Types and OOP Exceptions

Exceptions and Resource Safety

Klyn uses exceptions for exceptional failures, API boundary contracts, and resource cleanup. This page covers throw, try, catch, finally, and the language form that manages auto-closable resources.

Throwing an Exception

if value == 0:
    throw ZeroDivisionException("division by zero")

Try and Catch

try:
    riskyOperation()
catch e as Exception:
    print(e.message)

A catch block may bind the exception to a local name with catch e as Exception:. The exception name and variables first declared in that block are local to the catch block. Declare state before the try when both successful and exceptional paths must update it.

A catch type may be a concrete generic exception, such as catch error as Fault<Int>:. Generic arguments remain invariant: Fault<Int> does not catch Fault<String>. The type must extend Exception, and the bound variable retains that exact type.

import klyn.io

status = "pending"

try:
    loadConfiguration()
    status = "loaded"
catch e as IOException:
    status = "failed"
    diagnostic = e.message

print(status)
# print(e)           # SyntaxError: e belongs to the catch block.
# print(diagnostic)  # SyntaxError: diagnostic belongs to the catch block.

Finally

try:
    writer.writeLine("hello")
finally:
    writer.close()

Use finally for cleanup that must happen whether the protected code succeeds or not. A finally block can use variables declared in an enclosing scope, but it cannot read locals owned by the sibling try or catch blocks.

Try With Resource

try resource = Resource():
    resource.doWork()
finally:
    print("resource is closed here")

The try resource = ...: form is the language-level resource-management construct. It is the preferred pattern when the value implements AutoClosable.

Declaring `throws`

public class Parser:
    public parse(value as String) as Int throws ValueException:
        throw ValueException("invalid value")

Use throws when the API contract should make an exceptional path explicit. ValueException is the canonical exception for a value that is outside the accepted domain or cannot be converted to the requested type.

Practical Guidance