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Chapter 3: Advanced

Lesson 6: Using match condition

We now have already_exists variable. If the value of this variable is true, then we don’t want to allow the user to register, however, if it’s false then we do want to allow the user to register.

To implement this, we need to use a conditional structure which we learned in the previous chapter, the match keyword.

We can quickly revisit the syntax for the match expression.

We have a task for you!

  • Use match on the variable already_exists.
  • The two branches corresponding to match will be True and False
  • Make sure to use end to declare the end of the statements.

    • We’ll fill in the code for the individual branches over the next lessons.

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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
    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
      (* Start typing from the line below *)
      match already_exists with
      | True =>
      | 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


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

  (* Start typing from the line below *)

end
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    Chapter 3: Advanced

    Lesson 5: OR operation in Scilla

    Chapter 3: Advanced

    Lesson 7: Messages for the True condition