Constructor name

A contract is declared using the contract keyword that starts the scope of the contract. The next keyword passed is the contract name. Only one constructor is able to be defined per contract. When deploying a contract to the network, developers will be asked to define the variables passed into the constructor.

caution

A constructor (immutable) variable cannot be changed after deployment.

This example contract name is defined as IncrementingButton. A contract declaration is followed by the declaration of its immutable parameters, the scope of which is defined by ().

IncrementingButton currently does not have any immutable variables defined.

contract IncrementingButton()

We will improve our contract by providing an immutable variable called contract_owner

Each immutable parameter is declared in the following pattern vname: vtype where vname is the parameter name and vtype is the parameter type. Multiple parameters are delimited with a ,.

contract_owner defines the owner of the contract. There will be transitions defined later that only the contract_owner can access.

contract IncrementingButton(  contract_owner: ByStr20)

These initialisation parameters can be viewed publicly from the state of the contract.

Checking the immutable variables

It is possible to check that the immutable variables satisfy one or more conditions. For example, let's assume for a moment that our IncrementingButton contract has a second immutable variable max_num_clicks: Uint32 that limits the number of clicks allowed, and that we want this variable to be positive. In other words, the deployment should fail if the variable is set to 0. This is called a "contract constraint" and is put in a with ... => block right after the definition of the immutable variables.

contract IncrementingButton(  contract_owner: ByStr20,  max_num_clicks: Uint32)with  let zero = Uint32 0 in (* check max_num_clicks > 0 *)  uint32_gt max_num_clicks zero=> 

More examples can be found in the oSCILLAtor repo under "InitParams".

We are still missing required fields that stop our contract being compiled correctly. See the next topic about Scilla Versions.