A Scheme Interpreter

1. Introduction

In this project, we will develop an interpreter for a subset of the Scheme language. As we proceed, we think about the issues that arise in the design of a programming language; many quirks of languages are byproducts of implementation decisions in interpreters and compilers.

We will also implement some small programs in Scheme. Scheme is a simple but powerful functional language. Since we only include a subset of the language, our interpreter will not exactly match the behaviour of other interpreters such as STk.

2. Core Concepts

2.1) Reading Scheme expressions

To test scheme reader:

$ python3 scheme_reader.py

Every time you type in a value into the prompt, both the str and repr values of the parsed expression are printed. e.g.:

read> 42
str : 42
repr: 42
read> '(1 2 3)
str : (quote (1 2 3))
repr: Pair('quote', Pair(Pair(1, Pair(2, Pair(3, nil))), nil))
read> nil
str : ()
repr: nil
read> '()
str : (quote ())
repr: Pair('quote', Pair(nil, nil))
read> (1 (2 3) (4 (5)))
str : (1 (2 3) (4 (5)))
repr: Pair(1, Pair(Pair(2, Pair(3, nil)), Pair(Pair(4, Pair(Pair(5, nil), nil)), nil)))
read> (1 (9 8) . 7)
str : (1 (9 8) . 7)
repr: Pair(1, Pair(Pair(9, Pair(8, nil)), 7))
read> (hi there . (cs . (student)))
str : (hi there cs student)
repr: Pair('hi', Pair('there', Pair('cs', Pair('student', nil))))

2.2) Symbol evaluation

scm> +
#[+]
scm> odd?
#[odd?]
scm> display
#[display]

2.3) Calling built-in procedures

scm> (+ 1 2)
3
scm> (* 3 4 (- 5 2) 1)
36
scm> (odd? 31)
True

2.4) Definitions

scm> (define x 15)
x
scm> (define y (* 2 x))
y
scm> y
30
scm> (+ y (* y 2) 1)
91
scm> (define x 20)
x
scm> x
20
scm> (eval (define tau 6.28))
6.28

It can also evaluate quoted expressions:

scm> 'hello
hello
scm> '(1 . 2)
(1 . 2)
scm> '(1 (2 three . (4 . 5)))
(1 (2 three 4 . 5))
scm> (car '(a b))
a
scm> (eval (cons 'car '('(1 2))))
1

2.5) Lambda expressions and procedure definition

A begin expression is evaluated by evaluating all sub-expressions in order. The value of the begin expression is the value of the final sub-expression:

scm> (begin (+ 2 3) (+ 5 6))
11
scm> (define x (begin (display 3) (newline) (+ 2 3)))
3
x
scm> (+ x 3)
8
scm> (begin (print 3) '(+ 2 3))
3
(+ 2 3)

Lambda expressions:

scm> (lambda (x y) (+ x y))
(lambda (x y) (+ x y))

2.6) Calling user-defined procedures

User-defined procedures:

scm> (define f (lambda (x) (* x 2)))
f

However, we'd like to be able to use the shorthand form of defining named procedures:

scm> (define (f x) (* x 2))
f

2.7) Evaluation of special forms

This function should evaluate either the second (consequent) or third (alternative) expression of the if expression, depending on whether the value of the first (predicate) expression is true.

scm> (if (= 4 2) 'a 'b)
b
scm> (if (= 4 4) (* 1 2) (+ 3 4))
2

It is legal to pass in just two expressions to the if special form. In this case, you should return the second expression if the first expression evaluates to a true value. Otherwise, return the special okay value, which represents an undefined value.

scm> (if (= 4 2) 'a)
okay

The logical forms and and or are short-circuiting. For and, the interpreter should evaluate each sub-expression from left to right, and if any of these evaluates to a false value, then False is returned. Otherwise, it should return the value of the last sub-expression. If there are no sub-expressions in an and expression, it evaluates to True.

