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In some cases it is necessary to interrupt the flow of execution with a failure: this is where the predefined function failwith comes in.

The failwith function

The failwith function raises an error that cannot be caught, which terminates the contract.

let main = (p: unit, s: unit) : [list<operation>, unit] =>
failwith("This contract always fails");

The call to failwith should be annotated with a type as the type-checker cannot infer the correct type yet.


Assertions can be used to ensure a certain condition is met when running a contract. The predefined function assert is used to check whether a given a Boolean condition is true. The function assert_some is used to check if an option value is not None. The function assert_some_with_error is like assert_some but an error message can be given. When a condition is not met, the contract will stop executing and display an error.

let main = (p: bool, s: unit) : [list<operation>, unit] => {
let u: unit = assert(p);
return [list([]), s];
let some = (o: option<unit>) => {

You can use assert_with_error or assert_some_with_error to use a custom error message

let main2 = (p: bool, s: unit) : [list<operation>, unit] => {
assert_with_error (p, "My custom error message.");
return [list([]), s];