12-error-handling.md

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12. Error Handling

When something goes wrong — an out-of-range index, a division by zero, an explicit failure — QuantoScript reports a runtime error. You can either let it stop the program, or catch it with try / oops.

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Catching errors with try / oops

try runs a block. oops is the language's word for "catch": if an error occurs inside try, control jumps to the oops block instead of stopping the program.

items = ["red", "green", "blue"]

try {
    print("first:", items[1])
    print("bad:", items[10])     # out of range
    print("never reached")
} oops {
    print("something went wrong, but the program is still alive")
}

print("after try/oops")

The oops block runs only when the try block fails:

try {
    print("safe work")
} oops {
    print("this does not run")
}

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Binding the error

Add a name after oops to capture a description of the error:

try {
    result = 10 / 0
} oops e {
    print("caught: " + e)
}

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Raising an error with oops(...)

Used as a call, oops raises an error with your message:

func safe_div(a, b) {
    if b == 0 {
        oops("division by zero")
    }
    return a / b
}

try {
    print(safe_div(10, 0))
} oops e {
    print("caught: " + e)
}

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Assertions

assert(condition, message) checks that a condition holds and stops the program with the message when it does not:

age = 25
assert(age > 0, "age must be positive")
print("assert passed")     # reached

assert_eq and assert_ne compare two values:

assert_eq(1 + 1, 2)
assert_ne(1, 2)

These are handy in tests.

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A complete example

func check_positive(n) {
    if n <= 0 {
        oops("expected a positive number")
    }
    return n
}

try {
    print(check_positive(42))
    print(check_positive(-1))
} oops e {
    print("failed:", e)
}

Single-line try / oops blocks are also accepted:

try { print(check_positive(42)) } oops e { print("failed:", e) }

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Next steps

Imports shows how to reuse code across files.

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