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Chapter 3: Advanced

Lesson 4: Map – Existence of a value

In order to check whether or not the user id or wallet id being sent by a user have been previously registered, we’ll need to read the value from the map. In this particular case, we simply need to check whether such value exists or not in a given map.

This is also called In-place existence check. It check whether in map m, any value corresponding to key k exists or not and accordingly returns a Bool which in above example is stored in variable b.

Existence checks through nested maps is supported with the syntax v <- exists m[k1][k2][...]. If one or more of the intermediate key(s) do not exist in the corresponding map, the result is False.

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b <- exists m[k]

We have a task for you!

In the transition register_user do the following two actions:

  • Check if the value of the variable user_address exists in the map users through the keyword exists, and then store the value in a new variable user_exists. This will all be done in a single line as shown in the example above.

  • Then, in the next line, check if the value of the variable twitter_username exists in the map used_usernames through the keyword exists, and then store the value in a new variable username_exists. This will all be done in a single line as shown in the example above.

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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
    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)
      (* Start typing from the line below *)
      user_exists <- exists users[user_address];
      username_exists <- exists used_usernames[twitter_username]
    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)

  (* Start typing from the line below *)

end
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    Chapter 3: Advanced

    Lesson 3: Map – Declaration

    Chapter 3: Advanced

    Lesson 5: OR operation in Scilla