scm> (and)
True
scm> (and 4 5 6)  ; all operands are true values
6
scm> (and 4 5 (+ 3 3))
6
scm> (and True False 42 (/ 1 0))  ; short-circuiting behavior of and
False

For or, evaluate each sub-expression from left to right. If any sub-expression evaluates to a true value, return that value. Otherwise, return False. If there are no sub-expressions in an or expression, it evaluates to False.

scm> (or)
False
scm> (or 5 2 1)  ; 5 is a true value
5
scm> (or False (- 1 1) 1)  ; 0 is a true value in Scheme
0
scm> (or 4 True (/ 1 0))  ; short-circuiting behavior of or
4

cond form returns the value of the first result sub-expression corresponding to a true predicate, or the sub-expression corresponding to else.

scm> (cond ((= 4 3) 'nope)
           ((= 4 4) 'hi)
           (else 'wait))
hi
scm> (cond ((= 4 3) 'wat)
           ((= 4 4))
           (else 'hm))
True
scm> (cond ((= 4 4) 'here (+ 40 2))
           (else 'wat 0))
42

The value of a cond is undefined if there are no true predicates and no else. In such a case, do_cond_form should return okay.

scm> (cond (False 1) (False 2))
okay

The let special form binds symbols to values locally, giving them their initial values. For example:

scm> (define x 'hi)
x
scm> (define y 'bye)
y
scm> (let ((x 42)
           (y (* 5 10)))
       (list x y))
(42 50)
scm> (list x y)
(hi bye)

do_mu_form is a non-standard Scheme expression type. A mu expression is similar to a lambda expression, but evaluates to a MuProcedure instance that is dynamically scoped:

scm> (define f (mu (x) (+ x y)))
f
scm> (define g (lambda (x y) (f (+ x x))))
g
scm> (g 3 7)
13

2.8) Implementing Scheme procedures.

The enumerate procedure takes in a list of values and returns a list of two-element lists, where the first element is the index of the value, and the second element is the value itself.

scm> (enumerate '(3 4 5 6))
((0 3) (1 4) (2 5) (3 6))
scm> (enumerate '())
()

The list-change procedure lists all of the ways to make change for a positive integer total amount of money, using a list of currency denominations, which is sorted in descending order. The resulting list of ways of making change should also be returned in descending order.

To make change for 10 with the denominations (25, 10, 5, 1), we get the possibliites:

10
5, 5
5, 1, 1, 1, 1, 1
1, 1, 1, 1, 1, 1, 1, 1, 1, 1

To make change for 5 with the denominations (4, 3, 2, 1), we get the possibilities:

4, 1
3, 2
3, 1, 1
2, 2, 1
2, 1, 1, 1
1, 1, 1, 1, 1

3. Data Types

Scheme Data Type	Our Internal Representation
Numbers	Python's built-in int and float data types.
Symbols	Python's built-in string data type.
Booleans (#t, #f)	Python's built-in True, False values.
Pairs	The Pair class, defined in scheme_reader.py.
nil	The nil object, defined in scheme_reader.py.

4. Files

Files in this project:

• scheme.py: the Scheme evaluator
• scheme_reader.py: the Scheme syntactic analyser
• questions.scm: a collection of functions (User-defined procedures) written in Scheme
• tests.scm: a collection of test cases written in Scheme
• scheme_tokens.py: a tokenizer for Scheme
• scheme_primitives.py primitive Scheme procedures
• buffer.py: a buffer implementation
• ucb.py: utility functions for 61A

5. Running the Scheme Interpreter

To run your Scheme interpreter in an interactive session, type:

$ python3 scheme.py

You can use your Scheme interpreter to evaluate the expressions in an input file by passing the file name as a command-line argument to scheme.py:

$ python3 scheme.py tests.scm

To exit the Scheme interpreter, press Ctrl-d or evaluate the exit procedure:

scm> (exit)

To test scheme reader, yvery time you type in a value into the prompt, both the str and repr values of the parsed expression are printed:

$ python3 scheme_reader.py

6. Class Project Site

here