *)
                            | 'Pascal'    (* upper *)
    (* Indicates how the names of static members should be further qualified *)
    Qualification         ::= 'None'
                            | 'Full'
                            | 'Type'

NOTE: When importing instace methods, we allow the unmanaged JavaScript function to have been written in
      'OO' style (PassInstanceAsArgument = false):
           instance.f = function(arg) { ... this ... }
      or 'Functional' style (PassIntanceAsArgument = true):
           function f(instance, arg) { ... instance ... }

      However, exported instance methods must always be in 'OO' style. (This restriction comes from
      the managed interop implementation rather than the JavaScript interop. It is difficult to
      construct the global-function to instance-function forwarder without using reflection, which
      seems a lot of complexity for small benifit.)

If an attribute has positional arguments, they are considered equivalent to the following property binding forms:

  Proxy(str, ...)        ==> Proxy(Script = str, ...)
  Keyed(str, ...)        ==> Keyed(Script = str, ...)
  Import(str, ...)       ==> Import(Script = str, ...) (* similarly for the other import names *)
  Export(str, ...)       ==> Export(Script = str, ...) (* similarly for the other export names *)
  NotSupported(str, ...) ==> NotSupported(Reason = str, ...)

InteropAttributes may be declared at any of the following CLR scopes:
 - Assembly
 - Member, which includes the specialized scopes:
    - Type
    - Property
    - Event
    - Method, which includes the specialized scopes:
       - InstanceInitializer
       - PropertyGetter
       - PropertySetter
       - EventAdder
       - EventRemover

Properties have some special cases for convenience:

    [Import]extern int this[int i] { get; set; }
        ==> implement as index into unmanaged JavaScript object

    [Import]extern int G { get; set; }
        ==> use "G" (after name adjustment) as name of field on unmanaged object
            (similarly for static properties)

    extern int G { [Import]get; [Import]set;}
 == [ImportGet][ImportSet]extern int G { get; set; }
        ==> use imported "get_G" and "set_G" functions (after name adjustment) to implement property

    [Import(Script = "g")]extern int G { get; set; }
        ==> use "g" as name of field on unmanaged object
            (similarly for static properties)

    extern int G { [Import(Script = "get")]get; [Import(Script = "set")]set; }
 == [ImportGet(Script = "get")][ImportSet(Script = "set")]extern int G { get; set; }
        ==> use imported "get" and "set" functions to implement property
            (similarly for static properties)

    [Export]int G { get; set; }
    int G { [Export]get; [Export]set;}
 == [ExportGet][ExportSet]int G { get; set; }
        ==> export as "get_G" and "set_G" (after name adjustment)
            (similarly for static properties)

    [Export(Script = "h.g")]int G { get; set; }
        ==> export as "h.get_g" and "h.set_g" (possibly without underscore)
            (similarly for static properties)

    int G { [Export(Script = "get")]get; [Export(Script = "set")]set; }
 == [ExportGet(Script = "get")][ExportSet(Script = "set")]int G { get; set; }
        ==> export as "get" and "set"
            (similarly for static properties)

Similarly, events have some special cases:

    [Import]extern event T E { add; remove; }
        ==> use "E" (after name adjustment) as name of field holding event handler in unmanaged object
            (similarly for static events)

    [Import(Script = "e")]extern event T E { add; remove; }
        ==> use "e" as name of field holding event handler in unmanaged object
            (similarly for static events)

    [ImportAdd][ImportRemove]extern event T E { add; remove; }
        ==> use "add_E" and"remove_E" functions (after name adjustment) to install and uninstall
            event handler in unmanaged object
            (similarly for static events)

    [ImportAdd(Script = "add")][ImportRemove(Script = "remove")]extern event T E { add; remove; }
        ==> use "add" and"remove" functions to install and uninstall event handler in unmanaged object
            (similarly for static events)

    [Export]event T E { add; remove; }
        ==> export as "add_E" and "remove_E" (after name adjustment)
            (similarly for static events)

