first-steps5 min

Handling Errors

Things go wrong. A user enters something unexpected, a calculation hits an impossible case, a resource isn't available. Clean Language gives you a clean, readable way to deal with it.

You raise an error with error(), and catch it with onError:

functions:
    integer divide(integer a, integer b)
        if b == 0
            error("Cannot divide by zero")
        return a / b

start:
    integer result = divide(10, 2) onError -1
    print(result.toString())

    integer bad = divide(10, 0) onError -1
    print(bad.toString())

5
-1

The onError -1 part means: if anything goes wrong, use -1 instead. No crash, no mystery — just a predictable fallback.

For more than just a fallback value, use onError: with a block:

functions:
    integer divide(integer a, integer b)
        if b == 0
            error("Cannot divide by zero")
        return a / b

start:
    integer result = divide(10, 0) onError:
        print("Something went wrong — using default")
        -1

    print(result.toString())

Something went wrong — using default
-1

The last line of the onError: block is the value that gets used as the result. You can do any amount of work — logging, cleanup — and then provide the fallback on the last line.

Quick recap

Use error("message") inside a function to raise an error
expression onError fallbackValue — catch and provide a simple fallback
expression onError: with a block — catch and run any code, last line is the fallback
Errors propagate up the call chain until something catches them
If nothing catches an error, the program stops with an error message