Throwing Exceptions

Exceptions in Coco signal failures during endpoint execution. You can throw a plain String, or throw a class instance whose except() special method produces the error message. When an exception is thrown, execution stops immediately and the error is returned to the caller; any subsequent code in the endpoint is skipped. Use exceptions for invariant violations and permission checks rather than control flow.

Catching Exceptions

try...catch...finally statement is not implemented in Coco 0.7.0 or PISA 0.5.0

exception.coco

coco Throw
class Person:
    field age U64

    method adult() -> (is_adult Bool):
        is_adult = self.age >=18

    method __except__() -> (err String):
        yield err f"underage: only {self.age} years"

// Simplest way to throw an exception is to just throw a string
endpoint TestThrow(s String):
    throw s

endpoint invoke AgeVerification(age U64):
    memory person = Person{age: age}

    if !(is_adult) <- person.adult():
        throw person.__except__()