Message Board Contract (clarity)

(define-map messages { user: principal } {message: (string-utf8 100)})


(define-public (write-message (msg (string-utf8 100)))
  (begin
    (map-set messages { user: tx-sender } { message: msg })
    (ok true)
  )
)

(define-public (read-message (user principal))
  (match (map-get? messages { user: user })
    value (ok (get message value))
    (err u404)
  )
)

(define-public (read-message (user principal))
  (match (map-get? messages { user: user })
    value (ok (get message value))
    (err u404)
  )
)

This smart contract allows users to write a single message tied to their address (like a profile status), and allows others to read those messages. All data is stored on-chain, meaning it’s transparent and permanent until overwritten.

---

🧠 Brief Overview

• Purpose: Store one personal message per user on the blockchain.
• Data Storage: A map that links user addresses to their messages.
• Functions:
  • write-message: Save/update your message.
  • read-message: Read another user’s message.
  • see-message: Low-level view of a user’s message record (for dev/debug).

---

🔍 Code Breakdown

🗃️ 1. Define the Message Map

(define-map messages { user: principal } {message: (string-utf8 100)})

• This map stores messages per user.
• Key: { user: principal } — user’s blockchain address.
• Value: { message: (string-utf8 100) } — a UTF-8 string up to 100 characters.

---

✍️ 2. Write a Message

(define-public (write-message (msg (string-utf8 100)))
  (begin
    (map-set messages { user: tx-sender } { message: msg })
    (ok true)
  )
)

• Public function that lets users write/update their message.
• Uses tx-sender to identify the writer.
• Stores { message: msg } in the messages map.
• Returns (ok true) to confirm the update.

---

📖 3. Read Someone’s Message

(define-public (read-message (user principal))
  (match (map-get? messages { user: user })
    value (ok (get message value))
    (err u404)
  )
)

• Takes a user (their address) as input.
• Tries to get their message from the map.
• If found: extracts the message and returns it with ok.
• If not: returns (err u404) (message not found).

---

🔍 4. Developer View: See Raw Message Record

(define-read-only (see-message (recipient principal))
	(ok (map-get? messages recipient))
)

• Read-only helper function.
• Returns the full record stored in the map, not just the message string.
• Useful for debugging or exploring the raw map value (including key/field names).

---

🧭 Step-by-Step: How It Works

🧍 User A writes a message

(write-message "Hello, Stacks!")

• The contract saves:

  { user: UserA } → { message: "Hello, Stacks!" }

---

🧑‍🤝‍🧑 User B reads User A’s message

(read-message UserA)

• If User A has written a message: returns ok "Hello, Stacks!".
• If not: returns err u404.

---

🧪 Developer reads raw data

(see-message UserA)

• Output:

  ok (some { message: "Hello, Stacks!" })

  or ok none if there’s no message.

---

📘 Summary Table

Component	Purpose
messages map	Stores one message per user
write-message	Lets users post or update their message
read-message	Lets anyone read another user’s message
see-message	Returns the full map entry (debug-style)
err u404	Error if no message found for a user

---

🧩 Real-World Analogy

Imagine a global public board where:

• Each user gets one sticky note to post a message.
• Anyone can read your note.
• You can update it any time.
• Others can’t change your note — only you can.