XO1402: loop without a provable bound

error · reported by the checker

Every loop of a real-time function (core 6.5) needs a bound known at compile time: a for over a range whose ends are constants is bounded by its length; any other for and every while needs a bound N clause after the iterated value or the condition, N a constant Int. A bound is a contract (core 3.5): when the iteration count is not constant, the loop faults with XO-F006 when it would start iteration N + 1, and xo test fuzzes the function. This code is also reported in any function when a bound is not a constant, is negative, or is smaller than a constant range it bounds.

Example

fn sum(xs: List[Int]) -> Int uses realtime {
  var t = 0
  for x in xs { t += x }
  t
}

Fix

const MAX = 64

fn sum(xs: List[Int]) -> Int uses realtime {
  var t = 0
  for x in xs bound MAX { t += x }
  t
}

Run xo explain XO1402 for this text in a terminal, or see section 10.6 of the specification.

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XO1402: loop without a provable bound

Example

Fix