Exceptions

Exceptions signal failures during execution. When thrown, execution stops immediately and the error is returned to the caller.

Throwing Exceptions

Throw a String

endpoint Withdraw(amount U64):
    if amount > balance:
        throw "Insufficient balance"

Throw with Class

Classes can implement __except__ for custom error messages:

class Person:
    field age U64

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

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

    if person.age < 18:
        throw person  // Calls __except__ automatically

Use Cases

Use exceptions for:

Invariant violations — conditions that should never occur
Permission checks — unauthorized access attempts
Validation failures — invalid input data

warning

Don't use exceptions for control flow. They immediately halt execution.

Can I catch exceptions in Coco?

try, catch, and finally are reserved keywords for future use. Exception handling is not yet implemented.

Error Types

When testing, errors appear in the format !<error_type>::<error_message>:

Error Type	When It Occurs
`builtin.AccessError`	Map key doesn't exist, index out of bounds
`builtin.ArithmeticError`	Integer overflow/underflow
`builtin.RuntimeError`	General runtime errors
`user`	From `throw "message"`

Throwing Exceptions​

Throw a String​

Throw with Class​

Use Cases​

Can I catch exceptions in Coco?​

Error Types​