Error handling

In the below example we implement the ability to raise basic errors and stop execution.

Firstly, a Composite Type (ADT) Type Error is defined with the constructor values we want to raise as errors.

Secondly, a library function called make_error is defined. The function takes a variable called result of type Error and matches that to an error code. This is wrapped in a JSON object storing both the error name and error code.

Lastly, we define a procedure ThrowError which takes an instance of a variable with type Error. It then calls the library function and returns the error. The keyword throw stops the execution of the smart contract with the specific error passed.

tip

Throwing errors with descriptive names helps users of your smart contracts debug what's wrong.

scilla_version 0library IncrementingButton
let uint128_zero = Uint128 0let uint128_one  = Uint128 1
(* Error exception *)type Error =  | NotContractOwner  | NotUniqueClicker
let make_error =  fun (result : Error) =>    let result_code =       match result with      | NotContractOwner             => Int32 -1      | NotUniqueClicker             => Int32 -2      end    in    { _exception : "Error"; code : result_code }      contract IncrementingButton(  contract_owner: ByStr20)
field current_clicker    : ByStr20 = contract_ownerfield total_count_clicks : Uint128 = uint128_zero
(*   Dev: Emit Errors *)procedure ThrowError(err : Error)  e = make_error err;  throw eend

IncrementingButton

In the previous section, we defined a pattern matching syntax that allowed branching depending on what the type was. For some paths of execution we want the logic to fail and stop executing. The procedures IsContractOwner and IsPreviousClicker should throw an error in some cases.

We define wrapping of a new error variable by referencing the ADT type of error err = NotContractOwner and then calling the ThrowError procedure with the err. ThrowError calls to the library function make_error to wrap the error and then finally the error is thrown, stopping the execution.

...
procedure ThrowError(err : Error)  e = make_error err;  throw eend
procedure IsContractOwner()  is_contract_owner = builtin eq contract_owner _sender;  match is_contract_owner with  | True =>     (* No Operation - Continue contract execution *)  | False =>    err = NotContractOwner;    ThrowError err  endend
procedure IsPreviousClicker(new_clicker: ByStr20)  previous_clicker <- current_clicker;  is_previous_clicker = builtin eq previous_clicker _sender;  match is_previous_clicker with  | True =>     err = NotUniqueClicker;    ThrowError err  | False =>    (* No Operation - Continue contract execution *)  endend

In the next section, we will create logic that users can interact with to press the button, we will be concerned with the functionality and security of our button, combining the atomic logical blocks we have built with procedures.