put sketch files into `prototype` and important structural improve on sketch.py

2 files changed, 1148 insertions, 0 deletions

diff --git a/prototype/sketch.py b/prototype/sketch.py
new file mode 100644
index 0000000..c2007ee
--- /dev/null
+++ b/prototype/sketch.py
@@ -0,0 +1,540 @@
+#! /bin/env python
+
+class EvalObj(object):
+    def prepare(self, pc):
+        pass
+    def __str__(self):
+        return "#<Object>"
+
+class UnspecObj(EvalObj):
+    def __str__(self):
+        return "#<Unspecified>"
+    def ext_repr(self):
+        return self.__str__()
+
+class NumberObj(EvalObj):
+    def __str__(selfl):
+        return "#<Number>"
+
+class IntObj(NumberObj):
+    def __init__(self, num):
+        self.val = int(num)
+    def __str__(self):
+        return "#<Integer>"
+    def ext_repr(self):
+        return str(self.val)
+
+class FloatObj(NumberObj):
+    def __init__(self, num):
+        self.val = float(num)
+    def __str__(self):
+        return "#<Float>"
+    def ext_repr(self):
+        return str(self.val)
+
+class BoolObj(EvalObj):
+    def __init__(self, b):
+        self.val = b
+    def __str__(self):
+        return "#<Boolean>"
+    def ext_repr(self):
+        if self.val:
+            return "#t"
+        else:
+            return "#f"
+
+class SymObj(EvalObj):
+    def __init__(self, string):
+        self.name = string
+    def __str__(self):
+        return "#<Symbol: " + self.name + ">"
+    def get_name():
+        return self.name
+
+class OptObj(EvalObj):
+    pass
+
+class ProcObj(OptObj):
+    def __init__(self, body, envt, para_list):
+        self.body = body
+        self.envt = envt
+        self.para_list = para_list
+    def call(self, arg_list, envt, cont, stack, top, ret_addr):
+        # Create a new continuation
+        ncont = Continuation(envt, ret_addr, cont, self.body)  
+        cont = ncont                         # Add to the cont chain
+        envt = Environment(self.envt)        # Create local env and recall the closure
+        #TODO: Compare the arguments to the parameters
+        for i in xrange(1, len(arg_list)):
+            envt.add_binding(self.para_list[i - 1], arg_list[i])      
+                                             # Create bindings
+        stack[top] = RetAddr(False)          # Continuation mark 
+        pc = self.body[0]                         # Move to the proc entry point
+        return (pc, top, envt, cont)
+
+    def ext_repr(self):
+        return "#<Procedure>"
+    def __str__(self):
+        return self.ext_repr()
+
+class SpecialOptObj(OptObj):
+        pass
+
+class BuiltinProcObj(OptObj):
+    def __init__(self, f, name):
+        self.handler = f
+        self.name = name
+    def ext_repr(self):
+        return "#<Builtin Procedure: " + self.name + ">"
+    def __str__(self):
+        return self.ext_repr()
+    def call(self, arg_list, envt, cont, stack, top, ret_addr):
+        stack[top] = self.handler(arg_list[1:])
+        return (ret_addr.func.next, top, envt, cont)
+
+# Convert an obj to boolean
+def to_bool(obj):
+    if obj.val is False:
+        return BoolObj(False)
+    else:
+        return BoolObj(True)
+
+# Mark all children of pc as flag
+def _fill_marks(pc, flag):
+    pc = pc.cdr
+    while not (pc is empty_list):
+        pc.func.skip = flag
+        pc = pc.cdr
+
+class _builtin_if(SpecialOptObj):
+    def prepare(self, pc):
+        # TODO: check number of arguments 
+        # Evaluate the condition first
+        self.state = 0 # Prepared
+        pc = pc.cdr
+        pc.func.skip = False
+        pc.cdr.func.skip = True
+        if not (pc.cdr.cdr is empty_list):
+            pc.cdr.cdr.func.skip = True
+
+    def pre_call(self, arg_list, pc, envt, cont):
+        pc = pc.car
+        # Condition evaluated and the decision is made
+        self.state = 1 
+        if (to_bool(arg_list[1])).val:
+            pc = pc.cdr
+            pc.func.skip = True
+            pc.cdr.func.skip = False
+            if not (pc.cdr.cdr is empty_list):
+                # Eval the former
+                pc.cdr.cdr.func.skip = True
+        else:
+            pc = pc.cdr
+            pc.func.skip = True
+            pc.cdr.func.skip = True
+            if not (pc.cdr.cdr is empty_list):
+                # Eval the latter
+                pc.cdr.cdr.func.skip = False
+
+    def post_call(self, arg_list, pc, envt, cont):
+        # Value already evaluated, so just return it
+        return arg_list[1]
+
+    def call(self, arg_list, envt, cont, stack, top, ret_addr):