    [Export Script="h.e"]event T E { add; remove; }
        ==> export as "h.add_e" and "h.remove_e" (possibly without underscore)
            (similarly for static events)

    [ExportAdd][ExportRemove]event T E { add; remove; }
        ==> export as "add_E" and "remove_E" (after name adjustment)
            (similarly for static events)

    [ExportAdd(Script = "add")][ExportRemove(Script = "remove")]event T E { add; remove; }
        ==> export as "add" and "remove"
            (similarly for static events)

In the following we assume attributes and properties have been simplified according to these rules:

 - An 'Import' or 'Export' attribute on a PropertyGetter (PropertySetter) is replaced by a 'ImportGet' or 'ExportGet'
   ('ImportSet' or 'ExportSet') attribute on the containing Property, with the same properties.

Attributes are subject to the following restrictions:
 - If a node has attributes A and A', then A must not be a sub attribute of A'.
 - Type attributes may only appear at Assembly and Type scope.
 - 'ImportKey' may only appear:
     - at Property scope, and;
     - on exactly once such Property per type, and;
     - within classes of 'Keyed' style whose base type is not 'Keyed'.
 - 'ImportSet', 'ImportGet', 'ExportSet' and 'ImportGet' may only appear at Property scope.
 - 'ImportAdd', 'ImportRemove', 'ExportAdd', 'ExportRemove' may only appear at Event scope.
 - Polymorphic methods may not be exported.
 - Static methods and constructors of higher-kinded types may not be exported.
 - Instance methods of higher-kinded types may be exported. The types at which the higher-kinded type is instantiated
   are recovered from the instance object.
 - Polymorphic methods may be imported. The type parameters at which the polymorphic method is instantiated are not
   available to the JavaScript world.
 - Methods of higher-kinded types may be imported. The type parameters at which the higher-kinded type is instantiated
   are not available to the JavaScript world.
 
Properties are subject to the following restrictions:
 - 'Script' cannot be bound to a FunctionExpression:
    - on 'Import' attributes at Method scope if the method is virtual.
    - on 'Import' or 'ImportKey' attributes at Property scope (but ok to be a FunctionExpression on 'Import'
      attributes at 'PropertyGetter' and 'PropertySetter' scope, and ok on 'ImportGet' and 'ImportSet'
      attributes at Property scope).
    - on 'Import' attributes at Events scope (but ok to be a FunctionExpression on 'Import' attributes at
      'EventAdder' and 'EventRemover' scope, and ok on 'ImportAdd' and 'ImportRemove' attributes at
      Event scope).

We say the the parent of:
 - a top-level Type is its containing Assembly
 - a nested Type is its containing Assembly (and not its declaring Type)
 - an overridding Method is its overridden Method, provided the overridden method is imported
 - a Method (not covered by above), InstanceInitializer, StaticInitializer, Property or Event is its declaring Type
 - a PropertyGetter or PropertySetter is its declaring Property
 - an EventAdder, EventRemover or EventCaller is its declaring Event

The 'sub attribute' relationship is defined as:
 - 'ImportGet', 'ImportSet', 'ImportAdd', 'ImportRemove' and 'ImportKey' are sub attributes of 'Import'.
 - similarly for the export attribute family.
 - 'Import' and 'Export' are sub attributes of 'Naming'.
 - plus reflexivity and transitivity.

We say node N has attribute A, written N.IsA, iff
 - N has an attribute A', where A' is a sub attribute of A.
 - otherwise, if N has a parent M, then M has attribute A.
 - otherwise, false.

