CS 441 Program 3: Expression Evaluator with Environment

Student: Gabriel Morais (Senior, Computer Science)
Course: CS 441
Assignment: Program 3 - Expression Evaluator with Environment

A Racket-based expression evaluator that supports variable management and stateful computation using Either/Result types instead of Maybe.

Development Note: This project was developed with AI assistance (GitHub Copilot with Claude Sonnet 4.5), but the AI did not do the work for me. I provided detailed requirements, reviewed all code, identified and fixed bugs, validated functionality, and ensured deep understanding of all concepts.

Features

• Either/Result Type System: Returns (success value) or (failure message) instead of Maybe's (just value) or #<nothing>
• Environment Management: Maintains state of defined variables and their values
• Variable Operations:
  • define: Create new variables (with or without initial values)
  • assign: Assign values to undefined variables (immutable data)
  • remove: Remove variables from environment
  • id: Reference variable values in expressions
• Arithmetic Operations: add, sub, mult, div with proper error handling
• Interactive REPL: Read-Eval-Print Loop for interactive expression evaluation

Usage

Running Tests

racket -e '(require "expression-evaluator.rkt") (run-tests)'

Starting the REPL

Edit the file and uncomment the last line:

(repl)

Then run:

racket expression-evaluator.rkt

Or from the REPL:

racket
> (require "expression-evaluator.rkt")
> (repl)

REPL Commands

• (quit) - Exit the REPL
• (env) - Display current environment state
• Any expression to evaluate

Expression Grammar

Numeric Literals

'(num 5)

Arithmetic Operations

'(add (num 5) (num 3))        ; 5 + 3 = 8
'(sub (num 10) (num 4))       ; 10 - 4 = 6
'(mult (num 3) (num 7))       ; 3 * 7 = 21
'(div (num 20) (num 4))       ; 20 / 4 = 5

Variable Management

Define (with value)

'(define x (num 10))          ; Define x and assign 10

Define (without value)

'(define y)                   ; Define y as undefined

Assign

'(assign y (num 20))          ; Assign 20 to previously undefined y

Remove

'(remove x)                   ; Remove x from environment

Reference Variable

'(id x)                       ; Get value of x
'(add (id x) (num 5))        ; Use x in expression

Examples

> '(define a (num 10))
Success: Defined a = 10

> '(add (id a) (num 5))
Success: 15

> '(define b)
Success: Defined b (undefined)

> '(assign b (mult (id a) (num 2)))
Success: Assigned b = 20

> '(sub (id b) (id a))
Success: 10

> '(remove a)
Success: Removed a

> '(id a)
Error: Identifier not defined: a

Identifier Rules

• Must start with an alphabetic character
• Can contain letters, digits, hyphens (-), and underscores (_)
• Valid examples: x, my-var, count_2, totalSum
• Invalid examples: 123, _test, -var

Error Handling

The evaluator returns descriptive errors for:

• Division by zero
• Undefined identifiers
• Redefinition of existing identifiers
• Assignment to non-existent or already-assigned identifiers
• Removal of non-existent identifiers
• Invalid identifier names
• Unknown operations

Implementation Details

State Management

• Uses immutable hash tables for functional state management
• State is threaded through all evaluations
• On success: returns updated state
• On failure: returns original state unchanged

Either/Result Type

(list success value)    ; Success case
(list failure message)  ; Failure case

Helper functions:

• (from-success default result) - Extract success value
• (from-failure default result) - Extract failure message

Immutability

• All variables are immutable once assigned
• Can define → assign → use, but cannot reassign
• Must remove and redefine to change a value

Project Structure

program3/
├── expression-evaluator.rkt       # Main implementation with evaluator, REPL, and tests
├── run-repl.rkt                   # Simple launcher to start the REPL
├── README.md                      # This file - complete project documentation
├── QUICK_REFERENCE.md             # Quick reference card for REPL usage
├── LLM_PROMPTS.md                 # Documentation of AI prompts used
├── MACHINE_GENERATED_CODE.md      # Tracking of AI-generated code
└── VIDEO_GUIDE.md                 # Guide for creating the video presentation

Testing

The implementation includes comprehensive tests covering:

• Basic numeric expressions
• Arithmetic operations
• Division by zero handling
• Variable definition and usage
• Variable assignment
• Variable removal
• Error cases (undefined variables, redefinition, etc.)

Run tests with: racket -e '(require "expression-evaluator.rkt") (run-tests)'