gitignore nit* in bin/

[nit.git] / src / naive_interpreter.nit

# This file is part of NIT ( http://www.nitlanguage.org ).
#
# Copyright 2012 Jean Privat <jean@pryen.org>
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# Interpretation of a Nit program directly on the AST
module naive_interpreter

import literal
import typing
import auto_super_init

redef class ToolContext
        # --discover-call-trace
        var opt_discover_call_trace: OptionBool = new OptionBool("Trace calls of the first invocation of a method", "--discover-call-trace")

        redef init
        do
                super
                self.option_context.add_option(self.opt_discover_call_trace)
        end
end

redef class ModelBuilder
        # Execute the program from the entry point (Sys::main) of the `mainmodule'
        # `arguments' are the command-line arguments in order
        # REQUIRE that:
        #   1. the AST is fully loaded.
        #   2. the model is fully built.
        #   3. the instructions are fully analysed.
        fun run_naive_interpreter(mainmodule: MModule, arguments: Array[String])
        do
                var time0 = get_time
                self.toolcontext.info("*** START INTERPRETING ***", 1)

                var interpreter = new NaiveInterpreter(self, mainmodule, arguments)
                init_naive_interpreter(interpreter, mainmodule)

                var time1 = get_time
                self.toolcontext.info("*** END INTERPRETING: {time1-time0} ***", 2)
        end

        private fun init_naive_interpreter(interpreter: NaiveInterpreter, mainmodule: MModule) do
                var sys_type = mainmodule.sys_type
                if sys_type == null then return # no class Sys
                var mainobj = new MutableInstance(sys_type)
                interpreter.mainobj = mainobj
                interpreter.init_instance(mainobj)
                var initprop = mainmodule.try_get_primitive_method("init", sys_type)
                if initprop != null then
                        interpreter.send(initprop, [mainobj])
                end
                interpreter.check_init_instance(mainobj)
                var mainprop = mainmodule.try_get_primitive_method("main", sys_type)
                if mainprop != null then
                        interpreter.send(mainprop, [mainobj])
                end
        end
end

# The visitor that interprets the Nit Program by walking on the AST
private class NaiveInterpreter
        # The modelbuilder that know the AST and its associations with the model
        var modelbuilder: ModelBuilder

        # The main moduleof the program (used to lookup methoda
        var mainmodule: MModule

        # The command line arguments of the interpreted program
        # arguments.first is the program name
        # arguments[1] is the first argument
        var arguments: Array[String]

        var mainobj: nullable Instance

        init(modelbuilder: ModelBuilder, mainmodule: MModule, arguments: Array[String])
        do
                self.modelbuilder = modelbuilder
                self.mainmodule = mainmodule
                self.arguments = arguments
                self.true_instance = new PrimitiveInstance[Bool](mainmodule.bool_type, true)
                self.false_instance = new PrimitiveInstance[Bool](mainmodule.bool_type, false)
                self.null_instance = new MutableInstance(mainmodule.model.null_type)
        end

        # Subtype test in the context of the mainmodule
        fun is_subtype(sub, sup: MType): Bool
        do
                return sub.is_subtype(self.mainmodule, self.frame.arguments.first.mtype.as(MClassType), sup)
        end

        fun force_get_primitive_method(name: String, recv: MType): MMethod
        do
                return self.modelbuilder.force_get_primitive_method(self.frame.current_node, name, recv, self.mainmodule)
        end

        # Is a return executed?
        # Set this mark to skip the evaluation until the end of the specified method frame
        var returnmark: nullable Frame = null

        # Is a break executed?
        # Set this mark to skip the evaluation until a labeled statement catch it with `is_break'
        var breakmark: nullable EscapeMark = null

        # Is a continue executed?
        # Set this mark to skip the evaluation until a labeled statement catch it with `is_continue'
        var continuemark: nullable EscapeMark = null

        # Is a return or a break or a continue executed?
        # Use this function to know if you must skip the evaluation of statements
        fun is_escaping: Bool do return returnmark != null or breakmark != null or continuemark != null

        # The value associated with the current return/break/continue, if any.
        # Set the value when you set a escapemark.
        # Read the value when you catch a mark or reach the end of a method
        var escapevalue: nullable Instance = null

        # If there is a break and is associated with `escapemark', then return true an clear the mark.
        # If there is no break or if `escapemark' is null then return false.
        # Use this function to catch a potential break.
        fun is_break(escapemark: nullable EscapeMark): Bool
        do
                if escapemark != null and self.breakmark == escapemark then
                        self.breakmark = null
                        return true
                else
                        return false
                end
        end

        # If there is a continue and is associated with `escapemark', then return true an clear the mark.
        # If there is no continue or if `escapemark' is null then return false.
        # Use this function to catch a potential continue.
        fun is_continue(escapemark: nullable EscapeMark): Bool
        do
                if escapemark != null and self.continuemark == escapemark then
                        self.continuemark = null
                        return true
                else
                        return false
                end
        end

        # Evaluate `n' as an expression in the current context.
        # Return the value of the expression.
        # If `n' cannot be evaluated, then aborts.
        fun expr(n: AExpr): nullable Instance
        do
                var frame = self.frame
                var old = frame.current_node
                frame.current_node = n
                #n.debug("IN Execute expr")
                var i = n.expr(self)
                if i == null and not self.is_escaping then
                        n.debug("inconsitance: no value and not escaping.")
                end
                #n.debug("OUT Execute expr: value is {i}")
                #if not is_subtype(i.mtype, n.mtype.as(not null)) then n.debug("Expected {n.mtype.as(not null)} got {i}")
                frame.current_node = old
                return i
        end

        # Evaluate `n' as a statement in the current context.
        # Do nothing if `n' is sull.
        # If `n' cannot be evaluated, then aborts.
        fun stmt(n: nullable AExpr)
        do
                if n != null then
                        var frame = self.frame
                        var old = frame.current_node
                        frame.current_node = n
                        #n.debug("Execute stmt")
                        n.stmt(self)
                        frame.current_node = old
                end
        end

        # Map used to store values of nodes that must be evaluated once in the system (AOnceExpr)
        var onces: Map[ANode, Instance] = new HashMap[ANode, Instance]

        # Return the boolean instance associated with `val'.
        fun bool_instance(val: Bool): Instance
        do
                if val then return self.true_instance else return self.false_instance
        end

