5.1.21. Error Handling

This tutorial covers daslang’s error-handling mechanisms: panic for fatal errors, try/recover for catching panics, assertions for invariants, and defer for cleanup actions.

5.1.21.1. panic

panic("message") halts execution immediately with a runtime error. Use it for unrecoverable situations:

panic("something went horribly wrong")

Certain operations (null dereference, out-of-bounds access) cause implicit panics.

5.1.21.2. try / recover

try/recover catches panics and continues execution. It is not general exception handling — only panics are caught:

try {
    risky_operation()
} recover {
    print("recovered from panic\n")
}

Note

You cannot return from inside try or recover blocks. Assign results to a variable instead.

5.1.21.3. assert and verify

assert(condition, "message") checks a condition at runtime. It may be stripped in release builds:

assert(x > 0, "x must be positive")

verify(condition, "message") is never stripped — side effects in the condition are always evaluated:

verify(initialize(), "init failed")

5.1.21.4. static_assert and concept_assert

static_assert checks conditions at compile time:

static_assert(typeinfo(sizeof type<int>) == 4, "expected 4-byte int")

concept_assert works like static_assert but reports the error at the caller’s line — useful inside generic functions:

def sum(arr : auto(TT)[]) {
    concept_assert(typeinfo is_numeric(type<TT>), "need numeric type")
}

5.1.21.5. finally and defer

finally runs cleanup code when a block exits:

for (i in range(3)) {
    total += i
} finally {
    print("loop done\n")
}

defer (from daslib/defer) schedules cleanup that runs when the enclosing scope exits. Multiple defers run in LIFO order:

require daslib/defer

def example {
    defer() { print("third\n") }
    defer() { print("second\n") }
    defer() { print("first\n") }
}
// prints: first, second, third (reverse order)