+        if self.state == 0:
+            self.pre_call(arg_list, ret_addr, envt, cont)
+            top += 1
+            pc = ret_addr.car.func.next     # Invoke again
+        else:
+            stack[top] = self.post_call(arg_list, ret_addr, envt, cont)
+            pc = ret_addr.func.next
+        return (pc, top, envt, cont)
+
+    def ext_repr(self):
+        return "#<Builtin Macro: if>"
+    def __str__(self):
+        return self.ext_repr()
+
+class _builtin_lambda(SpecialOptObj):
+
+    def prepare(self, pc):
+        # TODO: check number of arguments 
+        # Do not evaulate anything
+        _fill_marks(pc, True)
+
+    def call(self, arg_list, envt, cont, stack, top, ret_addr):
+        pc = ret_addr.car
+        para_list = list()              # paramter list
+        par = pc.cdr.car                # Switch to the first parameter
+        while not (par is empty_list): 
+            para_list.append(par.car)
+            par = par.cdr
+
+        # Clear the flag to avoid side-effects (e.g. proc calling)
+        _fill_marks(pc, False)
+
+        pc = pc.cdr.cdr     # Move pc to procedure body
+        #TODO: check body
+        body = list()       # store a list of expressions inside <body>
+
+        while not (pc is empty_list):
+            body.append(pc)
+            pc.func.next = None # Make each expression a orphan 
+                                # in order to ease the exit checking
+            pc = pc.cdr
+
+        stack[top] = ProcObj(body, envt, para_list)
+        return (ret_addr.func.next, top, envt, cont)
+
+    def ext_repr(self):
+        return "#<Builtin Macro: lambda>"
+    def __str__(self):
+        return self.ext_repr()
+
+class _builtin_define(SpecialOptObj):
+
+    def prepare(self, pc):
+        if is_arg(pc.cdr):              # Simple value assignment
+            pc.cdr.func.skip = True     # Skip the identifier
+            pc.cdr.cdr.func.skip = False
+        else:                       # Procedure definition
+            _fill_marks(pc, True)   # Skip all parts
+
+    def call(self, arg_list, envt, cont, stack, top, ret_addr):
+        pc = ret_addr.car
+        # TODO: check identifier
+        if is_arg(pc.cdr):          # Simple value assignment
+            id = pc.cdr.car             
+            obj = arg_list[1]
+        else:                       # Procedure definition
+            id = pc.cdr.car.car
+            para_list = list()      # Parameter list
+            par = pc.cdr.car
+            if not (par.cdr is empty_list):
+                # If there's at least one parameter
+                par = par.cdr
+                while not (par is empty_list):
+                    para_list.append(par.car)
+                    par = par.cdr
+
+            # Clear the flag to avoid side-effects (e.g. proc calling)
+            _fill_marks(pc, False)  
+            pc = pc.cdr.cdr         # Move pc to procedure body
+            #TODO: check body
+            body = list()           # store a list of expressions inside <body>
+
+            while not (pc is empty_list):
+                body.append(pc)
+                pc.func.next = None
+                pc = pc.cdr
+
+            obj = ProcObj(body, envt, para_list)
+        
+        envt.add_binding(id, obj)
+        stack[top] = UnspecObj()
+        return (ret_addr.func.next, top, envt, cont)
+
+    def ext_repr(self):
+        return "#<Builtin Macro: define>"
+    def __str__(self):
+        return self.ext_repr()
+
+class _builtin_set(SpecialOptObj):
+    def prepare(self, pc):
+        # TODO: check number of arguments 
+        pc.cdr.func.skip = True     # Skip the identifier
+        pc.cdr.cdr.func.skip = False  
+
+    def call(self, arg_list, envt, cont, stack, top, ret_addr):
+        pc = ret_addr.car
+        id = pc.cdr.car
+        if envt.has_obj(id):
+            envt.add_binding(id, arg_list[1])
+        stack[top] = UnspecObj()
+        return (ret_addr.func.next, top, envt, cont)
+
+    def ext_repr(self):
+        return "#<Builtin Macro: set!>"
+    def __str__(self):
+        return self.ext_repr()
+
+class Tokenizor():
+
+    def __init__(self):
+        self.data = ""                  # string buffer
+        self.tokenized = list()         # tokens
+
+    def feed(self, data):               # Store the data in the buffer
+        self.data = data
+
+    def read(self): 
+        if len(self.tokenized) == 0:        # no tokens available, let's produce
+            if len(self.data) == 0:         # I'm hungry, feed me!