We say the value of property P of attribute A on node N, written N.A::P, is:
 - the value of P as declared on an attribute A' of node N, where A' is a sub attribute of A.
 - otherwise, if N has a parent M, then the value of property P of attribute A on node M.
 - otherwise the default value for P given by the following table:

    Property                    Value
    --------------------------  ------------------
    Script                      null
    PassRootAsArgument          false
    PassInstanceAsArgument      true if Script is a function, otherwise false 
    Creation                    Constructor
    Qualification               None
    NamespaceCasing             Exact
    TypeNameCasing              Exact
    PrefixNameCasing            Camel
    MemberNameCasing            Camel
    RemoveAccessorPrefix        false
    RemoveAccessorUnderscore    false
    DefaultKey                  null
    SimulateMulticastEvents     false
    Reason                      null

Given a class C we determine the style of C as follows:
 - If C has the 'Proxy' attribute, it is 'Proxied'.
 - Otherwise if C has the 'Keyed' attribute, it is 'Keyed'.
 - Otherwise if C is derived from type T with style S and S is not 'Normal', then C is style S.
 - Otherwise if C has any imported or exported instance methods, or an imported constructor, then C is 'Keyed'.
 - Otherwise C is 'Normal'.

The 'key name' of class C, written C.Key, is determined as follows:
 - If C or a supertype of C has a 'ImportKey' attribute on property 'prop', then the key name is:
       prop.Import::Script ?? RecaseMember(prop)
 - Otherwise:
       C.Naming::DefaultKey

Often the names of imported JavaScript definitions can be systematically derived from the CLR name
representing them. The 'Qualification', 'NamespaceCasing', 'TypeNameCasing',
'PrefixNameCasing', 'MemberNameCasing' and 'RemoveAccessorUnderscore' properties allow this
derivation to be controlled.

    Recase(string str, c : Casing) : string =
        if c == 'Exact' then
            str
        else if c == 'Pascal' then
            UppercaseFirstChar(str)
        else (* c == 'Camel' *)
            LowercaseFirstChar(str)

    RecaseMember(member : Member) : DottedIdentifier =
        Recase(member.Name, member.Naming::MemberNameCasing)

    RecaseType(member : Member) : DottedIdentifier =
        Recase(member.DeclaringType.Name, member.Naming::TypeNameCasing)

    RecaseProperty(getter : PropertyGetter) : DottedIdentifier =
        Recase(getter.DeclaringProperty.Name, getter.Naming::MemberNameCasing)

    RecaseProperty(setter : PropertySetter) : DottedIdentifier =
        Recase(setter.DeclaringProperty.Name, setter.Naming::MemberNameCasing)

    RecaseEvent(adder : EventAdder) : DottedIdentifier =
        Recase(adder.DeclaringEvent.Name, adder.Naming::MemberNameCasing)

    RecaseEvent(remover : EventRemover) : DottedIdentifier =
        Recase(remover.DeclaringEvent.Name, remover.Naming::MemberNameCasing)

    ReprefixName(member : Member, prefix : string, id : string) : DottedIdentfier =
        let id' = if id begins with (prefix + "_") then id without initial (prefix + "_") else id in
        return Recase(prefix, member.Naming::PrefixNameCasing) +
               (member.Naming::RemoveAccessorUnderscore ? "" : "_") +
               Recase(id', member.Naming::MemberNameCasing)

    ExpandedName(member : Member, prefix : string, d : DottedIdentifier) =
        let id1 '.' ... '.' idn = d in
        let idn' = Recase(prefix, member.Naming::PrefixNameCasing +
                   (member.Naming::RemoveAccessorUndescore ? "" : "_" ) +
                   idn in
        id1 '.' ... '.' idn-1 '.' idn'

    QualifiedName(member : Member, script : Script) : Script =
        if script is DottedIdentifier && (member is InstanceInitializer || member.IsStatic) then
            let t = member.DeclaringType in
            let nc = member.Naming::Qualification in
            if nc == 'None' then
                script
            else if nc =='Type' then
                Recase(t.Name, member.Naming::TypeNameCasing) '.' script
            else (* nc == 'Full' *)
                Recase(t.Namespace, member.Naming::NamespaceCasing) '.'
                Recase(t.Name, member.Naming::TypeNameCasing) '.' script
        else
            script