        # Return the integer instance associated with `val'.
        fun int_instance(val: Int): Instance
        do
                var ic = self.mainmodule.get_primitive_class("Int")
                return new PrimitiveInstance[Int](ic.mclass_type, val)
        end

        # Return the char instance associated with `val'.
        fun char_instance(val: Char): Instance
        do
                var ic = self.mainmodule.get_primitive_class("Char")
                return new PrimitiveInstance[Char](ic.mclass_type, val)
        end

        # Return the float instance associated with `val'.
        fun float_instance(val: Float): Instance
        do
                var ic = self.mainmodule.get_primitive_class("Float")
                return new PrimitiveInstance[Float](ic.mclass_type, val)
        end

        # The unique intance of the `true' value.
        var true_instance: Instance

        # The unique intance of the `false' value.
        var false_instance: Instance

        # The unique intance of the `null' value.
        var null_instance: Instance

        # Return a new array made of `values'.
        # The dynamic type of the result is Array[elttype].
        fun array_instance(values: Array[Instance], elttype: MType): Instance
        do
                assert not elttype.need_anchor
                var nat = new PrimitiveInstance[Array[Instance]](self.mainmodule.get_primitive_class("NativeArray").get_mtype([elttype]), values)
                var mtype = self.mainmodule.get_primitive_class("Array").get_mtype([elttype])
                var res = new MutableInstance(mtype)
                self.init_instance(res)
                self.send(self.force_get_primitive_method("with_native", mtype), [res, nat, self.int_instance(values.length)])
                self.check_init_instance(res)
                return res
        end

        # Return a new native string initialized with `txt'
        fun native_string_instance(txt: String): Instance
        do
                var val = new Buffer.from(txt)
                val.add('\0')
                var ic = self.mainmodule.get_primitive_class("NativeString")
                return new PrimitiveInstance[Buffer](ic.mclass_type, val)
        end

        # The current frame used to store local variables of the current method executed
        fun frame: Frame do return frames.first

        # The stack of all frames. The first one is the current one.
        var frames: List[Frame] = new List[Frame]

        # Return a stack stace. One line per function
        fun stack_trace: String
        do
                var b = new Buffer
                b.append(",---- Stack trace -- - -  -\n")
                for f in frames do
                        b.append("| {f.mpropdef} ({f.current_node.location})\n")
                end
                b.append("`------------------- - -  -")
                return b.to_s
        end

        # Exit the program with a message
        fun fatal(message: String)
        do
                if frames.is_empty then
                        print message
                else
                        self.frame.current_node.fatal(self, message)
                end
                exit(1)
        end

        # Debug on the current node
        fun debug(message: String)
        do
                if frames.is_empty then
                        print message
                else
                        self.frame.current_node.debug(message)
                end
        end

        # Store known method, used to trace methods as thez are reached
        var discover_call_trace: Set[MMethodDef] = new HashSet[MMethodDef]

        # Execute `mpropdef' for a `args' (where args[0] is the receiver).
        # Return a falue if `mpropdef' is a function, or null if it is a procedure.
        # The call is direct/static. There is no message-seding/late-bindng.
        fun call(mpropdef: MMethodDef, args: Array[Instance]): nullable Instance
        do
                if self.modelbuilder.toolcontext.opt_discover_call_trace.value and not self.discover_call_trace.has(mpropdef) then
                        self.discover_call_trace.add mpropdef
                        self.debug("Discovered {mpropdef}")
                end
                var vararg_rank = mpropdef.msignature.vararg_rank
                if vararg_rank >= 0 then
                        assert args.length >= mpropdef.msignature.arity + 1 # because of self
                        var rawargs = args
                        args = new Array[Instance]

                        args.add(rawargs.first) # recv

                        for i in [0..vararg_rank[ do
                                args.add(rawargs[i+1])
                        end

                        var vararg_lastrank = vararg_rank + rawargs.length-1-mpropdef.msignature.arity
                        var vararg = new Array[Instance]
                        for i in [vararg_rank..vararg_lastrank] do
                                vararg.add(rawargs[i+1])
                        end
                        # FIXME: its it to late to determine the vararg type, this should have been done during a previous analysis
                        var elttype = mpropdef.msignature.mparameters[vararg_rank].mtype.anchor_to(self.mainmodule, args.first.mtype.as(MClassType))
                        args.add(self.array_instance(vararg, elttype))

                        for i in [vararg_lastrank+1..rawargs.length-1[ do
                                args.add(rawargs[i+1])
                        end
                end
                if args.length < mpropdef.msignature.arity + 1 or args.length > mpropdef.msignature.arity + 1 + mpropdef.msignature.mclosures.length then
                        fatal("NOT YET IMPLEMENTED: Invalid arity for {mpropdef}. {args.length} arguments given.")
                end
                if args.length < mpropdef.msignature.arity + 1 + mpropdef.msignature.mclosures.length then
                        fatal("NOT YET IMPLEMENTED: default closures")
                end

                # Look for the AST node that implements the property
                var mproperty = mpropdef.mproperty
                if self.modelbuilder.mpropdef2npropdef.has_key(mpropdef) then
                        var npropdef = self.modelbuilder.mpropdef2npropdef[mpropdef]
                        return npropdef.call(self, mpropdef, args)
                else if mproperty.name == "init" then
                        var nclassdef = self.modelbuilder.mclassdef2nclassdef[mpropdef.mclassdef]
                        return nclassdef.call(self, mpropdef, args)
                else
                        fatal("Fatal Error: method {mpropdef} not found in the AST")
                        abort
                end
        end

        fun call_closure(closure: ClosureInstance, args: Array[Instance]): nullable Instance
        do
                var nclosuredef = closure.nclosuredef
                var f = closure.frame
                for i in [0..closure.nclosuredef.mclosure.mtype.as(MSignature).arity[ do
                        var variable = nclosuredef.variables[i]
                        f.map[variable] = args[i]
                end

                self.frames.unshift(f)

                self.stmt(nclosuredef.n_expr)

                self.frames.shift

                if self.is_continue(nclosuredef.escapemark) then
                        var res = self.escapevalue
                        self.escapevalue = null
                        return res
                end
                return null
        end

