Functions

In addition to defining fields, types, and constructors for contracts, Polylang also allows developers to define custom functions within a contract.

These functions can be used for various purposes - modifying/updating field values, adding additional logic and validation, access control, deleting the record itself, interacting with other contracts and records et al.

Modifying or Updating records

Field values of records can only be updated via functions. Consider the following contract, for instance:

contract Person {
    id: string;
    name: string;
    age: number;
 
    constructor (id: string, name: string, age: number) {
      this.id = id;
      this.name = name;
      this.age = age;
    }
 
    setName(newName: string) {
      this.name = newName;
    }
 
    setAge(newAge: number) {
      this.age = newAge;
    }
}

The setName and setAge functions can be used to update the values for the name and age fields respectively.

Validation

You can have custom validation logic inside functions. In the following example, we check that account we're transferring money from has sufficient funds to carry out that operation:

contract Account {
    id; string;
    balance: number;
 
    constructor (id: string) {
      this.id = id;
      this.balance = 0;
    }
 
    transfer (to: Account, amount: number) {
      // ensure we have enough money in our account
      if (this.balance < amount) {
        error('Insufficient funds for transfer');
      }
 
      this.balance -= amount;
      to.balance += amount;
    }
}

Access Control

We can also provide tighter checks on who can modify the record. Taking the same Account contract as above, we modify like so:

contract Account {
    id; string;
    balance: number;
    publicKey: PublicKey;
 
    constructor (id: string) {
      this.id = id;
      this.publicKey = ctx.publicKey;
      this.balance = 0;
    }
 
    transfer (to: Account, amount: number) {
      // first check that we are authorized to attempt this transaction
      if (this.publicKey != ctx.publicKey) {
        error('User is not authorized to transfer funds from this account');
      }
 
      // ensure we have enough money in our account
      if (this.balance < amount) {
        error('Insufficient funds for transfer');
      }
 
      this.balance -= amount;
      to.balance += amount;
    }
}

In the example above, we initialize the publicKey field with the public key of the user who created the Account, and when the transfer function is invoked to transfer funds out of this account, we check whether the public key of the user attempting this transactions matches that of the user who created this account.

Deleting a record

Polylang allows deletion of records using the selfdestruct built-in function. This built-in function must be, of course, called from within a Polylang function. This function can be named anything, but by convention, such a deletion function is called del.

For example (building on the same contract):

contract Account {
    id; string;
    balance: number;
    publicKey: PublicKey;
 
    constructor (id: string) {
      this.id = id;
      this.publicKey = ctx.publicKey;
      this.balance = 0;
    }
 
    transfer (to: Account, amount: number) {
      // first check that we are authorized to attempt this transaction
      if (this.publicKey != ctx.publicKey) {
        error('User is not authorized to transfer funds from this account');
      }
 
      // ensure we have enough money in our account
      if (this.balance < amount) {
        error('Insufficient funds for transfer');
      }
 
      this.balance -= amount;
      to.balance += amount;
    }
 
    // the deletion function for this contract
    del() {
      if (this. publicKey != ctx.publicKey) {
        error('User is not authorized to delete this account');
      }
 
      // call the built-in function
      selfdestruct();
    }
}

Constructors Control Flow