Import and Export Scripts
~~~~~~~~~~~~~~~~~~~~~~~~~

We define the underlying script representing each imported or exported member as follows:

    FinalImportScript(method : Method, script : Expression) : Expression =
        if method.Import::PassRootAsArgument then
            let n = <arity of method> in
            "function (x1, ..., xn) { return script(root, x1, ..., xn); }"
        else
            script

    (* XREF1153 *)
    ImportScript(method : Method) : Expression =
        let f = QualifiedName(method, method.Import::Script ?? RecaseMember(method)) in
        if method.IsStatic || f is FunctionExpression || method.Import::PassInstanceAsArgument then
            FinalImportScript(method, f)
        else
            let n = <arity of method> in
            if method.Import::PassRootAsArgument then
                "function (x1, x2, ..., xn) { return x1.f(root, x2, ..., xn); }"
            else
                "function (x1, x2, ..., xn) { return x1.f(x2, ..., xn); }"

    FinalExportScript(method : Method, script : Expression) : Expression =
        if script is FunctionExpression then
            if method.Export::PassRootAsArgument then
                if method.IsStatic then
                    "function (v) { script(root, v); }"
                else
                    "function (i, v) { script(root, i, v); }"
            else
                script
        else
            if method.IsStatic then
                "function(v) { script = v; }"
            else
                "function(i, v) { i.script = v; }"

    (* XREF1163 *)
    ExportScript(method : Method) : Expression =
        FinalExportScript(method, QualifiedName(method, method.Export::Script ?? RecaseMember(method)))

    (* XREF1171 *)
    ImportScript(init : InstanceInitializer) : Expression =
        let n = <arity of init> in
        if init.Import::Script == null then
            let oi = init.Import::Creation in
            if oi == 'Array' then
                "function(x2, ..., xn) { return [x2, ..., xn]; }"
            else if oi == 'Object' then
                if n > 1 then
                    undefined
                else
                    "function () { return {}; }"
            else (* oi == 'Constructor' *)
                let f = QualifiedName(init, RecaseType(init)) in
                "function (x2, ..., xn) { return new f(x2, ..., xn); }"
        else if init.Import::Script is a DottedExpression then
            let f = QualifiedName(init, init.Import::Script) in
            "function(x2, ..., xn) { return new f(x2, ..., xn); }"
        else
            FinalImportScript(init, init.Import::Script)

    (* XREF1181 *)
    ExportScript(init : InstanceInitializer) : Expression =
        let f = QualifiedName(init, init.Export::Script ?? RecaseType(init)) in
        if f is FunctionExpression then
            if init.Export::PassRootAsArgument then
                "function (v) { f(root, v); }"
            else
                f
        else
            "function(v) { f = v; }"

    (* XREF1187 *)
    ImportScript(getter : PropertyGetter) : Expression =
        if getter.IsImportGet then
            FinalImportScript(getter, QualifiedName(getter, getter.Import::Script ?? ReprefixName(getter, "get", getter.Name)))
        else if getter.Import::Script == null && getter.Arity == 2 && !getter.IsStatic then
            "function (i, x) { return i[x]; }"
        else
            let f = QualifiedName(getter, getter.Import::Script ?? RecaseProperty(getter)) in
            if getter.IsStatic && n == 0 then
                "function () { return f; }"
            else if !getter.IsStatic && n == 1 then
                "function (i) { return i.f; }"
            else
                undefined

    ImportScript(setter : PropertySetter) : Expression =
        if setter.IsImportSet then
            FinalImportScript(setter, QualifiedName(setter, setter.Import::Script ?? ReprefixName(setter, "set", setter.Name)))
        else if setter.Import::Script == null && setter.Arity == 3 && !setter.IsStatic then
            "function (i, x, v) { i[x] = v; }"
        else
            let f = QualifiedName(setter, setter.Import::Script ?? RecaseProperty(setter)) in
            if setter.IsStatic && n == 1 then
                "function (v) { f = v; }"
            else if !setter.IsStatic && n == 2 then
                "function (i, v) { return i.f = v; }"
            else
                undefined

    (* XREF1193 *)
    ExportScript(getter : PropertyGetter) : Expression =
        if getter.IsExportGet then
