Immutability

Nymph values and local bindings are immutable. Nymph has no mut binding, mut T, mut func, field assignment, index assignment, or compound assignment. Construct a new value instead of changing an old one; persistent collections may share storage internally while preserving the old value.

struct Counter(value: int)

func increment(counter: Counter): Counter = Counter(value = counter.value + 1)
func demo(): #(int, int) = {
  let before = Counter(value = 0)
  let after = increment(before)
  #(before.value, after.value)
}

Updating repeated state

Use an immutable state loop. Its header creates fresh loop-carried bindings for each iteration, and continue(name = value) replaces them simultaneously. Use fold when the operation is naturally a reduction.

func sum_to(limit: int): int = loop@sum (
  let next = 1
  let total = 0
) {
  if (next > limit) { break@sum total }
  continue(next = next + 1, total = total + next)
}

There is no source while. JavaScript emitted by the compiler may use mutation and host loops as an unobservable optimization; that does not make either operation part of Nymph source semantics.