function
assert
assert(cond: bool, msg: string)
When cond is false, prints file:line: assertion failed: <msg> to standard error and aborts (exit code 1); when it is true, execution continues. It is the guarded form of panic — for the programmer-bug cases the language should catch loudly rather than let corrupt the world: an index that must be in range, a value that must be non-negative, a state that must hold before a step. The location is baked in at compile time, so a failure points at the exact assertion. Assertions document and enforce the invariants a system relies on; keep them for "this must be true" checks, not for recoverable runtime conditions.
Parameters
condthe invariant that must hold (a bool)msga message describing the invariant (a string)Example
program Demo {
function withdraw(balance: int, amount: int) -> int {
assert(amount >= 0, "amount must be non-negative")
assert(amount <= balance, "cannot overdraw")
return balance - amount
}
entry {
print(withdraw(100, 30)) # 70
print(withdraw(100, 250)) # aborts: Demo.ludic:4: assertion failed: cannot overdraw
}
}