            FinalExportScript(getter, QualifiedName(getter, getter.Export::Script ?? ReprefixName(getter, "get", getter.Name)))
        else
            FinalExportScript(getter, QualifiedName(getter, ExpandedName(getter, "get", getter.Export::Script ?? RecaseProperty(getter))))

    ExportScript(setter : PropertySetter) : Expression =
        if setter.IsExportSet then
            FinalExportScript(setter, QualifiedName(setter, setter.Export::Script ?? ReprefixName(setter, "set", setter.Name)))
        else
            FinalExportScript(setter, QualifiedName(setter, ExpandedName(setter, "set", setter.Export::Script ?? RecaseProperty(setter))))

    (* XREF1201 *)
    ImportScript(adder : EventAdder) : Expression =
        if adder.IsImportAdd then
            FinalImportScript(adder, QualifiedName(adder, adder.Import::Script ?? ReprefixName(adder, "add", adder.Name)))
        else
            let f = QualifiedName(adder, adder.Import::Script ?? RecaseEvent(adder)) in
            if adder.IsStatic then
                "function(d) { f = d; }"
            else
                "function(i, d) { i.f = d; }

    ImportScript(remover : EventRemover) : Expression =
        if remover.IsImportRemove then
            FinalImportScript(remover, QualifiedName(remover, remover.Import::Script ?? ReprefixName(remover, "remove", remover.Name)))
        else
            let f = QualifiedName(remover, remover.Import::Script ?? RecaseEvent(remover)) in
            if remover.IsStatic then
                "function(d) { f = d; }"
            else
                "function(i, d) { i.f = d; }"

    (* XREF1213 *)
    ExportScript(adder : EventAdder) : Expression =
        if adder.IsExportAdd then
            FinalExportScript(adder, QualifiedName(adder, adder.Export::Script ?? ReprefixName(adder, "add", adder.Name)))
        else
            FinalExportScript(adder, QualifiedName(adder, ExpandedName(adder, "add", adder.Export::Script ?? RecaseEvent(adder))))

    ExportScript(remover : EventRemover) : Expression =
        if remover.IsExportRemove then
            FinalExportScript(QualifiedName(adder, remover.Export::Script ?? ReprefixName(remover, "remove", remover.Name)))
        else
            FinalExportScript(QualifiedName(adder, ExpandedName(remover, "remove", remover.Export::Script ?? RecaseEvent(remover))))


Imported and Exported Members
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We may ask the following questions of each definition node:

    IsSupported : Node -> bool
        Is definition available to programs when targetting JavaScript?
        If not, all the following are undefined.

    Style : Type -> Style
        What representation style do instance of type use for JavaScript interop, where Style is
        'Normal', 'Keyed' or 'Closed'.

    TypeClassifier : Type -> Script
        If type has style 'Keyed' or 'Proxied', the type classifier function on root of type, or null
        type classifier function is specified.

    IsImported : Node -> bool
        Is definition's implementation supplied by unmanaged JavaScript?
        If not, the result of Import is undefined.

    Import : Node -> Expression
        The JavaScript function representing the imported unmanaged definition, ready to be bound into
        the appropriate managed JavaScript structure and be invoked from the managed world.

    IsExported : Node -> bool
        Is managed definition available to unmanaged JavaScript?
        If not, the result of Export is undefined.

    Export : Node -> Expression
        The JavaScript function which will effect the export by binding the exported definition
        into the global unmanaged JavaScript scope or the unmanaged part corresponding to a managed object.

These questions are answered by the following definitions:

    (*
     * Common
     *)

    (* XREF1103 *)
    (* We may assume all type arguments of method and its declaring type are in scope, and thus
       at1 .. atn and rt will be well-defined w.r.t. the run-time type specialization of the method
       and its declaring type. *)
    PrimImport(method : Method, script : Expression) : Expression =
        let n = <arity of method> in
        let at1 = <type structure of method argument 1 type> in
        ...
        let atn = <type structure of method argument n type> in
        let rt = <type structure of method result type> in
        let t = <type structure of method.DeclaringType> in
        if method.IsStatic then
            "function (x1, ..., xn) {
                 try {