        # Execute `mproperty' for a `args' (where args[0] is the receiver).
        # Return a falue if `mproperty' is a function, or null if it is a procedure.
        # The call is polimotphic. There is a message-seding/late-bindng according to te receiver (args[0]).
        fun send(mproperty: MMethod, args: Array[Instance]): nullable Instance
        do
                var recv = args.first
                var mtype = recv.mtype
                if mtype isa MNullType then
                        if mproperty.name == "==" then
                                return self.bool_instance(args[0] == args[1])
                        else if mproperty.name == "!=" then
                                return self.bool_instance(args[0] != args[1])
                        end
                        #fatal("Reciever is null. {mproperty}. {args.join(" ")} {self.frame.current_node.class_name}")
                        fatal("Reciever is null")
                        abort
                end
                var propdefs = mproperty.lookup_definitions(self.mainmodule, mtype)
                if propdefs.length > 1 then
                        fatal("NOT YET IMPLEMETED ERROR: Property conflict: {propdefs.join(", ")}")
                        abort
                end
                assert propdefs.length == 1 else
                        fatal("Fatal Error: No property '{mproperty}' for '{recv}'")
                        abort
                end
                var propdef = propdefs.first
                return self.call(propdef, args)
        end

        # Read the attribute `mproperty' of an instance `recv' and return its value.
        # If the attribute in not yet initialized, then aborts with an error message.
        fun read_attribute(mproperty: MAttribute, recv: Instance): Instance
        do
                assert recv isa MutableInstance
                if not recv.attributes.has_key(mproperty) then
                        fatal("Uninitialized attribute {mproperty.name}")
                        abort
                end
                return recv.attributes[mproperty]
        end

        # Collect attributes of a type in the order of their init
        fun collect_attr_propdef(mtype: MType): Array[AAttrPropdef]
        do
                var cache = self.collect_attr_propdef_cache
                if cache.has_key(mtype) then return cache[mtype]

                var res = new Array[AAttrPropdef]
                for cd in mtype.collect_mclassdefs(self.mainmodule)
                do
                        var n = self.modelbuilder.mclassdef2nclassdef[cd]
                        for npropdef in n.n_propdefs do
                                if npropdef isa AAttrPropdef then
                                        res.add(npropdef)
                                end
                        end
                end

                cache[mtype] = res
                return res
        end

        var collect_attr_propdef_cache = new HashMap[MType, Array[AAttrPropdef]]

        # Fill the initial values of the newly created instance `recv'.
        # `recv.mtype' is used to know what must be filled.
        fun init_instance(recv: Instance)
        do
                for npropdef in collect_attr_propdef(recv.mtype) do
                        npropdef.init_expr(self, recv)
                end
        end

        # Check that non nullable attributes of `recv' are correctly initialized.
        # This function is used as the last instruction of a new
        # FIXME: this will work better once there is nullable types
        fun check_init_instance(recv: Instance)
        do
                if not recv isa MutableInstance then return
                for npropdef in collect_attr_propdef(recv.mtype) do
                        if npropdef.n_expr == null then
                                # Force read to check the initialization
                                self.read_attribute(npropdef.mpropdef.mproperty, recv)
                        end
                end
        end

        # This function determine the correct type according the reciever of the current definition (self).
        fun unanchor_type(mtype: MType): MType
        do
                return mtype.anchor_to(self.mainmodule, self.frame.arguments.first.mtype.as(MClassType))
        end
end

# An instance represents a value of the executed program.
abstract class Instance
        # The dynamic type of the instance
        # ASSERT: not self.mtype.is_anchored
        var mtype: MType

        # return true if the instance is the true value.
        # return false if the instance is the true value.
        # else aborts
        fun is_true: Bool do abort

        # Return true if `self' IS `o' (using the Nit semantic of is)
        fun eq_is(o: Instance): Bool do return self is o

        # Human readable object identity "Type#number"
        redef fun to_s do return "{mtype}#{object_id}"

        # Return the integer value if the instance is an integer.
        # else aborts
        fun to_i: Int do abort

        # Return the integer value if the instance is a float.
        # else aborts
        fun to_f: Float do abort

        # The real value encapsulated if the instance is primitive.
        # Else aborts.
        fun val: Object do abort
end

# A instance with attribute (standards objects)
class MutableInstance
        super Instance

        # The values of the attributes
        var attributes: Map[MAttribute, Instance] = new HashMap[MAttribute, Instance]
end

# Special instance to handle primitives values (int, bool, etc.)
# The trick it just to encapsulate the <<real>> value
class PrimitiveInstance[E: Object]
        super Instance

        # The real value encapsulated
        redef var val: E

        init(mtype: MType, val: E)
        do
                super(mtype)
                self.val = val
        end

        redef fun is_true
        do
                if val == true then return true
                if val == false then return false
                abort
        end

        redef fun ==(o)
        do
                if not o isa PrimitiveInstance[Object] then return false
                return self.val == o.val
        end

        redef fun eq_is(o)
        do
                if not o isa PrimitiveInstance[Object] then return false
                return self.val is o.val
        end

        redef fun to_s do return "{mtype}#{val.object_id}({val})"

        redef fun to_i do return val.as(Int)

        redef fun to_f do return val.as(Float)
end

private class ClosureInstance
        super Instance

        var frame: Frame

        var nclosuredef: AClosureDef

        init(mtype: MType, frame: Frame, nclosuredef: AClosureDef)
        do
                super(mtype)
                self.frame = frame
                self.nclosuredef = nclosuredef
        end
end

# Information about local variables in a running method
private class Frame
        # The current visited node
        # The node is stored by frame to keep a stack trace
        var current_node: ANode
        # The executed property.
        # A Method in case of a call, an attribute in case of a default initialization.
        var mpropdef: MPropDef
        # Arguments of the method (the first is te receiver
        var arguments: Array[Instance]
        # Mapping betwen a variable an the current value
        var map: Map[Variable, Instance] = new HashMap[Variable, Instance]
end

redef class ANode
        # Aborts the program with a message
        # `v' is used to know if a colored message is displayed or not
        private fun fatal(v: NaiveInterpreter, message: String)
        do
                if v.modelbuilder.toolcontext.opt_no_color.value == true then
                        print("Runtime error: {message} ({location.file.filename}:{location.line_start})")
                else
                        print("{location}: Runtime error: {message}\n{location.colored_line("0;31")}")
                        print(v.stack_trace)
                end
                exit(1)
        end
end

redef class APropdef
        # Execute a `mpropdef' associated with the current node.
        private fun call(v: NaiveInterpreter, mpropdef: MMethodDef, args: Array[Instance]): nullable Instance
        do
                fatal(v, "NOT YET IMPLEMENTED method kind {class_name}. {mpropdef}")
                abort
        end
end

redef class AConcreteMethPropdef
        redef fun call(v, mpropdef, args)
        do
                var f = new Frame(self, self.mpropdef.as(not null), args)
                for i in [0..mpropdef.msignature.arity[ do
                        var variable = self.n_signature.n_params[i].variable
                        assert variable != null
                        f.map[variable] = args[i+1]
                end
                for i in [0..mpropdef.msignature.mclosures.length[ do
                        var c = mpropdef.msignature.mclosures[i]
                        var variable = self.n_signature.n_closure_decls[i].variable
                        assert variable != null
                        f.map[variable] = args[i + 1 + mpropdef.msignature.arity]
                end

                v.frames.unshift(f)