+                return None
+            self.tokenized = self.data.replace('(', '( ')\
+                                    .replace(')', ' )')\
+                                    .split()
+            self.data = ""                  # Clear the buffer
+            if len(self.tokenized) == 0:    # You feed me with the air, bastard!
+                return None
+        return self.tokenized.pop(0)
+
+class Prog(object):
+    def __init__(self, skip = False, next = None):
+        self.skip = skip
+        self.next = next
+
+class Data(object):
+    pass
+
+class Cons(object):                     # Construction
+    def __init__(self, car, cdr, func = Data()):
+        self.car = car
+        self.cdr = cdr
+        self.func = func
+    def ext_repr(self):
+        return "#<Cons>"
+    def __str__(self):
+        return self.ext_repr()
+    def print_(self):
+        print "car: " + str(self.car) + "\n" + \
+              "cdr: " + str(self.cdr) + "\n"
+
+class EmptyList(object):
+    def ext_repr(self):
+        return "()"
+    def __str__(self):
+        return self.ext_repr()
+
+empty_list = EmptyList()
+
+class RetAddr(object):                  # Return Adress (Refers to an Cons)
+    def __init__(self, addr):
+        self.addr = addr
+    def __str__(self):
+        return "#<Return Address>"
+
+class ASTGenerator(EvalObj):            # Abstract Syntax Tree Generator
+
+    def to_obj(self, obj):              # Try to convert a string to EvalObj
+        try: return IntObj(obj)
+        except Exception:
+            try: return FloatObj(obj)
+            except Exception: return SymObj(obj)
+ 
+    def __init__(self, stream):
+        self.stream = stream
+        self.stack = list()  # Empty stack
+
+    def absorb(self):
+        stack = self.stack
+        while True:
+            if len(stack) > 0 and stack[0] != '(':
+                return Cons(stack.pop(0), empty_list, Prog(0))   
+                                                    # An AST is constructed
+            token = self.stream.read()              # Read a new token 
+            if token is None: return None           # Feed me!
+            if token == '(':
+                stack.append(token) 
+            elif token == ')':                      # A list is enclosed
+                lst = list()
+                while stack[-1] != '(':
+                    lst = stack[-1:] + lst
+                    del stack[-1]
+                if len(lst) > 0:                    # At least one elem
+                    root = Cons(lst[0], None, Prog())          # The operator in the list
+                    # Collect the operands
+                    ref = root
+                    for i in lst[1:]:
+                        ref.cdr = Cons(i, None, Prog())
+                        ref.func.next = ref.cdr
+                        ref = ref.cdr
+                    ref.cdr = empty_list
+                    stack[-1] = root
+                else:                               # Null list
+                    stack[-1] = empty_list
+            else:
+                stack.append(self.to_obj(token))    # Found an EvalObj
+
+def is_id(string):
+    return isinstance(string, SymObj)
+def is_arg(pc):
+    return isinstance(pc.car, EvalObj)
+def is_ret_addr(val):
+    return isinstance(val, RetAddr)
+
+class Environment(object):                      # Store all bindings
+    def __init__(self, prev_envt = None):
+        self.prev_envt = prev_envt
+        self.binding = dict()
+
+    def add_binding(self, id_obj, eval_obj):    # Bind id_obj to eval_obj
+        self.binding[id_obj.name] = eval_obj
+
+    def get_obj(self, id_obj):
+        if is_id(id_obj):                # Resolve the id
+            ptr = self
+            while ptr:                   # Lookup the id in the chain
+                try:
+                    return ptr.binding[id_obj.name]
+                except KeyError:
+                    ptr = ptr.prev_envt
+            raise KeyError
+        else:
+            return id_obj                # id_obj is inherently an EvalObj
+
+    def has_obj(self, id_obj):
+        ptr = self
+        while ptr:
+            try:
+                t = ptr.binding[id_obj.name]
+                return True
+            except KeyError: 
+                ptr = ptr.prev_envt
+        return False
+
+class Continuation(object):             # Store the state of the interpreter
+    def __init__(self, envt, pc, old_cont, proc_body, body_cnt = 0):    