                     return rt.Import(script(at1.Export(x1), ..., atn.Export(xn)));
                 }
                 catch (e) {
                     throw root.ImportException(e);
                 }
             }"
        else
            "function (x1, ..., xn) {
                 try {
                     return rt.Import(script(at1.ExportThis(x1), at2.Export(x2)..., atn.Export(xn)));
                 }
                 catch (e) {
                     throw root.ImportException(e);
                 }
             }"

    (* XREF1109 *)
    (* We may assume all type arguments of method's declaring type are in scope, but method itself
       must be monomorphic. *)
    PrimExport(method : Method, script : Expression) : Expression =
        let n = <arity of method> in
        let at1 = <type structure of method argument 1 type> in
        ...
        let atn = <type structure of method argument n type> in
        let rt = <type structure of method result type> in
        let s = <slot for method> in
        let t = <type structure of method.DeclaringType> in
        if method.IsStatic then
            "function () {
                 script(function(x1, ..., xn) {
                            try {
                                return rt.Export(t.s(at1.Import(x1), ..., atn.Import(xn)));
                            }
                            catch (e) {
                                throw root.ExportException(e);
                            }
                        });
             }"
        else
            "function(x1) {
                 script(at1.Export(x1),
                        function(x2, ..., xn) {
                            try {
                                return rt.Export(t.s(x1, at1.Import(x2), ..., atn.Import(xn)));
                            }
                            catch (e) {
                                throw root.ExportException(e);
                            }
                        });
             }"

    (*
     * General
     *)

    IsSupported(node : Node) : bool = node.IsNotSupported

    (*
     * Types
     *)

    Style(type : Type) : Style = <the style of type as per above definition>

    TypeClassifier(type : Type) : Script =
        let s = Style(type) in
        if s == 'Normal' then
            null
        else if s == 'Proxied' then
            let root = <the root of type> in
            root.Proxy::Script
        else (* s == 'Keyed' *)
            let root = <the root of type> in
            root.Keyed::Script

    (*
     * Methods
     *)

    IsImported(method : Method) : bool = method.IsImport && method.IsExtern || method has Import attribute

    Import(method : Method) : Expression = PrimImport(method, ImportScript(method))

    IsExported(method : Method) : bool =
        method.IsExport && !method.IsExtern && !method.IsNotExported ||
        method.IsVirtual && method.OverriddenMethod != null && IsImported(method.MethodWhichIntroducedVirtualSlot)

    Export(method : Method) : Expression = PrimExport(method, ExportScript(method))

    (*
     * Constructors
     *)

    IsImported(init : InstanceInitializer) : bool = init.IsImport && init.IsExtern || init has Import attribute

    (* XREF1117 *)
    Import(init : InstanceInitializer) : Expression =
        let n = <arity of init> in
        let at1 = <type structure of init argument 1 type> in
        ...
        let atn = <type structure of init argument n type> in
        let t = <type structure of init.DeclaringType> in
        let u = <type structure of init.DeclaringType.BaseType> in
        let js = <type structure of JSObject> in
        let script = ImportScript(init) in
        let importingCtor = if init.DeclaringType has ctor(InteropStyle, JSObject, <init argument 2 type>, ..., <init argument n type>) then
                                "t.<ctor>(x1, null, js.Import(y), x2, ..., xn);"
                            else if init.DeclaringType has ctor(InteropStyle, <init argument 2 type>, ..., <init argument n type>) then
                                "t.<ctor>(x1, null, x2, ..., xn);"
                            else if init.DeclaringType has ctor(IteropStyle, JSObject) then
                                "t.<ctor>(x1, null, js.Import(y));"
                            else if init.DeclaringType has ctor(InteropStyle) then
                                "t.<ctor>(x1, null);"
                            else if init.DeclaringType.BaseType has ctor(InteropStyle) then
                                "u.<ctor>(x1, null);"