                # Call the implicit super-init
                var auto_super_inits = self.auto_super_inits
                if auto_super_inits != null then
                        var selfarg = [args.first]
                        for auto_super_init in auto_super_inits do
                                if auto_super_init.intro.msignature.arity == 0 then
                                        v.send(auto_super_init, selfarg)
                                else
                                        v.send(auto_super_init, args)
                                end
                        end
                end

                v.stmt(self.n_block)
                v.frames.shift
                if v.returnmark == f then
                        v.returnmark = null
                        var res = v.escapevalue
                        v.escapevalue = null
                        return res
                end
                return null
        end
end

redef class AInternMethPropdef
        redef fun call(v, mpropdef, args)
        do
                var pname = mpropdef.mproperty.name
                var cname = mpropdef.mclassdef.mclass.name
                if pname == "output" then
                        var recv = args.first
                        recv.val.output
                        return null
                else if pname == "object_id" then
                        var recv = args.first
                        if recv isa PrimitiveInstance[Object] then
                                return v.int_instance(recv.val.object_id)
                        else
                                return v.int_instance(recv.object_id)
                        end
                else if pname == "output_class_name" then
                        var recv = args.first
                        print recv.mtype.as(MClassType).mclass
                        return null
                else if pname == "native_class_name" then
                        var recv = args.first
                        var txt = recv.mtype.as(MClassType).mclass.to_s
                        return v.native_string_instance(txt)
                else if pname == "==" then
                        # == is correclt redefined for instances
                        return v.bool_instance(args[0] == args[1])
                else if pname == "!=" then
                        return v.bool_instance(args[0] != args[1])
                else if pname == "is_same_type" then
                        return v.bool_instance(args[0].mtype == args[1].mtype)
                else if pname == "exit" then
                        exit(args[1].to_i)
                        abort
                else if pname == "sys" then
                        return v.mainobj
                else if cname == "Int" then
                        if pname == "unary -" then
                                return v.int_instance(-args[0].to_i)
                        else if pname == "succ" then
                                return v.int_instance(args[0].to_i + 1)
                        else if pname == "prec" then
                                return v.int_instance(args[0].to_i - 1)
                        else if pname == "+" then
                                return v.int_instance(args[0].to_i + args[1].to_i)
                        else if pname == "-" then
                                return v.int_instance(args[0].to_i - args[1].to_i)
                        else if pname == "*" then
                                return v.int_instance(args[0].to_i * args[1].to_i)
                        else if pname == "%" then
                                return v.int_instance(args[0].to_i % args[1].to_i)
                        else if pname == "/" then
                                return v.int_instance(args[0].to_i / args[1].to_i)
                        else if pname == "<" then
                                return v.bool_instance(args[0].to_i < args[1].to_i)
                        else if pname == ">" then
                                return v.bool_instance(args[0].to_i > args[1].to_i)
                        else if pname == "<=" then
                                return v.bool_instance(args[0].to_i <= args[1].to_i)
                        else if pname == ">=" then
                                return v.bool_instance(args[0].to_i >= args[1].to_i)
                        else if pname == "<=>" then
                                return v.int_instance(args[0].to_i <=> args[1].to_i)
                        else if pname == "ascii" then
                                return v.char_instance(args[0].to_i.ascii)
                        else if pname == "to_f" then
                                return v.float_instance(args[0].to_i.to_f)
                        else if pname == "lshift" then
                                return v.int_instance(args[0].to_i.lshift(args[1].to_i))
                        else if pname == "rshift" then
                                return v.int_instance(args[0].to_i.rshift(args[1].to_i))
                        end
                else if cname == "Char" then
                        var recv = args[0].val.as(Char)
                        if pname == "ascii" then
                                return v.int_instance(recv.ascii)
                        else if pname == "succ" then
                                return v.char_instance(recv.succ)
                        else if pname == "prec" then
                                return v.char_instance(recv.prec)
                        else if pname == "<" then
                                return v.bool_instance(recv < args[1].val.as(Char))
                        else if pname == ">" then
                                return v.bool_instance(recv > args[1].val.as(Char))
                        else if pname == "<=" then
                                return v.bool_instance(recv <= args[1].val.as(Char))
                        else if pname == ">=" then
                                return v.bool_instance(recv >= args[1].val.as(Char))
                        else if pname == "<=>" then
                                return v.int_instance(recv <=> args[1].val.as(Char))
                        end
                else if cname == "Float" then
                        if pname == "unary -" then
                                return v.float_instance(-args[0].to_f)
                        else if pname == "+" then
                                return v.float_instance(args[0].to_f + args[1].to_f)
                        else if pname == "-" then
                                return v.float_instance(args[0].to_f - args[1].to_f)
                        else if pname == "*" then
                                return v.float_instance(args[0].to_f * args[1].to_f)
                        else if pname == "/" then
                                return v.float_instance(args[0].to_f / args[1].to_f)
                        else if pname == "to_i" then
                                return v.int_instance(args[0].to_f.to_i)
                        end
                else if cname == "NativeString" then
                        var recvval = args.first.val.as(Buffer)
                        if pname == "[]" then
                                var arg1 = args[1].to_i
                                if arg1 >= recvval.length or arg1 < 0 then
                                        debug("Illegal access on {recvval} for element {arg1}/{recvval.length}")
                                end
                                return v.char_instance(recvval[arg1])
                        else if pname == "[]=" then
                                var arg1 = args[1].to_i
                                if arg1 >= recvval.length or arg1 < 0 then
                                        debug("Illegal access on {recvval} for element {arg1}/{recvval.length}")
                                end
                                recvval[arg1] = args[2].val.as(Char)
                                return null
                        else if pname == "copy_to" then
                                # sig= copy_to(dest: NativeString, length: Int, from: Int, to: Int)
                                var destval = args[1].val.as(Buffer)
                                var lenval = args[2].to_i
                                var fromval = args[3].to_i
                                var toval = args[4].to_i
                                if fromval < 0 then
                                        debug("Illegal access on {recvval} for element {fromval}/{recvval.length}")
                                end
                                if fromval + lenval >= recvval.length then
                                        debug("Illegal access on {recvval} for element {fromval}+{lenval}/{recvval.length}")
                                end
                                if toval < 0 then
                                        debug("Illegal access on {destval} for element {toval}/{destval.length}")
                                end
                                if toval + lenval >= destval.length then
                                        debug("Illegal access on {destval} for element {toval}+{lenval}/{destval.length}")
                                end
                                recvval.copy(fromval, lenval, destval, toval)
                                return null
                        else if pname == "atoi" then
                                return v.int_instance(recvval.to_i)
                        end
                else if pname == "calloc_string" then
                        return v.native_string_instance("!" * args[1].to_i)
                else if cname == "NativeArray" then
                        var recvval = args.first.val.as(Array[Instance])
                        if pname == "[]" then
                                if args[1].to_i >= recvval.length or args[1].to_i < 0 then
                                        debug("Illegal access on {recvval} for element {args[1].to_i}/{recvval.length}")
                                end
                                return recvval[args[1].to_i]
                        else if pname == "[]=" then
                                recvval[args[1].to_i] = args[2]
                                return null
                        else if pname == "copy_to" then
                                recvval.copy(0, args[2].to_i, args[1].val.as(Array[Instance]), 0)
                                return null
                        end
                else if pname == "calloc_array" then
                        var recvtype = args.first.mtype.as(MClassType)
                        var mtype: MType
                        mtype = recvtype.supertype_to(v.mainmodule, recvtype, v.mainmodule.get_primitive_class("ArrayCapable"))
                        mtype = mtype.arguments.first
                        var val = new Array[Instance].filled_with(v.null_instance, args[1].to_i)
                        return new PrimitiveInstance[Array[Instance]](v.mainmodule.get_primitive_class("NativeArray").get_mtype([mtype]), val)
                end
                fatal(v, "NOT YET IMPLEMENTED intern {mpropdef}")
                abort
        end
end