+        self.envt = envt                # envt pointer
+        self.pc = pc                    # pc pointer
+        self.old_cont = old_cont        # previous state
+        self.proc_body = proc_body      # procedure expression list
+        self.body_cnt = 0               # how many exp have been evaluated
+       
+# Miscellaneous builtin procedures #
+def _builtin_plus(arg_list):
+    res = 0
+    for i in arg_list:
+        res += i.val
+    return IntObj(res)
+
+def _builtin_minus(arg_list):
+    res = arg_list[0].val
+    for i in arg_list[1:]:
+        res -= i.val
+    return IntObj(res)
+
+def _builtin_times(arg_list):
+    res = 1
+    for i in arg_list:
+        res *= i.val
+    return IntObj(res)
+
+def _builtin_div(arg_list):
+    res = arg_list[0].val
+    for i in arg_list[1:]:
+        res /= i.val
+    return IntObj(res)
+
+def _builtin_lt(arg_list):
+    #TODO: need support to multiple operands
+    return BoolObj(arg_list[0].val < arg_list[1].val)
+
+def _builtin_gt(arg_list):
+    return BoolObj(arg_list[0].val > arg_list[1].val)
+
+def _builtin_eq(arg_list):
+    return BoolObj(arg_list[0].val == arg_list[1].val)
+
+def _builtin_display(arg_list):
+    print "Display: " + arg_list[0].ext_repr()
+    return UnspecObj()
+# Miscellaneous builtin procedures #
+
+_default_mapping = {
+        SymObj("+") : BuiltinProcObj(_builtin_plus, "+"),
+        SymObj("-") : BuiltinProcObj(_builtin_minus, "-"),
+        SymObj("*") : BuiltinProcObj(_builtin_times, "*"),
+        SymObj("/") : BuiltinProcObj(_builtin_div, "/"),
+        SymObj("<") : BuiltinProcObj(_builtin_lt, "<"),
+        SymObj(">") : BuiltinProcObj(_builtin_gt, ">"),
+        SymObj("=") : BuiltinProcObj(_builtin_eq, "="),
+        SymObj("display") : BuiltinProcObj(_builtin_display, "display"),
+        SymObj("lambda") : _builtin_lambda(),
+        SymObj("if") : _builtin_if(),
+        SymObj("define") : _builtin_define(),
+        SymObj("set!") : _builtin_set()}
+
+class Evaluator(object):
+
+    def _add_builtin_routines(self, envt):
+        for sym in _default_mapping:
+            envt.add_binding(sym, _default_mapping[sym])
+
+    def __init__(self):
+        self.envt = Environment() # Top-level Env
+        self._add_builtin_routines(self.envt)
+    def run_expr(self, prog):
+        stack = [0] * 100       # Eval Stack 
+        top = -1                # stack top
+
+        pc = prog               # pc register
+        cont = None             # continuation register
+        envt = self.envt        # environment register
+
+        def push(pc, top):        # Push EvalObj to the stack
+            ntop = top
+            if is_arg(pc):        # Pure operand 
+                ntop += 1
+                stack[ntop] = envt.get_obj(pc.car)  # Try to find the binding
+                stack[ntop].prepare(pc)
+                npc = pc.func.next      # Move to the next instruction
+            else:                       # Found an Operator
+                ntop += 1
+                stack[ntop] = RetAddr(pc)           # Push return address
+                npc = pc.car
+
+            return (npc, ntop)
+
+        (pc, top) = push(pc, top)
+
+        while is_ret_addr(stack[0]):    # Still need to evaluate
+            while pc and pc.func.skip: 
+                pc = pc.func.next       # Skip the masked branches
+
+            if pc is None:              # All arguments are evaluated, exiting
+               arg_list = list()
+               # Collect all arguments
+               while not is_ret_addr(stack[top]):
+                   arg_list = [stack[top]] + arg_list
+                   top -= 1
+               opt = arg_list[0]            # the operator
+               ret_addr = stack[top].addr   # Return address
+
+                # Fake return (one of the expressions are evaluated)
+               if ret_addr is False:        
+                    body = cont.proc_body
+                    cont.body_cnt += 1
+                    ncur = cont.body_cnt
+                    if ncur == len(body):   # All exps in the body are evaled
+                        stack[top] = arg_list[0]
+                        envt = cont.envt
+                        pc = cont.pc.func.next
+                        cont = cont.old_cont
+                    else:
+                        pc = body[ncur]     # Load the next exp
+                    # Revert to the original cont.
