Precondition enforcing type system

gistfile1.txt

def sqrt(i: Int >= 0): Int >= 0 {
  ...
}

def good_input() {
  i = Int.parse(stdio.read_line)
  if i >= 0 {
    print "sqrt: #{sqrt(i)}
  } else {
    print "Cannot take the sqrt of a negative number"
  }
}

def bad_input() {
  i = Int.parse(stdio.read_line)
  print "sqrt: #{sqrt(i)}"
  # Compiler error: sqrt takes (Int >= 0), not (Int)
}

def ok_input() {
  i = Int.parse(stdio.read_line)
  assert(i >= 0)  # Allows the next line to compile
  print "sqrt: #{sqrt(i)}"
}