Assignment 4 (PostScript Interpreter - Part 1) Solution

An Interpreter for a Simple Postscript-like Language
The Problem

In this assignment you will write an interpreter in Python for a simplified PostScript-like language, concentrating on key computational features of the abstract machine, omitting all PS features related to graphics, and using a somewhat-simplified syntax. The simplified language, SPS, has the following features of PS:
▪    integer constants, e.g. 1, 2, 3, -4, -5
    • boolean constants, e.g. true, false
▪    array constants, e.g. [1 2 3 4], [-1 2 3 -4], [1 x 3 4 add 2 sub],

[1 2 x 4] where x is a variable. For simplicity we will assume that SPS arrays are not nested (can’t have subarrays).

    • name constants, e.g. /fact: start with a / and letter followed by an arbitrary sequence of letters and numbers

    • names to be looked up in the dictionary stack, e.g. fact: as for name constants, without the /

    • code constants: code between matched curly braces { ... }
▪    built-in operators on numbers: add, sub, mul, eq, lt, gt

    • built-in operators on boolean values: and, or, not (we will call these psAnd, psOr, and psNot)

    • built-in operators on array values: length, get, getinterval, put, putinterval, forall. See the lecture notes for more information on array functions (you will implement forall operator in Part2).

    • built-in conditional operators: if, ifelse (you will implement if/ifelse operators in Part2)
    • built-in loop operator: repeat (you will implement repeat operator in Part 2).

    • stack operators: dup, copy, count, pop, clear, exch, mark, cleartomark, counttomark

    • dictionary creation operator: dict; takes one operand from the operand stack, ignores it, and creates a new, empty dictionary on the operand stack (we will call this psDict)

    • dictionary stack manipulation operators: begin, end.

        ◦ begin requires one dictionary operand on the operand stack; end has no operands.
    • name definition operator: def.

    • defining (using def; we will call this psDef) and calling functions

    • stack printing operator (prints contents of stack without changing it): stack

Part 1 - Requirements

In Part 1 you will build some essential pieces of the interpreter but not yet the full interpreter. The pieces you build will be driven by Python test code rather than actual Postscript programs. The pieces you are going to build first are:

        1. The operand stack
        2. The dictionary stack
        3. Defining variables (with def) and looking up names

        4. The operators that don't involve code arrays: all of the operators except repeat loop operator, if/ifelse operators, forall operator, and calling functions (You will complete these in Part 2)

    1. The Operand Stack - opstack

The operand stack should be implemented as a Python list. The list will contain Python integers, strings, and later in Part 2 code arrays. Python integers and lists on the stack represent Postscript integer constants and array constants. Python strings which start with a slash / on the stack represent names of Postscript variables. When using a list as a stack, assume that the top of the stack is the end of the list (i.e., the pushing and popping happens at the end of the list).
2. The Dictionary Stack - dictstack

The dictionary stack is also implemented as a Python list. It will contain Python dictionaries which will be the implementation for Postscript dictionaries. The dictionary stack needs to support adding and removing dictionaries at the top of the stack (i.e., end of the list), as well as defining and looking up names.

    3. define and lookup

You will write two helper functions, define and lookup, to define a variables and to lookup the value of a variable, respectively.

The define function adds the “name:value” pair to the top dictionary in the dictionary stack. Your psDef function ( i.e., your implementation of the Postscript def operator) should pop the name and value from operand stack and call the “define” function.

You should keep the ‘/’ in the name constant when you store it in the dictStack.

def define(name, value):
pass
#add name:value pair to the top dictionary in the dictionary stack.

The lookup function should look-up the value of a given variable in the dictionary stack. In Part 2, when you interpret simple Postscript expressions, you will call this function for variable lookups and function calls.

def lookup(name):
pass
            ▪ return the value associated with name
        ◦ What is your design decision about what to do when there is no definition for

“name”? If “name” is not defined, your program should not break, but should give an appropriate error message.


    4. Array constants

In our SPS interpreter we will represent array constants as Python lists. Remember that, the operators and variables in arrays will be evaluated before the array constant is pushed onto the stack. For example, the SPS array [1 2 3 true 5] will be represented as the Python list [1,2,3,True,5] when pushed onto the opstack. Additional examples:

-    SPS array [1 2 3 add 5] will be represented as Python list [1,5,5]
-    SPS array [1 true false and not 5] will be represented as Python list [1,true,5]

    • SPS array [1 x y 5] will be represented as Python list [1,2,3,5] where x’s value is 2 and y’s value is 3.

Important note: In part-1, we will assume that array constants are already evaluated and include only integers and boolean values when they are pushed onto the stack. In part-2, when we interpret SPS code, we will evaluate the constant arrays before we push them onto the stack.
5. Operators

Operators will be implemented as zero-argument Python functions that manipulate the operand and dictionary stacks. For example, the add operator could be implemented as follows.

#pop 2 values from stack; check if they are numerical (int type); add them; push the result back to stack.
def add():
if len(opstack) > 1:
op2 = opPop()
op1 = opPop()
if(isinstance(op1,int) and isinstance(op2,int)):
opPush(op1+op2)
else:
print("Error: add - one of the operands is not a numerical value")
opPush(op1)
opPush(op2)
else:
print("Error: add expects 2 operands")


    • The begin and end operators are a little different in that they manipulate the dictionary stack in addition to or instead of the operand stack. Remember that the dict operator (i.e., psDict function)affects only the operand stack.