                            else if init.DeclaringType.BaseType has ctor() then
                                "u.<ctor>(x1);"
                            else
                                ERROR in
        let body = let s = Style(init.DeclaringType) in
                   if s == 'Primitive' then
                       "root.InheritProperties(x1, y);"
                   else if s == 'Keyed' then
                       "var k = t.Root.GetKeyField(y);
                        if (k == null) {
                            k = x1.Id;
                            t.Root.SetKeyField(y, k);
                        }
                        t.Root.KeyToObject[k.toString()] = x1;
                        x1.Unmanaged = y;
                        importingCtor"
                   else (* s == 'Proxied' *)
                       "x1.Unmanaged = y;
                        importingCtor" in
        if init.DeclaringType.IsValueType then
            if Style(init.DeclaringType) == 'Primitive' then
                "function (x1, x2, ..., xn) {
                     try {
                         script(at1.Export(x1), at2.Export(x2), ..., atn.Export(xn));
                     }
                     catch (e) {
                         throw root.ImportException(e);
                     }
                 }"
            else
                ERROR
        else
            "function (x1, x2, ..., xn) {
                 var y;
                 try {
                     y = script(at2.Export(x2), ..., atn.Export(xn));
                 }
                 catch (e) {
                     throw root.ImportException(e);
                 }
                 body
             }"

    IsExported(init : InstanceInitializer) : bool = init.IsExport && !init.IsExtern && !init.IsNotExported

    (* XREF1123 *)
    Export(init : InstanceInitializer) : Expression =
        let n = <arity of init> in
        let at1 = <type structure of init argument 1 type> in
        ...
        let atn = <type structure of init argument n type> in
        let t = <type structure of init.DeclaringType> in
        let body = if init.DeclaringType is a value type then
                       "t.<ctor>(root.NewPointerToVariable(x1, t), at2.Import(x2), ..., atn.Import(xn))"
                   else
                       "t.<ctor>(x1, at2.Import(x2), ..., atn.Import(xn))" in
        let script = ExportScript(init) in
        "function() {
             script(function(x2, ..., xn) {
                        try {
                            var x1 = t.I(t);
                            body;
                            return t.Export(x1);  // used declaring type, not type of first arg
                        }
                        catch(e) {
                            throw root.ExportException(e);
                        }
                    });
         }"

    (*
     * Properties
     *)

    IsImported(getter : PropertyGetter) : bool = getter.IsImport && getter.IsExtern || getter has Import attribute

    Import(getter : PropertyGetter) : Expression = PrimImport(getter, ImportScript(getter))

    IsImported(setter : PropertySetter) : bool = setter.IsImport && setter.IsExtern || setter has Import attribute

    Import(setter : PropertySetter) : Expression = PrimImport(setter, ImportScript(setter))

    IsExported(getter : PropertyGetter) : bool = getter.IsExport && !getter.IsExtern && !init.IsNotExported

    Export(getter : PropertyGetter) : Expression = PrimExport(getter, ExportScript(getter))

    IsExported(setter : PropertySetter) : bool = setter.IsExport && !setter.IsExtern && !init.IsNotExported

    Export(setter : PropertySetter) : Expression = PrimExport(setter, ExportScript(setter))

    (*
     * Events
     *)

    IsImported(adder : EventAdder) : bool = adder.IsImport && adder.IsExtern || adder has Import attribute

    (* XREF1129 *)
    Import(adder : EventAdder) : Expression =
        let t = <type structure for adder.DeclaringType> in
        let dt = <type structure of event handler delegate type> in
        let s = <event slot for adder.DeclaringEvent> in
        let sl = if adder.IsStatic then
                     <type structure for adder.DeclaringType>
                 else
                     "i" in
        let eta = if adder.Import::SimulateMulticastEvents then
                      "dt.Export(sl.s)"
                  else
                      "dt.Export(d)" in
        let script = ImportScript(adder) in
        let doadd = if adder.IsStatic then
                        "script(eta)"
                    else