redef class AbstractArray[E]
        fun copy(start: Int, len: Int, dest: AbstractArray[E], new_start: Int)
        do
                self.copy_to(start, len, dest, new_start)
        end
end

redef class AExternInitPropdef
        redef fun call(v, mpropdef, args)
        do
                var pname = mpropdef.mproperty.name
                var cname = mpropdef.mclassdef.mclass.name
                if pname == "native_stdout" then
                        return new PrimitiveInstance[OStream](mpropdef.mclassdef.mclass.mclass_type, stdout)
                else if pname == "native_stdin" then
                        return new PrimitiveInstance[IStream](mpropdef.mclassdef.mclass.mclass_type, stdin)
                else if pname == "native_stderr" then
                        return new PrimitiveInstance[OStream](mpropdef.mclassdef.mclass.mclass_type, stderr)
                else if pname == "io_open_read" then
                        var a1 = args[1].val.as(Buffer)
                        return new PrimitiveInstance[IStream](mpropdef.mclassdef.mclass.mclass_type, new IFStream.open(a1.to_s))
                else if pname == "io_open_write" then
                        var a1 = args[1].val.as(Buffer)
                        return new PrimitiveInstance[OStream](mpropdef.mclassdef.mclass.mclass_type, new OFStream.open(a1.to_s))
                end
                fatal(v, "NOT YET IMPLEMENTED extern init {mpropdef}")
                abort
        end
end

redef class AExternMethPropdef
        super TablesCapable
        redef fun call(v, mpropdef, args)
        do
                var pname = mpropdef.mproperty.name
                var cname = mpropdef.mclassdef.mclass.name
                if cname == "Int" then
                        var recvval = args.first.val.as(Int)
                        if pname == "rand" then
                                var res = recvval.rand
                                return v.int_instance(res)
                        end
                else if cname == "NativeFile" then
                        var recvval = args.first.val
                        if pname == "io_write" then
                                var a1 = args[1].val.as(Buffer)
                                recvval.as(OStream).write(a1.substring(0, args[2].to_i))
                                return args[2]
                        else if pname == "io_read" then
                                var str = recvval.as(IStream).read(args[2].to_i)
                                var a1 = args[1].val.as(Buffer)
                                new Buffer.from(str).copy(0, str.length, a1, 0)
                                return v.int_instance(str.length)
                        else if pname == "io_close" then
                                recvval.as(IOS).close
                                return v.int_instance(0)
                        end
                else if cname == "NativeString" then
                        var recvval = args.first.val.as(Buffer)
                        if pname == "file_exists" then
                                return v.bool_instance(recvval.to_s.file_exists)
                        else if pname == "file_mkdir" then
                                recvval.to_s.mkdir
                                return null
                        else if pname == "get_environ" then
                                var txt = recvval.to_s.environ
                                return v.native_string_instance(txt)
                        else if pname == "system" then
                                var res = sys.system(recvval.to_s)
                                return v.int_instance(res)
                        end
                else if cname == "Int" then
                        if pname == "rand" then
                                return v.int_instance(args[0].to_i.rand)
                        end
                else if cname == "Float" then
                        if pname == "cos" then
                                return v.float_instance(args[0].to_f.cos)
                        else if pname == "sin" then
                                return v.float_instance(args[0].to_f.sin)
                        end
                else if pname == "native_argc" then
                        return v.int_instance(v.arguments.length)
                else if pname == "native_argv" then
                        var txt = v.arguments[args[1].to_i]
                        return v.native_string_instance(txt)
                else if pname == "get_time" then
                        return v.int_instance(get_time)
                else if pname == "atan2" then
                        return v.float_instance(atan2(args[1].to_f, args[2].to_f))
                else if pname == "pi" then
                        return v.float_instance(pi)
                else if pname == "lexer_goto" then
                        return v.int_instance(lexer_goto(args[1].to_i, args[2].to_i))
                else if pname == "lexer_accept" then
                        return v.int_instance(lexer_accept(args[1].to_i))
                else if pname == "parser_goto" then
                        return v.int_instance(parser_goto(args[1].to_i, args[2].to_i))
                else if pname == "parser_action" then
                        return v.int_instance(parser_action(args[1].to_i, args[2].to_i))
                end
                fatal(v, "NOT YET IMPLEMENTED extern {mpropdef}")
                abort
        end
end

redef class AAttrPropdef
        redef fun call(v, mpropdef, args)
        do
                var recv = args.first
                assert recv isa MutableInstance
                var attr = self.mpropdef.mproperty
                if args.length == 1 then
                        return v.read_attribute(attr, recv)
                else
                        assert args.length == 2
                        recv.attributes[attr] = args[1]
                        return null
                end
        end