(Note about dict: Remember that the dict operator takes an integer operand from the operand

stack and pushes an empty dictionary to the operand stack (affects only the operand stack). The initial size argument is ignored – Postscript requires it for backward compatibility of dict operator with the early Postscript versions).

    • The def operator (i.e., psDef function) takes two operands from the operand stack: a string (recall that strings that start with “/” in the operand stack represent names of postscript variables) and a value. It changes the dictionary at the top of the dictionary stack so that the string is mapped to the value by that dictionary. Notice that def does not change the number of dictionaries on the dictionary stack!

You may start your implementation using the below skeleton code (given in
HW4_part1_skeleton.py). Please make sure to use the function names given below.


#-------------------------
10% -------------------------------------
    • The operand stack: define the operand stack and its operations opstack = [] #assuming top of the stack is the end of the list

    • Now define the HELPER FUNCTIONS to push and pop values on the opstack
    • Remember that there is a Postscript operator called "pop" so we choose
    • different names for these functions.
    • Recall that `pass` in python is a no-op: replace it with your code.

def opPop():
pass
    • opPop should return the popped value.

    • The pop() function should call opPop to pop the top value from the opstack, but it will ignore the popped value.

def opPush(value):
pass
#--------------------------
20% -------------------------------------

    • The dictionary stack: define the dictionary stack and its operations dictstack = [] #assuming top of the stack is the end of the list

    • now define functions to push and pop dictionaries on the dictstack, to
    • define name, and to lookup a name

def dictPop():
pass
# dictPop pops the top dictionary from the dictionary stack.

def dictPush(d):
pass
#dictPush pushes the dictionary ‘d’ to the dictstack.

#Note that, your interpreter will call dictPush only when Postscript #“begin” operator is called. “begin” should pop the empty dictionary from #the opstack and push it onto the dictstack by calling dictPush.

def define(name, value):
pass
#add name:value pair to the top dictionary in the dictionary stack.
#Keep the '/' in the name constant.

#Your psDef function should pop the name and value from operand stack and #call the “define” function.

def lookup(name):
pass
    • return the value associated with name
    • What is your design decision about what to do when there is no definition for

“name”? If “name” is not defined, your program should not break, but should give an appropriate error message.

#---------------------------
10% -------------------------------------

    • Arithmetic, comparison, and boolean operators: add, sub, mul, eq, lt, gt, and, or, not

    • Make sure to check the operand stack has the correct number of parameters
    • and types of the parameters are correct.
def add():
pass

def sub():
pass

def mul():
pass

def eq():
pass

def lt():
pass

def gt():
pass

def psAnd():
pass

def psOr():
pass
def psNot():
pass

#---------------------------
25% -------------------------------------

    • Array operators: define the string operators length, get, getinterval, put, putinterval

def length(): pass

def get():
pass

def getinterval():
pass

def put():
pass

def putinterval():
pass

#---------------------------
15% -------------------------------------

    • Define the stack manipulation and print operators: dup, copy, count, pop, clear, exch, mark, cleartomark, counttotmark

def dup(): pass

def copy():
pass

def count():
pass

def pop():
pass

def clear():
pass

def exch():
pass

def mark():
pass

def cleartomark():
pass

def counttomark():
pass

def stack():
pass

#---------------------------
20% -------------------------------------
    • Define the dictionary manipulation operators: psDict, begin, end, psDef

    • name the function for the def operator psDef because def is reserved in Python. Similarly, call the function for dict operator as psDict.

    • Note: The psDef operator will pop the value and name from the opstack and call your own "define" operator (pass those values as parameters).
    • Note that psDef()won't have any parameters.
    • 
def psDict():
pass

def begin():
pass

def end():
pass

def psDef():
pass

Important Note: For all operators you need to implement basic checks, i.e., check whether there are sufficient number of values in the operand stack and check whether those values have correct types. For example,

    • def operator: the operand stack should have 2 values where the second value from top of the stack is a string starting with '/'

    • get operator : the operand stack should have 2 values; the top value on the stack should be an

integer and the second value should be an array constant.
Also, see the add implementation on page 3.

You will be deducted points if you don’t do error checking.

4. Testing Your Code

We will be using the unittest Python testing framework in this assignment. See https://docs.python.org/3/library/unittest.html for additional documentation.

The file HW3Sampletests_part1.py provides sample test cases for the SPS operators. This file imports the HW4_part1 module (HW4_part1.py file) which will include your implementations of the SPS operators.

For example:

def test_lookup(self):
opPush('/n1')
opPush(3)
psDef()
self.assertEqual(lookup('n1'),3)

def test_add(self):
opPush(1)
opPush(2)
add()
self.assertEqual(opPop(),3)


In Python unittest framework, each test function has a “test_” prefix. To run all tests, execute the following command on the command line.

python -m unittest HW4Sampletests_part1

You can run tests with more detail (higher verbosity) by passing in the -v flag:

python -m unittest -v HW4Sampletests_part1
Main Program

In this assignment, we simply write some unit tests to verify and validate the functions. If you would like to execute the code, you need to write the code for the "main" program. Unlike in C or Java, this is not done by writing a function with a special name. Instead the following idiom is used. This code is to be written at the left margin of your input file (or at the same level as the def lines if you've indented those.

if __name__ == '__main__':
...code to do whatever you want done...