+               else:
+                    (pc, top, envt, cont) = opt.call(arg_list, envt, cont, 
+                                                    stack, top, ret_addr)
+            else: 
+                (pc, top) = push(pc, top)
+
+        return stack[0]
+
+t = Tokenizor()
+e = Evaluator()
+
+import sys, pdb
+a = ASTGenerator(t)
+while True:
+    sys.stdout.write("Sonsi> ")
+    while True:
+        exp = a.absorb()
+        if exp: break
+        cmd = sys.stdin.readline()
+        t.feed(cmd)
+    try:
+        print e.run_expr(exp).ext_repr()
+    except Exception as exc:
+        print exc
diff --git a/prototype/sketch_obsolete.py b/prototype/sketch_obsolete.py
new file mode 100755
index 0000000..36be406
--- /dev/null
+++ b/prototype/sketch_obsolete.py
@@ -0,0 +1,608 @@
+#! /bin/env python
+
+class EvalObj(object):
+    def __str__(self):
+        return "#<Object>"
+
+class UnspecObj(EvalObj):
+    def __str__(self):
+        return "#<Unspecified>"
+    def ext_repr(self):
+        return self.__str__()
+
+class NumberObj(EvalObj):
+    def __str__(selfl):
+        return "#<Number>"
+
+class IntObj(NumberObj):
+    def __init__(self, num):
+        self.val = int(num)
+    def __str__(self):
+        return "#<Integer>"
+    def ext_repr(self):
+        return str(self.val)
+
+class FloatObj(NumberObj):
+    def __init__(self, num):
+        self.val = float(num)
+    def __str__(self):
+        return "#<Float>"
+    def ext_repr(self):
+        return str(self.val)
+
+class BoolObj(EvalObj):
+    def __init__(self, b):
+        self.val = b
+    def __str__(self):
+        return "#<Boolean>"
+    def ext_repr(self):
+        if self.val:
+            return "#t"
+        else:
+            return "#f"
+
+class IdObj(EvalObj):
+    def __init__(self, string):
+        self.name = string
+    def __str__(self):
+        return "#<Identifier: " + self.name + ">"
+    def get_name():
+        return self.name
+
+class OptObj(EvalObj):
+    pass
+
+class ProcObj(OptObj):
+    def __init__(self, body, envt, para_list):
+        self.body = body
+        self.envt = envt
+        self.para_list = para_list
+    def ext_repr(self):
+        return "#<Procedure>"
+    def __str__(self):
+        return self.ext_repr()
+
+class SpecialOptObj(OptObj):
+    def prepare(self, pc):
+        pass
+    def call(self, arg_list, pc, envt, cont):
+        pass
+
+class BuiltinProcObj():
+    def __init__(self, f, name):
+        self.handler = f
+        self.name = name
+    def ext_repr(self):
+        return "#<Builtin Procedure: " + self.name + ">"
+    def __str__(self):
+        return self.ext_repr()
+    def call(self, arg_list):
+        return self.handler(arg_list)
+
+# Convert an obj to boolean
+def to_bool(obj):
+    if obj.val is False:
+        return BoolObj(False)
+    else:
+        return BoolObj(True)
+
+# Mark all children of pc as flag
+def _fill_marks(pc, flag):
+    pc = pc.chd
+    while pc:
+        pc.skip = flag
+        pc = pc.sib
+
+class _builtin_if(SpecialOptObj):
+    def prepare(self, pc):
+        # TODO: check number of arguments 
+        # Evaluate the condition first
+        self.state = 0 # Prepared
+        pc = pc.chd
+        pc.skip = False
+        pc.sib.skip = True
+        if pc.sib.sib:
+            pc.sib.sib.skip = True
+
+    def pre_call(self, arg_list, pc, envt, cont):
+        # Condition evaluated and the decision is made
+        self.state = 1 
+        if (to_bool(arg_list[0])).val:
+            pc = pc.chd
+            pc.skip = True
+            pc.sib.skip = False
+            if pc.sib.sib:
+                # Eval the former
+                pc.sib.sib.skip = True
+            return (None, True) # Re-eval
+        else:
+            pc = pc.chd
+            pc.skip = True
+            pc.sib.skip = True
+            if pc.sib.sib:
+                # Eval the latter
+                pc.sib.sib.skip = False
+            return (None, True) # Re-eval
+
+    def post_call(self, arg_list, pc, envt, cont):
+        # Value already evaluated, so just return it
+        return (arg_list[0], False)
+
+    def call(self, arg_list, pc, envt, cont):
+        if self.state == 0:
+            return self.pre_call(arg_list, pc, envt, cont)
+        else:
+            return self.post_call(arg_list, pc, envt, cont)
+
+    def ext_repr(self):
+        return "#<Builtin Macro: if>"
+    def __str__(self):
+        return self.ext_repr()
+
+class _builtin_lambda(SpecialOptObj):
+
+    def prepare(self, pc):
+        # TODO: check number of arguments 
+        # Do not evaulate anything
+        _fill_marks(pc, True)
+
+    def call(self, arg_list, pc, envt, cont):
+        para_list = list()              # paramter list
+        par = pc.chd                    # Switch to the first parameter
+        if par.obj.obj:                     # If there is at least one parameter
+            para_list.append(par.obj.obj)
+            if par.chd:                 # More paramters?
+                par = par.chd
+                while par: 
+                    para_list.append(par.obj)
+                    par = par.sib
+
+        # Clear the flag to avoid side-effects (e.g. proc calling)
+        _fill_marks(pc, False)
+
+        pc = pc.chd.sib     # Move pc to procedure body
+        #TODO: check body
+        body = list()       # store a list of expressions inside <body>
+
+        while pc:
+            body.append(pc)
+            pc.next = None  # Make each expression a orphan 
+                            # in order to ease the exit checking
+            pc = pc.sib
+
+        return (ProcObj(body, envt, para_list), False)
+
+    def ext_repr(self):
+        return "#<Builtin Macro: lambda>"
+    def __str__(self):
+        return self.ext_repr()
+
+class _builtin_define(SpecialOptObj):
+
+    def prepare(self, pc):
+        if is_arg(pc.chd):          # Simple value assignment
+            pc.chd.skip = True      # Skip the identifier
+            pc.chd.sib.skip = False
+        else:                       # Procedure definition
+            _fill_marks(pc, True)   # Skip all parts
+
+    def call(self, arg_list, pc, envt, cont):
+        # TODO: check identifier
+        if is_arg(pc.chd):          # Simple value assignment
+            id = pc.chd.obj             
+            obj = arg_list[0]
+        else:                       # Procedure definition
+            id = pc.chd.obj.obj
+            para_list = list()      # Parameter list
+            par = pc.chd
+            if par.chd:             # If there's at least one parameter
+                par = par.chd
+                while par:
+                    para_list.append(par.obj)
+                    par = par.sib
+
+            # Clear the flag to avoid side-effects (e.g. proc calling)
+            _fill_marks(pc, False)  
+            pc = pc.chd.sib         # Move pc to procedure body
+            #TODO: check body
+            body = list()           # store a list of expressions inside <body>
+
+            while pc:
+                body.append(pc)
+                pc.next = None
+                pc = pc.sib
+
+            obj = ProcObj(body, envt, para_list)
+        
+        envt.add_binding(id, obj)
+        return (UnspecObj(), False)
+
+    def ext_repr(self):
+        return "#<Builtin Macro: define>"
+    def __str__(self):
+        return self.ext_repr()
+
+class _builtin_set(SpecialOptObj):
+    def prepare(self, pc):
+        # TODO: check number of arguments 
+        pc = pc.chd             
+        pc.skip = True              # Skip the identifier
+        pc.sib.skip = False  
+
+    def call(self, arg_list, pc, envt, cont):
+        id = pc.chd.obj
+        if envt.has_obj(id):
+            envt.add_binding(id, arg_list[0])
+        return (UnspecObj(), False)
+
+    def ext_repr(self):
+        return "#<Builtin Macro: set!>"
+    def __str__(self):
+        return self.ext_repr()
+
+class Tokenizor():
+
+    def __init__(self):
+        self.data = ""                  # string buffer
+        self.tokenized = list()         # tokens
+
+    def feed(self, data):               # Store the data in the buffer
+        self.data = data
+
+    def read(self): 
+        if len(self.tokenized) == 0:        # no tokens available, let's produce
+            if len(self.data) == 0:         # I'm hungry, feed me!
