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:
〈Program〉 ::= ( prog 〈Expr〉 )
〈Expr〉 ::= 〈Id〉
| 〈Const〉
| ( myignore 〈Expr〉 )
| ( myadd 〈Expr〉 〈Expr〉 )
| ( mymul 〈Expr〉 〈Expr〉 )
| ( myneg 〈Expr〉 )
| (mylet 〈Id〉 〈Expr〉 〈Expr〉 )
〈Id〉 ::= a | b | . . . | z
〈Const〉 ::= integer constant
Here are five valid PLAN program
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 sub-expression
evaluate to.
4. (mymul 〈Expr〉 〈Expr〉) evaluates to the product of whatever values the two sub-expression
evaluate to.
5. (myneg 〈Expr〉) evaluates to X · (−1), where X is the integer value that the sub-expression
evaluates to.
6. (mylet 〈Id〉 〈Expr〉1 〈Expr〉2) has the following semantics. First, 〈Expr〉1 is evaluated. The
resulting integer value is bound to the identifier 〈Id〉. Then the second sub-expression 〈Expr〉2
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 〈Expr〉2 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
( myinte rpre te r
’ ( (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 and/or simplify your interpreter’s 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
( d e f i n e (myadd x y ) (+ x y ) )
and then giving a PLAN program directly to the Scheme interpreter.
2. The only built-in 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 car/cdr variant such as cadadr. You should not use any other built-in
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:
〈Program〉 ::= ( prog 〈Expr〉 )
〈Expr〉 ::= 〈Id〉
| 〈Const〉
| ( myignore 〈Expr〉 )
| ( myadd 〈Expr〉 〈Expr〉 )
| ( mymul 〈Expr〉 〈Expr〉 )
| ( myneg 〈Expr〉 )
| (mylet 〈Id〉 〈Expr〉 〈Expr〉 )
| (mylet 〈Id〉 〈Function〉 〈Expr〉 )
〈Function〉 ::= ( myfunction 〈Id〉 〈Expr〉 )
〈Id〉 ::= a | b | . . . | z
〈Const〉 ::= 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 〈Id〉1 (myfunction 〈Id〉2 〈Expr〉1) 〈Expr〉2)
is evaluated, the myfunction expression is evaluated by binding the body of the function 〈Expr〉1
to 〈Id〉1. This binding is only active while 〈Expr〉2 is being evaluated.
If an expression (〈Id〉1 〈Expr〉) is encountered while evaluating 〈Expr〉2 and a new binding
for 〈Id〉1 has not been introduced, then the value of 〈Expr〉 is bound to 〈Id〉2 and 〈Expr〉1 is
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evaluated (once this finishes, the binding of the value of 〈Expr〉 and 〈Id〉2 is removed). The value
from evaluating 〈Expr〉1 is the value of (〈Id〉1 〈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.edu/coamresources.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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