        # Evaluate and set the default value of the attribute in `recv'
        private fun init_expr(v: NaiveInterpreter, recv: Instance)
        do
                assert recv isa MutableInstance
                var nexpr = self.n_expr
                if nexpr != null then
                        var f = new Frame(self, self.mpropdef.as(not null), [recv])
                        v.frames.unshift(f)
                        var val = v.expr(nexpr)
                        assert val != null
                        v.frames.shift
                        assert not v.is_escaping
                        recv.attributes[self.mpropdef.mproperty] = val
                        return
                end
                var mtype = self.mpropdef.static_mtype.as(not null)
                # TODO The needinit info is statically computed, move it to modelbuilder or whatever
                mtype = mtype.resolve_for(self.mpropdef.mclassdef.bound_mtype, self.mpropdef.mclassdef.bound_mtype, self.mpropdef.mclassdef.mmodule, true)
                if mtype isa MNullableType then
                        recv.attributes[self.mpropdef.mproperty] = v.null_instance
                end
        end
end

redef class ADeferredMethPropdef
        redef fun call(v, mpropdef, args)
        do
                fatal(v, "Deferred method called")
                abort
        end
end

redef class AClassdef
        # Execute an implicit `mpropdef' associated with the current node.
        private fun call(v: NaiveInterpreter, mpropdef: MMethodDef, args: Array[Instance]): nullable Instance
        do
                var super_inits = self.super_inits
                if super_inits != null then
                        assert args.length == 1
                        for su in super_inits do
                                v.send(su, args)
                        end
                        return null
                end
                var recv = args.first
                assert recv isa MutableInstance
                var i = 1
                # Collect undefined attributes
                for npropdef in self.n_propdefs do
                        if npropdef isa AAttrPropdef and npropdef.n_expr == null then
                                recv.attributes[npropdef.mpropdef.mproperty] = args[i]
                                i += 1
                        end
                end
                return null
        end
end

redef class AExpr
        # Evaluate the node as a possible expression.
        # Return a possible value
        # NOTE: Do not call this method directly, but use `v.expr'
        # This method is here to be implemented by subclasses.
        private fun expr(v: NaiveInterpreter): nullable Instance
        do
                fatal(v, "NOT YET IMPLEMENTED expr {class_name}")
                abort
        end

        # Evaluate the node as a statement.
        # NOTE: Do not call this method directly, but use `v.stmt'
        # This method is here to be implemented by subclasses (no need to return something).
        private fun stmt(v: NaiveInterpreter)
        do
                expr(v)
        end

end

redef class ABlockExpr
        redef fun stmt(v)
        do
                for e in self.n_expr do
                        v.stmt(e)
                        if v.is_escaping then return
                end
        end
end

redef class AVardeclExpr
        redef fun stmt(v)
        do
                var ne = self.n_expr
                if ne != null then
                        var i = v.expr(ne)
                        if i == null then return
                        v.frame.map[self.variable.as(not null)] = i
                end
        end
end

redef class AVarExpr
        redef fun expr(v)
        do
                return v.frame.map[self.variable.as(not null)]
        end
end

redef class AVarAssignExpr
        redef fun stmt(v)
        do
                var i = v.expr(self.n_value)
                if i == null then return
                v.frame.map[self.variable.as(not null)] = i
        end
end

redef class AVarReassignExpr
        redef fun stmt(v)
        do
                var vari = v.frame.map[self.variable.as(not null)]
                var value = v.expr(self.n_value)
                if value == null then return
                var res = v.send(reassign_property.mproperty, [vari, value])
                assert res != null
                v.frame.map[self.variable.as(not null)] = res
        end
end

redef class ASelfExpr
        redef fun expr(v)
        do
                return v.frame.arguments.first
        end
end

redef class AContinueExpr
        redef fun stmt(v)
        do
                var ne = self.n_expr
                if ne != null then
                        var i = v.expr(ne)
                        if i == null then return
                        v.escapevalue = i
                end
                v.continuemark = self.escapemark
        end
end

redef class ABreakExpr
        redef fun stmt(v)
        do
                var ne = self.n_expr
                if ne != null then
                        var i = v.expr(ne)
                        if i == null then return
                        v.escapevalue = i
                end
                v.breakmark = self.escapemark
        end
end

redef class AReturnExpr
        redef fun stmt(v)
        do
                var ne = self.n_expr
                if ne != null then
                        var i = v.expr(ne)
                        if i == null then return
                        v.escapevalue = i
                end
                v.returnmark = v.frame
        end
end

redef class AAbortExpr
        redef fun stmt(v)
        do
                fatal(v, "Aborted")
                exit(1)
        end
end

redef class AIfExpr
        redef fun stmt(v)
        do
                var cond = v.expr(self.n_expr)
                if cond == null then return
                if cond.is_true then
                        v.stmt(self.n_then)
                else
                        v.stmt(self.n_else)
                end
        end
end

redef class AIfexprExpr
        redef fun expr(v)
        do
                var cond = v.expr(self.n_expr)
                if cond == null then return null
                if cond.is_true then
                        return v.expr(self.n_then)
                else
                        return v.expr(self.n_else)
                end
        end
end

redef class ADoExpr
        redef fun stmt(v)
        do
                v.stmt(self.n_block)
                v.is_break(self.escapemark) # Clear the break (if any)
        end
end