                        "script(t.ExportThis(i), eta)" in
        let body = if adder.Import::SimulateMulticastEvents then
                       "if (sl.s == null) {
                            sl.s = d;
                            try { doadd } catch(e) { throw root.ImportException(e); }
                        }
                        else
                            sl.s = root.CombineDelegates(sl.s, d);"
                   else
                       "try { doadd } catch(e) { throw root.ImportException(e); }" in
        if adder.IsStatic then
            "function(d) { body }"
        else
            "function (i, d) { body }"

    IsImported(remover : EventRemover) : bool = remover.IsImport && remover.IsExtern || remover has Import attribute

    Import(remover : EventRemover) : Expression =
        let t = <type structure for remover.DeclaringType> in
        let dt = <type structure of event handler delegate type> in
        let s = <event slot for remover.DeclaringEvent> in
        let sl = if remover.IsStatic then
                     <type structure for remover.DeclaringType>
                 else
                     "i" in
        let eta = if remover.Import::SimulateMulticastEvents then
                      "null"
                  else
                      "dt.Export(d)" in
        let script = ImportScript(remover) in
        let doremove = if remover.IsStatic then
                           "script(eta)"
                       else
                           "script(<adder first arg type>.Export(i), eta)" in
        let body = if remover.Import::SimulateMulticastEvents then
                       "if (sl.s == null)
                            throw root.InvalidOperationException();
                        else {
                            sl.s = root.RemoveAllDelegates(sl.s, d);
                            if (sl.s == null) {
                                try { doremove } catch (e) { throw root.ImportException(e); }
                            }
                        }"
                   else
                       "try { doremove } catch(e) { throw root.ImportException(e); }" in
        if remover.IsStatic then
            "function(d) { body }"
        else
            "function(i, d) { body }"

    IsExported(adder : EventAdder) : bool = adder.IsExport && !adder.IsExtern && !init.IsNotExported

    (* XREF1151 *)
    Export(adder : EventAdder) : Expression =
        let dt = <type structure for event handler delegate type> in
        let t = <type structure for adder.DeclaringType> in
        let s = <event slot for adder.DeclaringEvent> in
        let sl = if adder.IsStatic then
                     <type structure for adder.DeclaringType>
                 else
                     "i" in
        let body = "var d = dt.Import(h);
                    if (sl.s == null)
                        sl.s = d;
                    else
                        sl.s = root.CombineDelegates(sl.s, d);" in
        let script = ExportScript(adder) in
        if adder.IsStatic then
            "function() { script(function(h) { try { body } catch (e) { throw root.ExportException(e); } }); }"
        else
            "function(i) { script(<adder first arg type>.Export(i),
                                  function(h) { try { body } catch (e) { throw root.ExportException(e); } });
                         }"

    IsExported(remover : EventRemover) : bool = remover.IsExport && !remover.IsExtern && !init.IsNotExported

    Export(remover : EventRemover) : Expression =
        let dt = <type structure for event handler delegate type> in
        let t = <type structure for remover.DeclaringType> in
        let s = <event slot for remover.DeclaringEvent> in
        let sl = if remover.IsStatic then
                     <type structure for remover.DeclaringType>
                 else
                     "i" in
        let body = "var d = dt.Import(h);  // ok since import is idempotent
                    if (sl.s == null)
                        throw root.InvalidOperationException();
                    else
                        sl.s = root.RemoveAllDelegates(sl.s, d);" in
        let script = ExportScript(remover) in
        if remover.IsStatic then
            "function() { script(function(h) { try { body } catch (e) { throw root.ExportException(e); }); }"
        else
            "function(i) { script(<removed first arg typ>.Export(i),
                                  function(h) { try { body } catch (e) { throw root.ExportException(e); } });
                         }"