+                return None
+            self.tokenized = self.data.replace('(', '( ')\
+                                    .replace(')', ' )')\
+                                    .split()
+            self.data = ""                  # Clear the buffer
+            if len(self.tokenized) == 0:    # You feed me with the air, bastard!
+                return None
+        return self.tokenized.pop(0)
+
+class Node(object):                     # AST Node
+    def __init__(self, obj, sib):
+        self.obj = obj
+        self.sib = sib
+        self.skip = None                # skip the current branch? (runtime)
+        self.next = sib                 # next instruction         (runtime)
+
+    def __str__(self):
+        return "#<AST Node>"
+    def __expr__(self):
+        return self.__str__()
+
+class ArgNode(Node):                    # AST Node (Argument)
+    def __init__(self, obj, sib = None):
+        super(ArgNode, self).__init__(obj, sib)
+    def __str__(self):
+        return "#<AST ArgNode>"
+    def __expr__(self):
+        return self.__str__()
+
+    def print_(self):
+        print \
+            "======================" + "\n" + \
+            "Obj: " + str(self.obj) + "\n" + \
+            "Sib: " + str(self.sib) + "\n" + \
+            "======================" 
+
+class OptNode(Node):                    # AST Node (Operator)
+    def __init__(self, obj, sib = None, chd = None):
+        super(OptNode, self).__init__(obj, sib)
+        self.chd = chd
+
+    def __str__(self):
+        return "#<AST OptNode>"
+    def __expr__(self):
+        return self.__str__()
+
+    def print_(self):
+        print \
+            "======================" + "\n" + \
+            "Obj: " + str(self.obj) + "\n" + \
+            "Sib: " + str(self.sib) + "\n" + \
+            "Chd: " + str(self.chd) + "\n" + \
+            "======================" 
+
+class RetAddr(object):                  # Return Adress (Refers to an AST Node)
+    def __init__(self, addr):
+        self.addr = addr
+    def __str__(self):
+        return "#<Return Address>"
+
+class ASTGenerator(EvalObj):            # Abstract Syntax Tree Generator
+
+    def to_obj(self, obj):              # Try to convert a string to EvalObj
+        if isinstance(obj, Node): return obj
+        try: return IntObj(obj)
+        except Exception:
+            try: return FloatObj(obj)
+            except Exception: return IdObj(obj)
+ 
+    def to_node(self, obj):             # Try to convert an EvalObj to AST Node
+        if isinstance(obj, Node): return obj
+        return ArgNode(obj)
+       
+    def __init__(self, stream):
+        self.stream = stream
+        self.stack = list()  # Empty stack
+
+    def absorb(self):
+        stack = self.stack
+        while True:
+            if len(stack) > 0 and stack[0] != '(':
+                return self.to_node(stack.pop(0))   # An AST is constructed
+            token = self.stream.read()              # Read a new token 
+            if token is None: return None           # Feed me!