redef class AWhileExpr
        redef fun stmt(v)
        do
                loop
                        var cond = v.expr(self.n_expr)
                        if cond == null then return
                        if not cond.is_true then return
                        v.stmt(self.n_block)
                        if v.is_break(self.escapemark) then return
                        v.is_continue(self.escapemark) # Clear the break
                        if v.is_escaping then return
                end
        end
end

redef class ALoopExpr
        redef fun stmt(v)
        do
                loop
                        v.stmt(self.n_block)
                        if v.is_break(self.escapemark) then return
                        v.is_continue(self.escapemark) # Clear the break
                        if v.is_escaping then return
                end
        end
end

redef class AForExpr
        redef fun stmt(v)
        do
                var col = v.expr(self.n_expr)
                if col == null then return
                #self.debug("col {col}")
                var iter = v.send(v.force_get_primitive_method("iterator", col.mtype), [col]).as(not null)
                #self.debug("iter {iter}")
                loop
                        var isok = v.send(v.force_get_primitive_method("is_ok", iter.mtype), [iter]).as(not null)
                        if not isok.is_true then return
                        if self.variables.length == 1 then
                                var item = v.send(v.force_get_primitive_method("item", iter.mtype), [iter]).as(not null)
                                #self.debug("item {item}")
                                v.frame.map[self.variables.first] = item
                        else if self.variables.length == 2 then
                                var key = v.send(v.force_get_primitive_method("key", iter.mtype), [iter]).as(not null)
                                v.frame.map[self.variables[0]] = key
                                var item = v.send(v.force_get_primitive_method("item", iter.mtype), [iter]).as(not null)
                                v.frame.map[self.variables[1]] = item
                        else
                                abort
                        end
                        v.stmt(self.n_block)
                        if v.is_break(self.escapemark) then return
                        v.is_continue(self.escapemark) # Clear the break
                        if v.is_escaping then return
                        v.send(v.force_get_primitive_method("next", iter.mtype), [iter])
                end
        end
end

redef class AAssertExpr
        redef fun stmt(v)
        do
                var cond = v.expr(self.n_expr)
                if cond == null then return
                if not cond.is_true then
                        v.stmt(self.n_else)
                        if v.is_escaping then return
                        var nid = self.n_id
                        if nid != null then
                                fatal(v, "Assert '{nid.text}' failed")
                        else
                                fatal(v, "Assert failed")
                        end
                        exit(1)
                end
        end
end

redef class AOrExpr
        redef fun expr(v)
        do
                var cond = v.expr(self.n_expr)
                if cond == null then return null
                if cond.is_true then return cond
                return v.expr(self.n_expr2)
        end
end

redef class AAndExpr
        redef fun expr(v)
        do
                var cond = v.expr(self.n_expr)
                if cond == null then return null
                if not cond.is_true then return cond
                return v.expr(self.n_expr2)
        end
end

redef class ANotExpr
        redef fun expr(v)
        do
                var cond = v.expr(self.n_expr)
                if cond == null then return null
                return v.bool_instance(not cond.is_true)
        end
end

redef class AOrElseExpr
        redef fun expr(v)
        do
                var i = v.expr(self.n_expr)
                if i == null then return null
                if i != v.null_instance then return i
                return v.expr(self.n_expr2)
        end
end

redef class AEeExpr
        redef fun expr(v)
        do
                var i = v.expr(self.n_expr)
                if i == null then return null
                var i2 = v.expr(self.n_expr2)
                if i2 == null then return null
                return v.bool_instance(i.eq_is(i2))
        end
end

redef class AIntExpr
        redef fun expr(v)
        do
                return v.int_instance(self.value.as(not null))
        end
end

redef class AFloatExpr
        redef fun expr(v)
        do
                return v.float_instance(self.value.as(not null))
        end
end

redef class ACharExpr
        redef fun expr(v)
        do
                return v.char_instance(self.value.as(not null))
        end
end

redef class AArrayExpr
        redef fun expr(v)
        do
                var val = new Array[Instance]
                for nexpr in self.n_exprs.n_exprs do
                        var i = v.expr(nexpr)
                        if i == null then return null
                        val.add(i)
                end
                var mtype = v.unanchor_type(self.mtype.as(not null)).as(MClassType)
                var elttype = mtype.arguments.first
                return v.array_instance(val, elttype)
        end
end

redef class AStringFormExpr
        redef fun expr(v)
        do
                var txt = self.value.as(not null)
                var nat = v.native_string_instance(txt)
                var res = new MutableInstance(v.mainmodule.get_primitive_class("String").mclass_type)
                v.init_instance(res)
                v.send(v.force_get_primitive_method("from_cstring", res.mtype), [res, nat])
                v.check_init_instance(res)
                return res
        end
end

redef class ASuperstringExpr
        redef fun expr(v)
        do
                var array = new Array[Instance]
                for nexpr in n_exprs do
                        var i = v.expr(nexpr)
                        if i == null then return null
                        array.add(i)
                end
                var i = v.array_instance(array, v.mainmodule.get_primitive_class("Object").mclass_type)
                var res = v.send(v.force_get_primitive_method("to_s", i.mtype), [i])
                assert res != null
                return res
        end
end

redef class ACrangeExpr
        redef fun expr(v)
        do
                var e1 = v.expr(self.n_expr)
                if e1 == null then return null
                var e2 = v.expr(self.n_expr2)
                if e2 == null then return null
                var mtype = v.unanchor_type(self.mtype.as(not null))
                var res = new MutableInstance(mtype)
                v.init_instance(res)
                v.send(v.force_get_primitive_method("init", mtype), [res, e1, e2])
                v.check_init_instance(res)
                return res
        end
end

redef class AOrangeExpr
        redef fun expr(v)
        do
                var e1 = v.expr(self.n_expr)
                if e1 == null then return null
                var e2 = v.expr(self.n_expr2)
                if e2 == null then return null
                var mtype = v.unanchor_type(self.mtype.as(not null))
                var res = new MutableInstance(mtype)
                v.init_instance(res)
                v.send(v.force_get_primitive_method("without_last", mtype), [res, e1, e2])
                v.check_init_instance(res)
                return res
        end
end

redef class ATrueExpr
        redef fun expr(v)
        do
                return v.bool_instance(true)
        end
end

redef class AFalseExpr
        redef fun expr(v)
        do
                return v.bool_instance(false)
        end
end

redef class ANullExpr
        redef fun expr(v)
        do
                return v.null_instance
        end
end

