Question

Modify the make-account procedure so that it creates password-protected accounts. That is, make-account should take a symbol as an additional argument, as in (define acc (make-account 100 'secret-password)) The resulting account object should process a request only if it is accompanied by the password with which the account was created, and should otherwise return a complaint: ((acc' 'secret-password 'withdraw) 40) 60 ((acc' 'some-other-password 'deposit) 50) "Incorrect password"

Short answer

Create an account object using `make-account`, verify password for every operation, and handle operations if the verification is successful.

Step-by-step solution

Define the Procedure

To create a password-protected account, start by defining the procedure `make-account`. This procedure should have an additional parameter to accept the password, say `password`. So, it will be defined as `(define (make-account balance password))`.

Define the Account Functionality

Inside the `make-account` procedure, define an internal function that checks the provided password. If the password matches, it performs the requested operation; otherwise, it returns an error message. The internal function should take a password attempt, an operation ('withdraw or 'deposit), and an amount.

Implement Password Verification

In your internal function, first check if the provided password attempt equals the original password. Use an `if` statement: `(if (equal? provided-password password) ...)`. If the passwords do not match, return "Incorrect password".

Handle Withdraw and Deposit

When the password is correct, use a `cond` expression to handle the operations. For 'withdraw, check if the balance is sufficient before deducting the amount. For 'deposit, simply add the amount to the balance.

Return the Account Object

The entire account functionality (password verification and operations handling) is encapsulated in the internal function. The `make-account` procedure should return this function to provide an account object that can be interacted with the methods provided.

Key concepts

Scheme programming

Scheme is a simple and elegant programming language that's popular in academic settings. It's a dialect of Lisp and emphasizes minimalism, primarily employing functions and recursion to solve problems.
Scheme is highly valued in education because it encourages clear thinking about code structure and function. Here are some core features you might find useful:

• **Simple Syntax:** Scheme's syntax is minimal, which makes it easier for learners to understand.
• **Powerful Abstractions:** It allows you to express your ideas in powerful constructs, such as higher-order functions.
• **First-Class Procedures:** Functions in Scheme can be treated just like any other data, allowing them to be passed as arguments or returned from other functions.

Overall, Scheme's main benefit lies in its ability to help you focus on how a program should work logically, rather than getting caught up in complex syntax or unnecessary details.
When developing programs like the password-protected account exercise, these features aid in creating precise and efficiently operating code.

higher-order functions

In Scheme, higher-order functions are a standard feature. These are functions that either take other functions as arguments or return them as results. Such flexibility is key in functional programming, and it allows you to build more abstract and reusable code.
In the case of the password-protected account, the `make-account` procedure acts as a higher-order function by returning another function. This returned function embodies the entire account object, encapsulating behavior and state management.

• **Function as Data:** Treating functions as data allows for more expressive programs.
• **Abstraction and Encapsulation:** You can hide details and complexity, exposing only necessary interfaces.
• **Code Reuse:** Encourages writing generic, reusable software components.

Understanding higher-order functions is crucial for mastering Scheme programming. They are an essential tool for building complex systems in a modular and maintainable way.

procedure definition

A procedure definition in Scheme specifies the behavior of a block of code that can be executed by calling it with given arguments. It's akin to defining a 'task' or 'action' that you can perform multiple times with different data.
In the given exercise, the `make-account` procedure outlines the steps necessary to create an account with password protection. This involves defining internal logic to handle operations securely and efficiently.

• **Defining Procedures:** You use the `define` keyword to create procedures.
• **Modular Code:** Procedures allow you to modularize your program, cleaning up complex code into manageable pieces.
• **Parameters:** Within a procedure, parameters serve as placeholders for the actual data passed during the function call.

By mastering procedure definitions, you get to organize your code into logical sections. This can make it easier to read, reduce errors, and promote reuse. In functional programming languages like Scheme, efficient and organized procedure definition is a key aspect of skillful programming.