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Chapter 3: Advanced

Lesson 7: Messages for the True condition

In case already_exists variable is true, then we simply need to let the users know about that in a message.

We’ll be sending that message in the same way as we did in the second chapter.

We have a task for you!

  • First of all, we need to define the code for such an eventuality in the library. So, in the library section define a variable user_exists_code of Uint32 type and value 2.

  • In the True branch of the conditions, declare a variable msg with the following values:
  • For _tag, value should be empty, i.e., "".
  • For the _recipient, we want to send the message to the initial sender, so the value should be: _sender.
  • For the _amount, the value should be zero. Since we’ve defined it as a Uint128 0 variable in the library already, use the variable name zero.
  • For the _code, use the code that we have defined in the library to indicate that the user already exists, i.e., user_exists_code.

    • Finally, after you have defined msg, put this message in the list defined previously (i.e. one_msg variable type) and then send it.

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    Solution
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    scilla_version 0
    import BoolUtils
    library SocialMediaPayment
    let one_msg = 
      fun (msg: Message) => 
      let nil_msg = Nil {Message} in
      Cons {Message} msg nil_msg
    let zero = Uint128 0
    let not_owner_code = Uint32 1
    let accepted_code = Uint32 0
    (* Start typing from the line below to define the variable ‘user_exists_code’ of type Uint32 and value ‘2’*)
    let user_exists_code = Uint32 2
    contract SocialMediaPayment (owner: ByStr20)
    field users: Map ByStr20 String
      = Emp ByStr20 String
    field used_usernames: Map String Bool
      = Emp String Bool
    transition deposit()
      sender_is_owner = builtin eq _sender owner;
      match sender_is_owner with
      | False =>
        msg = { _tag: "";
                _recipient: _sender;
                _amount: zero;
                code: not_owner_code};
        msgs = one_msg msg;
        send msgs
      | True =>
        accept;
        msg = { _tag: "";
                _recipient: _sender;
                _amount: zero;
                code: accepted_code};
        msgs = one_msg msg;
        send msgs
      end
    end
    transition register_user (user_address: ByStr20, twitter_username: String)
      user_exists <- exists users[user_address];
      username_exists <- exists used_usernames[twitter_username]
      already_exists = orb user_exists username_exists
      match already_exists with
      | True =>
        (* Start typing from the line below *)
        msg = {_tag: ""; _recipient: _sender; _amount: zero; code: user_exists_code};
        msgs = one_msg msg;
        send msgs
      | False =>
      end
    end
    scilla_version 0
import BoolUtils

library SocialMediaPayment

let one_msg = 
  fun (msg: Message) => 
  let nil_msg = Nil {Message} in
  Cons {Message} msg nil_msg

let zero = Uint128 0
let not_owner_code = Uint32 1
let accepted_code = Uint32 0

(* Start typing from the line below to define the variable ‘user_exists_code’ of type Uint32 and value ‘2’*)

contract SocialMediaPayment (owner: ByStr20)


field users: Map ByStr20 String
  = Emp ByStr20 String

field used_usernames: Map String Bool
  = Emp String Bool

transition deposit()
  sender_is_owner = builtin eq _sender owner;
  match sender_is_owner with
  | False =>

    msg = { _tag: "";
            _recipient: _sender;
            _amount: zero;
            code: not_owner_code};
    msgs = one_msg msg;
    send msgs

  | True =>
    accept;
    msg = { _tag: "";
            _recipient: _sender;
            _amount: zero;
            code: accepted_code};
    msgs = one_msg msg;
    send msgs

  end
end

transition register_user (user_address: ByStr20, twitter_username: String)

  user_exists <- exists users[user_address];
  username_exists <- exists used_usernames[twitter_username]
  already_exists = orb user_exists username_exists

  match already_exists with
  | True =>

    (* Start typing from the line below *)

  | False =>

  end
end
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    Chapter 3: Advanced

    Lesson 6: Using match condition

    Chapter 3: Advanced

    Lesson 8: Map – Adding entries