+            if token == '(':
+                stack.append(token) 
+            elif token == ')':                      # A list is enclosed
+                lst = list()
+                while stack[-1] != '(':
+                    lst = stack[-1:] + lst
+                    del stack[-1]
+                if len(lst) > 0:                    # At least one elem
+                    root = OptNode(lst[0])          # The operator in the list
+                    # Collect the operands
+                    if len(lst) > 1:
+                        root.chd = lst[1]
+                        ref = root.chd
+                        for i in lst[2:]:
+                            ref.sib = i
+                            ref.next = ref.sib
+                            ref = ref.sib
+                    stack[-1] = root
+                else:                               # Null list
+                    stack[-1] = OptNode(ArgNode(None))
+            else:
+                stack.append(ArgNode(self.to_obj(token)))    # Found an EvalObj
+
+def is_id(string):
+    return isinstance(string, IdObj)
+def is_arg(node):
+    return isinstance(node, ArgNode)
+def is_ret_addr(val):
+    return isinstance(val, RetAddr)
+def is_builtin_proc(val):
+    return isinstance(val, BuiltinProcObj)
+def is_special_opt(val):
+    return isinstance(val, SpecialOptObj)
+def is_user_defined_proc(val):
+    return isinstance(val, ProcObj)
+
+class Environment(object):                      # Store all bindings
+    def __init__(self, prev_envt = None):
+        self.prev_envt = prev_envt
+        self.binding = dict()
+
+    def add_binding(self, id_obj, eval_obj):    # Bind id_obj to eval_obj
+        self.binding[id_obj.name] = eval_obj
+
+    def get_obj(self, id_obj):
+        if is_id(id_obj):                # Resolve the id
+            ptr = self
+            while ptr:                   # Lookup the id in the chain
+                try:
+                    return ptr.binding[id_obj.name]
+                except KeyError:
+                    ptr = ptr.prev_envt
+            raise KeyError
+        else:
+            return id_obj                # id_obj is inherently an EvalObj
+
+    def has_obj(self, id_obj):
+        ptr = self
+        while ptr:
+            try:
+                t = ptr.binding[id_obj.name]
+                return True
+            except KeyError: 
+                ptr = ptr.prev_envt
+        return False
+
+class Continuation(object):             # Store the state of the interpreter
+    def __init__(self, envt, pc, old_cont, proc_body, body_cnt = 0):    
+        self.envt = envt                # envt pointer
+        self.pc = pc                    # pc pointer
+        self.old_cont = old_cont        # previous state
+        self.proc_body = proc_body      # procedure expression list
+        self.body_cnt = 0               # how many exp have been evaluated
+       
+# Miscellaneous builtin procedures #
+def _builtin_plus(arg_list):
+    res = 0
+    for i in arg_list:
+        res += i.val
+    return IntObj(res)
+
+def _builtin_minus(arg_list):
+    res = arg_list[0].val
+    for i in arg_list[1:]:
+        res -= i.val
+    return IntObj(res)
+
+def _builtin_times(arg_list):
+    res = 1
+    for i in arg_list:
+        res *= i.val
+    return IntObj(res)
+
+def _builtin_div(arg_list):
+    res = arg_list[0].val
+    for i in arg_list[1:]:
+        res /= i.val
+    return IntObj(res)
+
+def _builtin_lt(arg_list):
+    #TODO: need support to multiple operands
+    return BoolObj(arg_list[0].val < arg_list[1].val)
+
+def _builtin_gt(arg_list):
+    return BoolObj(arg_list[0].val > arg_list[1].val)
+
+def _builtin_eq(arg_list):
+    return BoolObj(arg_list[0].val == arg_list[1].val)
+
+def _builtin_display(arg_list):
+    print "Display: " + arg_list[0].ext_repr()
+    return UnspecObj()
+# Miscellaneous builtin procedures #
+
+_default_mapping = {
+        IdObj("+") : BuiltinProcObj(_builtin_plus, "+"),
+        IdObj("-") : BuiltinProcObj(_builtin_minus, "-"),
+        IdObj("*") : BuiltinProcObj(_builtin_times, "*"),
+        IdObj("/") : BuiltinProcObj(_builtin_div, "/"),
+        IdObj("<") : BuiltinProcObj(_builtin_lt, "<"),
+        IdObj(">") : BuiltinProcObj(_builtin_gt, ">"),
+        IdObj("=") : BuiltinProcObj(_builtin_eq, "="),
+        IdObj("display") : BuiltinProcObj(_builtin_display, "display"),
+        IdObj("lambda") : _builtin_lambda(),
+        IdObj("if") : _builtin_if(),
+        IdObj("define") : _builtin_define(),
+        IdObj("set!") : _builtin_set()}
+
+class Evaluator(object):
+
+    def _add_builtin_routines