bridge.rs

type Bid = Int where self >= 1 && self <= 7;

fn doubled(bid: Bid) -> Int {
  bid * 2
}
// The return type of ^^^ could be `Int where self >= 2 && self <= 14`, but since
// Int is a supertype of that, just returning `Int` is fine

fn main() {
  n = get_console_input.to_int().or_go_boom()
  if n < 1 && n > 7 {
    print("A bid's gotta be between 1 and 7")
  } else {
    print(doubled(n))
  }
}

fn bad_main() {
  n = get_console_input().to_int().or_go_boom()
  print(doubled(n))
  // ^^^ Compile error: N is an `Int`, but double wants a `Bid` (`Int where self >= 1 && self <= 7`)
  // OR
  // pass --check-things-at-runtime-instead-of-compile-time
  // or --dont-bother-checking-my-types-my-code-is-perfect
  // to get this running while prototyping
}

fn assert_main() {
  n = get_console_input.to_int().or_go_boom()
  assert(n >= 1 && n <= 7)
  // ^^^ will blow up at runtime if it's not true.
  // Beforehand, n's type is `Int`, after that line, it's `Int where self >= 1 && self <= 7`.
  // Alternately, that line could be `i_solemnly_swear(n >= 1 && n <= 7)`, which changes the type
  // but _doesn't_ insert the runtime check.
  print(doubled(n))
}