Description
Problem Definition: To evaluate the programs, you need to understand the expressions of
the language. It is the same for computers; therefore, you saw in the lecture how you can invent
a language and define it for the computer to understand and evaluate.
In this project, you will define a language named MYLET that is similar to the simple LET
language covered in the class. The syntax for the MYLET language is given below.
Program ::= Expression
a-program (exp1)
Expression ::= Number
const-exp (num)
Expression ::= String
str-exp (str)
Expression ::= op(Expression, Expression, Number)
op-exp (exp1, exp2, num)
Expression ::= zero? (Expression)
zero?-exp (exp1)
Expression ::= if Expression then Expression
{elif Expression then Expression}*
else Expression
if-exp (exp1 exp2 conds exps exp3)
Expression ::= Indetifier
var-exp (var)
Expression ::= let Indetifier = Expression in Expression
let-exp (var exp1 body)
Figure 1. Syntax for the MYLET language
Part A. This part will prepare you for the following parts of the project. (15 pts)
(1) Write the 5 components of the language1
:
(2) For each component, specify where or which racket file (if it applies) we define and
handle them.
Part B. In this part, you will create an initial environment for programs to run. (10 pts)
(1) Create an initial environment that contains 3 different variables (x, y, and z).
(2) Using the environment abbreviation shown in the lectures, write how environment
changes at each variable addition.
Part C. Specify expressed and denoted values for MYLET language. (5 pts)
1Hint: review Lecture 10 slides
2
Part D. This is the main part of the project where you implement the MYLET language given
in the Figure 1 by adding the missing expressions.
(1) Add str-exp to the language. Strings are defined as any text starting and ending with
‘, e.g. ‘comp301’, ‘program’; strings are stored with ‘ symbols. (15 pts)
Hint: String is an expression that is similar to Number, understanding the addition
and implementation of Number may be helpful to complete this step.
(2) Add op-exp to the language. (15 pts) op-exp is similar to the diff-exp of the LET language; however, in LET language, the only possible operation was subtraction. op-exp
enables you to do 4 arithmetic operations via its third input (Number ), when third
input is:
• 1: perform addition (exp1 + exp2)
• 2: perform multiplication (exp1 * exp2)
• 3: perform division (exp1 / exp2)
• any other number: perform subtraction (exp1 – exp2)
(3) Add if-exp to the language. Unlike if-exp of the LET language, you can add multiple
conditions to be checked through elif-then extension. Starting from the condition of if,
conditions will be checked until a true condition is found, and expression corresponding
to the true condition will be evaluated as a result. If none of the if/elif conditions are
correct, the expression in the else statement will be evaluated. (15 pts)
(4) Add a custom expression to the language. The expression can be simple, but you need
to clearly explain what it does and how it works. You also need to provide the syntax
of the expression. (15 pts)
Note that the implementation of the other expressions, that are same with the LET language,
are already given in the .rkt file provided. We deleted the former implementations of if and
diff-exp.
Part E. Create the following test cases. (10 pts)
(1) custom expression: Write test cases that controls if the expression works according to
your explanation of the expression.
Note: We provided several test cases for you to try your implementation. Uncomment
corresponding test cases and run tests.rkt to test your implementation.
Bonus. Here is an alternative datatype ropes that allows manipulation of sequence of characters instead of the most commonly used strings. You can try to implement ropes instead of
strings as a bonus challenge.
Note: The bonus question is worth 2 points in your overall final grade and no partial credits will be awarded. To get full credit, please implement this problem using the second code
boilerplate (Project2BONUS) provided and write at least 6 test cases (two for each: fetch ith
character, concatenate, substring) in a clear way to your tests.rkt for us to run. Please
make sure that your test cases are clear and tests.rkt doesn’t give any errors, otherwise you
won’t be able to receive any credits for this question. Add your code for the bonus problem to
your submission as specified in the instructions.
Hint: Define your rope datatype similar to the way you did in the project, clearly define your
grammar and feel free to use any helper procedures.
3
