OurScheme – Project 2

A simplified Scheme interpreter (2017 Spring PL Project 2).
This project extends Project 1 by adding evaluation of expressions, definitions, conditionals, sequencing, and primitive functions.

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Features

• All primitive expressions can be evaluated.
• define supported (binding symbols to S-expressions).
• Conditionals supported (if, cond).
• Sequencing / functional composition supported (begin).
• Environment management with clean-environment.
• Maintains Project 1’s scanner, parser, pretty-printer, and error reporting.

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Program Workflow

1. Starts with:

   Welcome to OurScheme!
   

2. Prompts with > and waits for input.
3. Reads and evaluates an S-expression:

• If no syntax error:
  • Calls EvalSExp.
  • If no evaluation error → prints result.
  • Else → prints evaluation error.
• If syntax error → prints syntax error.

4. Terminates when (exit) is entered:

   Thanks for using OurScheme!
   

5. If EOF is reached before (exit), prints:

   ERROR (no more input) : END-OF-FILE encountered
   

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Supported Primitives

1. Constructors

• cons (2)
• list (>=0)

2. Quoting

• quote (1)
• ' (1)

3. Definition

• define (2) → binds a symbol to an S-expression
• Cannot redefine system primitives (e.g., cons, car).

4. Part Accessors

• car (1)
• cdr (1)

5. Predicates (1 argument each)

• atom?
• pair?
• list?
• null?
• integer?
• real?
• number? (same as real? in OurScheme)
• string?
• boolean?
• symbol?

6. Arithmetic, Logic, Strings

• Arithmetic: + - * / (≥ 2 arguments)
• Logical: not (1), and (≥ 2), or (≥ 2)
• Comparison: > >= < <= = (≥ 2 arguments)
• String operations:
  string-append, string>?, string<?, string=?

7. Equivalence

• eqv? (2)
• equal? (2)

8. Sequencing

• begin (≥ 1)

9. Conditionals

• if (2 or 3)
• cond (≥ 1) with else keyword

10. Environment Reset

• clean-environment (0)

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Example Interaction

Welcome to OurScheme!

> (cons 3 4)
( 3
.
4
)

> (define a 5)
a defined

> (cons a 10)
( 5
.
10
)

> (if (> a 3) 'good 'bad)
good

> (cond ((> a 10) 'big) (else 'small))
small

> (clean-environment)
environment cleaned

> (exit)
Thanks for using OurScheme!

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Error Handling

Four possible error types:

Project 1 errors (still valid in Project 2)

• ERROR (unexpected token) : atom or '(' expected ...
• ERROR (unexpected token) : ')' expected ...
• ERROR (no closing quote) : END-OF-LINE ...
• ERROR (no more input) : END-OF-FILE encountered New Project 2 errors
• ERROR (non-list) : ...
• ERROR (incorrect number of arguments) :
• ERROR (exit with arguments) : exit
• ERROR ( with incorrect argument type) :
• ERROR (attempt to apply non-function) :
• ERROR (no return value) :
• ERROR (unbound symbol) :
• ERROR (division by zero) : /
• ERROR (DEFINE format) : ( define ... )
• ERROR (COND format) : ( cond ... )

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Notes

• Scheme is case-sensitive (cons ≠ CONS).
• Primitives cannot be redefined.
• else is a keyword only in the last condition of cond.
• String formatting:
• Floats always printed with 3 decimal places.
• (clean-environment) clears all user definitions.

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