CSE 3341 Project 2 Overview
The goal of this lab is to write an interpreter for a simple functional language called PLAN. The interpreter itself should be written in Scheme. A PLAN program is a list, as defined by the following grammar:
P rogram Expr
::prog Expr:: I d
Const
myignore Expr
myadd Expr Expr
mymul Expr Expr
myneg Expr
mylet Id Expr Expr
::ab…z
:: integer constant Here are five valid PLAN program
Id C onst
1. prog 5
2. prog myadd myadd 7 myignore mymul 4 5 mymul 2 5
3. prog mylet z myadd 4 5 mymul z 2
4. prog mylet a 66 myadd mylet b mymul 2 4 myadd 2 b mymul 2 a 5. prog mylet x 66 myadd mylet x mymul 2 4 myadd 2 x mymul 2 x
Each PLAN program and expression evaluates to a particular integer value. The semantics of a program are defined as follows:
1. The entire program prog Expr evaluates to whatever Expr evaluates to.
2. myignore Expr evaluates to the integer value 0, regardless of what the subexpression
Expr looks like.
3. myadd Expr Expr evaluates to the sum of whatever values the two subexpression
evaluate to.
4. mymul Expr Expr evaluates to the product of whatever values the two subexpression evaluate to.
5. myneg Expr evaluates to X1, where X is the integer value that the subexpression evaluates to.
6. mylet Id Expr1 Expr2 has the following semantics. First, Expr1 is evaluated. The resulting integer value is bound to the identifier Id. Then the second subexpression Expr2 is evaluated, and the result of that evaluation serves as the value of the entire mylet expression. The binding between the id and the integer value is active only while Expr2 is being evaluated.
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7. Id evaluates to the value to which the identifier has been bound by a surrounding mylet expression. If there are multiple bindings for the identifier, the last i.e. latest, innermost such binding is used.
8. Const evaluates to the value of the integer constant.
Based on these rules, the five programs from above evaluate to:
1. 5 2. 17 3. 18 4. 142 5. 142
Implementation
Write a Scheme function called myinterpreter that takes as input a list of PLAN programs and produces a list of the corresponding values. For example, an invocation
myinterpreter
prog 5
prog mylet z myadd 4 5 mymul z 2
should produce the list 5 18.
Your implementation must work on scheme48 on stdlinux.
Instructions and suggestions intended to help you andor simplify your interpreters implemen
tation:
1. You do not need to write a scanner or a parser, we will let the scheme interpreter handle that for us.
2. You are guaranteed that the list given to the interpreter will not be empty, and will contain only valid PLAN programs. The programs will be valid both syntactically and semantically. Syntactically, you can assume that any program given is valid with respect to the grammar from above. Semantically, you can assume that any evaluation of an identifier has at least one existing binding for that identifier. Your implementation does not have to contain error handling code. Do not worry about arithmetic issues such as underflow or overflow.
3. Two very useful Scheme library functions for your interpreter to use are integer? and symbol?. The first one checks if its parameter is an integer constant, and the second one checks if its parameter is a symbol such as a, b, ect.
4. In order to maintain the set of bindings, consider using a list where each element of the list is one specific binding. A binding is really just a pair: the symbol and the integer value.
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5. Using load FILENAME or ,load FILENAME inside the scheme48 interpreter allows you to load a file named FILENAME with your implementation of myinterpreter and any other helper functions.
Instructions that limit what your interpreter can do:
1. A PLAN program is not a Scheme program. The PLAN program is input to your interpreter, not to the Scheme interpreter. For example, do not try to make the Scheme interpreter execute PLAN programs by defining Scheme functions like this
define myadd x yx y
and then giving a PLAN program directly to the Scheme interpreter.
2. The only builtin Scheme functions you are allowed to use are
define, let, equal?, car, cdr, cons, cond, if, quote, , , , null?, list?, symbol?, integer?
It is also ok to use any carcdr variant such as cadadr. You should not use any other builtin function.
3. Make sure your code is purely function: in particular, do not use define to try to create global variables!
Extra Credit
Only attempt this extra credit if you are fully confident your myinterpreter function described in the previous section is working correctly.
Create a myextra function which handles this modification of the grammar:
P rogram Expr
F unction Id C onst
::prog Expr:: I d
Const
myignore Expr
myadd Expr Expr
mymul Expr Expr
myneg Expr
mylet Id Expr Expr
mylet Id Function Expr
::myfunction Id Expr::ab…z
:: integer constant
This change allows users of the PLAN language to define their own functions. The semantics for the addition are as follows: When a mylet expression containing a myfunction expression like
mylet Id1 myfunction Id2 Expr1 Expr2
is evaluated, the myfunction expression is evaluated by binding the body of the function Expr1 to Id1. This binding is only active while Expr2 is being evaluated.
If an expression Id1 Expr is encountered while evaluating Expr2 and a new binding for Id1 has not been introduced, then the value of Expr is bound to Id2 and Expr1 is
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evaluated once this finishes, the binding of the value of Expr and Id2 is removed. The value from evaluating Expr1 is the value of Id1 Expr.
So for example,
1. prog mylet a myfunction b myadd b b a 5 evaluates to 10
2. prog mylet a myfunction b myadd b b mylet a 1 mymul a a evaluates to 1
Project Submission
On or before 11:59 pm November 22nd, you should submit a single file called myfns.ss containing all the function definitions needed for your project, including the main function myinter preter. If you are doing the extra credit, submit a zip file containing myfns.ss and myextra.ss, where myfns.ss contains all function definitions for the myinterpreter project, and myextra.ss contains all the function definitions for the myextra project. Do not use any other name for the file or for the main function. Other functions you define may have whatever names you choose. Use white spaces appropriately so that your function definitions are easy to read. Also, include some documentation in the same file not a separate README file. Comment lines in Scheme program start with a semicolon e.g. ;this is a scheme comment.
Submit your project to the dropbox on Carmen for Project 2.
If the time stamp on your submission is 12:00 am on November 23rd or later, you will receive a 10 reduction per day, for up to three days. If your submission is more than 3 days late, it will not be accepted and you will receive zero points for this project. If you resubmit your project, only the latest submission will be considered.
Grading
Your myinterpreter function will be tested against 10 valid test cases. The correct outputs for these test cases are worth 8 points each. An additional 20 points are for code readability and documentation. 100 points total.
If you chose to create a myextra function, it will be tested against 3 test cases, worth 4 points each. 12 points total.
Academic Integrity
The project you submit must be entirely your own work. Minor consultations with others in the class are OK, but they should be at a very high level, without any specific details. The work on the project should be entirely your own; all the design, programming, testing, and debugging should be done only by you, independently and from scratch. Sharing your code or documentation with others is not acceptable. Submissions that show excessive similarities for code or documentation will be taken as evidence of cheating and dealt with accordingly; this includes any similarities with projects submitted in previous instances of this course.
Academic misconduct is an extremely serious offense with severe consequences. Additional details on academic integrity are available from the Committee on Academic Misconduct see http:oaa.osu.educoamresources.html. If you have any questions about university policies or what constitutes academic misconduct in this course, please contact me immediately.
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