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Dok supports flow typing, i.e. if a variable is tested for being of a specific type, then in the rest of the code block, it is assumed of this type without a reassignation.

fun f(x::Int?)::Int {
  when x.isa(Int) {
    return x + 1
    #-NOTE `x` has type `Int` now
  } -else {
    return 0

assert f(0) == 1
assert f(Null) == 0