redef class AIsaExpr
        redef fun expr(v)
        do
                var i = v.expr(self.n_expr)
                if i == null then return null
                var mtype = v.unanchor_type(self.cast_type.as(not null))
                return v.bool_instance(v.is_subtype(i.mtype, mtype))
        end
end

redef class AAsCastExpr
        redef fun expr(v)
        do
                var i = v.expr(self.n_expr)
                if i == null then return null
                var mtype = v.unanchor_type(self.mtype.as(not null))
                if not v.is_subtype(i.mtype, mtype) then
                        #fatal(v, "Cast failed expected {mtype}, got {i}")
                        fatal(v, "Cast failed")
                end
                return i
        end
end

redef class AAsNotnullExpr
        redef fun expr(v)
        do
                var i = v.expr(self.n_expr)
                if i == null then return null
                var mtype = v.unanchor_type(self.mtype.as(not null))
                if i.mtype isa MNullType then
                        fatal(v, "Cast failed")
                end
                return i
        end
end

redef class AParExpr
        redef fun expr(v)
        do
                return v.expr(self.n_expr)
        end
end

redef class AOnceExpr
        redef fun expr(v)
        do
                if v.onces.has_key(self) then
                        return v.onces[self]
                else
                        var res = v.expr(self.n_expr)
                        if res == null then return null
                        v.onces[self] = res
                        return res
                end
        end
end

redef class ASendExpr
        redef fun expr(v)
        do
                var recv = v.expr(self.n_expr)
                if recv == null then return null
                var args = [recv]
                for a in self.raw_arguments.as(not null) do
                        var i = v.expr(a)
                        if i == null then return null
                        args.add(i)
                end
                for c in self.n_closure_defs do
                        var mtype = c.mclosure.mtype
                        var instance = new ClosureInstance(mtype, v.frame, c)
                        args.add(instance)
                end
                var mproperty = self.mproperty.as(not null)

                var res = v.send(mproperty, args)
                if v.is_break(self.escapemark) then
                        res = v.escapevalue
                        v.escapevalue = null
                end
                return res
        end
end

redef class ASendReassignFormExpr
        redef fun stmt(v)
        do
                var recv = v.expr(self.n_expr)
                if recv == null then return
                var args = [recv]
                for a in self.raw_arguments.as(not null) do
                        var i = v.expr(a)
                        if i == null then return
                        args.add(i)
                end
                var value = v.expr(self.n_value)
                if value == null then return

                var mproperty = self.mproperty.as(not null)
                var read = v.send(mproperty, args)
                assert read != null

                var write = v.send(self.reassign_property.mproperty, [read, value])
                assert write != null

                args.add(write)

                v.send(self.write_mproperty.as(not null), args)
        end
end

redef class ASuperExpr
        redef fun expr(v)
        do
                var recv = v.frame.arguments.first
                var args = [recv]
                for a in self.n_args.n_exprs do
                        var i = v.expr(a)
                        if i == null then return null
                        args.add(i)
                end
                if args.length == 1 then
                        args = v.frame.arguments
                end

                var mproperty = self.mproperty
                if mproperty != null then
                        if mproperty.intro.msignature.arity == 0 then
                                args = [recv]
                        end
                        # Super init call
                        var res = v.send(mproperty, args)
                        return res
                end

                # stantard call-next-method
                var mpropdef = v.frame.mpropdef
                # FIXME: we do not want an ugly static call!
                var mpropdefs = mpropdef.mproperty.lookup_super_definitions(mpropdef.mclassdef.mmodule, mpropdef.mclassdef.bound_mtype)
                if mpropdefs.length != 1 then
                        debug("Warning: NOT YET IMPLEMENTED: multiple MPRODFEFS for super {mpropdef} for {recv}: {mpropdefs.join(", ")}")
                end
                mpropdef = mpropdefs.first
                assert mpropdef isa MMethodDef
                var res = v.call(mpropdef, args)
                return res
        end
end

redef class ANewExpr
        redef fun expr(v)
        do
                var mtype = v.unanchor_type(self.mtype.as(not null))
                var recv: Instance = new MutableInstance(mtype)
                v.init_instance(recv)
                var args = [recv]
                for a in self.n_args.n_exprs do
                        var i = v.expr(a)
                        if i == null then return null
                        args.add(i)
                end
                var mproperty = self.mproperty.as(not null)
                var res2 = v.send(mproperty, args)
                if res2 != null then
                        #self.debug("got {res2} from {mproperty}. drop {recv}")
                        return res2
                end
                v.check_init_instance(recv)
                return recv
        end
end

redef class AAttrExpr
        redef fun expr(v)
        do
                var recv = v.expr(self.n_expr)
                if recv == null then return null
                var mproperty = self.mproperty.as(not null)
                return v.read_attribute(mproperty, recv)
        end
end

redef class AAttrAssignExpr
        redef fun stmt(v)
        do
                var recv = v.expr(self.n_expr)
                if recv == null then return
                var i = v.expr(self.n_value)
                if i == null then return
                var mproperty = self.mproperty.as(not null)
                assert recv isa MutableInstance
                recv.attributes[mproperty] = i
        end
end

redef class AAttrReassignExpr
        redef fun stmt(v)
        do
                var recv = v.expr(self.n_expr)
                if recv == null then return
                var value = v.expr(self.n_value)
                if value == null then return
                var mproperty = self.mproperty.as(not null)
                var attr = v.read_attribute(mproperty, recv)
                var res = v.send(reassign_property.mproperty, [attr, value])
                assert res != null
                assert recv isa MutableInstance
                recv.attributes[mproperty] = res
        end
end

redef class AIssetAttrExpr
        redef fun expr(v)
        do
                var recv = v.expr(self.n_expr)
                if recv == null then return null
                var mproperty = self.mproperty.as(not null)
                assert recv isa MutableInstance
                return v.bool_instance(recv.attributes.has_key(mproperty))
        end
end

redef class AClosureCallExpr
        redef fun expr(v)
        do
                var args = new Array[Instance]
                for a in self.n_args.n_exprs do
                        var i = v.expr(a)
                        if i == null then return null
                        args.add(i)
                end
                var i = v.frame.map[self.variable.as(not null)]
                assert i isa ClosureInstance
                var res = v.call_closure(i, args)
                return res
        end
end

redef class ADebugTypeExpr
        redef fun stmt(v)
        do
                # do nothing
        end
end