Token Vault (clarity)

(define-map balances { account: principal } uint)

;; #[allow(unchecked_data)]
(define-public (deposit (amount uint))
  (let ((current (default-to u0 (map-get? balances { account: tx-sender }))))
    (begin
      (map-set balances { account: tx-sender } (+ current amount))
      (ok true)
    )
  )
)

(define-public (withdraw (amount uint))
  (let ((current (default-to u0 (map-get? balances { account: tx-sender }))))
    (if (>= current amount)
        (begin
          (map-set balances { account: tx-sender } (- current amount))
          (ok true)
        )
        (err u402) ;; insufficient funds
    )
  )
)

🧠 Brief Overview

• Purpose: Track deposits and withdrawals of users on-chain.
• Data Storage: Each user’s balance is stored using a map.
• Functions:
  • deposit: Add tokens to the sender’s balance.
  • withdraw: Subtract tokens if the sender has enough.

---

📄 Code Breakdown

🗃️ 1. Define the Balance Map

(define-map balances { account: principal } uint)

• A map named balances stores user balances.
• Key: { account: principal } — each user’s address.
• Value: uint — the unsigned integer balance.

---

💸 2. Deposit Function

(define-public (deposit (amount uint))
  (let ((current (default-to u0 (map-get? balances { account: tx-sender }))))
    (begin
      (map-set balances { account: tx-sender } (+ current amount))
      (ok true)
    )
  )
)

• Public function — any user can call it.
• Accepts an amount to deposit.
• Fetches the current balance:
  • If none exists, defaults to 0.
• Adds the deposit amount to the current balance.
• Updates the map.
• Returns (ok true) to confirm success.

---

🏧 3. Withdraw Function

(define-public (withdraw (amount uint))
  (let ((current (default-to u0 (map-get? balances { account: tx-sender }))))
    (if (>= current amount)
        (begin
          (map-set balances { account: tx-sender } (- current amount))
          (ok true)
        )
        (err u402) ;; insufficient funds
    )
  )
)

• Also public — allows users to withdraw.
• Gets the user’s current balance.
• Checks: is the balance >= withdrawal amount?
  • ✅ If yes: subtracts and updates the balance.
  • ❌ If no: returns (err u402) — an error for insufficient funds.

---

🔄 Step-by-Step: How This Contract Works

Let’s walk through a real-world scenario:

🧍 User A wants to deposit 100 tokens:

1. Calls:

(deposit u100)

1. Contract:
   • Reads balance (e.g., u0 if new).
   • Adds 100.
   • Updates the map: { account: UserA } → 100.

---

🏧 Now User A wants to withdraw 30 tokens:

1. Calls:

(withdraw u30)

1. Contract:
   • Checks balance: 100.
   • 100 ≥ 30 → allowed.
   • Subtracts 30 → new balance = 70.
   • Updates the map.

---

⛔ User A tries to withdraw 100 (but only has 70):

• Fails the check.
• Returns:

(err u402)

Which can be interpreted in your frontend as “Insufficient Funds”.

---

🧩 Real-World Analogy

Think of it like a digital piggy bank for each user:

• You drop in coins (deposit).
• You can take out coins if there’s enough (withdraw).
• If it’s empty or has too little, you get an error.

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📘 Summary Table

Component	Purpose
balances	Map storing user balances
deposit	Adds tokens to the user’s vault
withdraw	Removes tokens if user has enough
err u402	Error code for “Insufficient Funds”