Exceptions

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.

cameligo

type storage = unit
type result = operation list * storage

[@entry]
let main (_param : unit) (_store : storage) : result =
  failwith "This contract always fails."

The call to failwith sometimes needs to be annotated with a type when the type-checker cannot infer the correct type, e.g. (failwith "message" : result).

jsligo

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

The call to failwith sometimes needs to be annotated with a type when the type-checker cannot infer the correct type, e.g. return (failwith("message") : result);.

Assertions

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.

cameligo

[@entry]
let main (p : bool) (s : unit) : operation list * unit =
  let u : unit = assert p
  in [], s

[@entry]
let some (o : unit option) (s : unit) : operation list * unit =
  let u : unit = assert_some o
  in [], s

jsligo

@entry
const main = (p: bool, s: unit) : [list<operation>, unit] => {
  let u: unit = assert(p);
  return [list([]), s];
};

@entry
const some = (o: option<unit>, s : unit) : [list<operation>, unit] => {
  assert_some(o);
  return [list([]), s]
};

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

cameligo

[@entry]
let main (p : bool) (s : unit) : operation list * unit =
  let () = assert_with_error p "My custom error message."
  in [], s

jsligo

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