version: 1
name: Swap in one tick
description: |
  Swapping an entity should be done in one tick.
  https://github.com/swarm-game/swarm/issues/684
objectives:
  - condition: |
      try {
        as base {
          l <- has "lambda";
          b <- has "bitcoin";
          return $ l && b
        }
      } {
        return false
      }
solution: |
  swap "bitcoin"; swap "gold coin";
robots:
  - name: base
    loc: [0,0]
    dir: [1,0]
    devices:
      - logger
      - scanner
      - grabber
      - fast grabber
    inventory:
      - [1, gold coin]
      - [1, bitcoin]
  - name: watcher
    loc: [0,0]
    dir: [1,0]
    system: true
    devices:
      - logger
    program: |
      def repeat = \c. c; repeat c end;
      repeat (
        d <- scan down;
        case d (\_.
          say "Fatal error: swap does not work atomically!"
        ) (\_.
          return ()
        )
      )
world:
  default: [blank]
  palette:
    '┌': [stone, upper left corner]
    '┐': [stone, upper right corner]
    '└': [stone, lower left corner]
    '┘': [stone, lower right corner]
    '─': [stone, horizontal wall]
    '│': [stone, vertical wall]
    'λ': [grass, lambda]
  upperleft: [-1, 1]
  map: |
    ┌─┐
    │λ│
    └─┘