version: 1
name: Invisible maze
author: Brent Yorgey
description: There is a maze, but it can't be seen, only sensed... can you
  program a robot to navigate it successfully?
objectives:
- goal:
  - There is a maze, but it can't be seen, only sensed... can you
    program a robot to navigate it successfully?  You need to get a robot
    to the goal square, labelled with an exclamation mark; you win by `grab`bing
    the `goal`.
  - In this challenge, it is guaranteed that the maze is a tree, that is,
    there are no loops within the maze.
  condition: |
    try {
      teleport self (27, -17);
      b <- ishere "goal";
      return (not b)
    } { return false }
solution: |
  def tL = turn left end;
  def tR = turn right end;
  def ifC = \test. \then. \else. b <- test; if b then else end;
  def forever = \c. c ; forever c end;
  def rightBlocked = tR; b <- blocked; tL; return b end;
  def mazeStep = ifC rightBlocked {ifC blocked {tL} {move}} {tR; move} end;
  build {
    require "treads"; require "branch predictor"; require "lambda";
    require "strange loop"; require "scanner"; require "grabber";   // #540

    forever (mazeStep; ifC (ishere "goal") {grab; return ()} {})
  }
robots:
  - name: base
    dir: [0,1]
    display:
      char: 'Ω'
      attr: robot
    devices:
      - dictionary
      - 3D printer
      - logger
      - grabber
    inventory:
      - [50, solar panel]
      - [50, treads]
      - [50, compass]
      - [50, scanner]
      - [50, lambda]
      - [50, branch predictor]
      - [50, strange loop]
      - [50, logger]
      - [50, grabber]
      - [0, boulder]
      - [0, goal]
entities:
  - name: wall
    display:
      invisible: true
    description:
    - An invisible wall.
    properties: [unwalkable, known]
  - name: goal
    display:
      char: '!'
      attr: device
    description:
    - The place you're trying to reach!  You win by executing `grab` on this item.
    properties: [known, portable]
world:
  default: [grass]
  palette:
    'Ω': [grass, null, base]
    '.': [grass, null]
    'x': [grass, wall]
    '@': [grass, boulder]
    '!': [grass, goal]
  upperleft: [-1,1]
  map: |
    @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
    @Ω........x...x..............@
    @xx.xxxxx.x.x.xxx....xxxxxxx.@
    @.x.x...x.x.x...xxxx.......x.@
    @.....x.x.x.x.x....xxxxxxxxx.@
    @xxxxxxxx.x.x.xxxx...........@
    @.......x...x....xxxxxxxxxxxx@
    @.xxxxx.xxxxxxxx.x...........@
    @.x.x.x..x.....x.x.xxxxxxxxx.@
    @.x.x.xx.x.xxxxx.x.x.....x...@
    @.x.x.xx.x.x...x.x.x.x.x.x.xx@
    @......x...x.x...x.x.x.x.x...@
    @xxxxxxx.xxx.xxxxx.x.x.x.xxx.@
    @....................x.x.x.x.@
    @xxxxxxxxxxxxxxxxxxxxx.x.x.x.@
    @..x...x...x...x.....x.x.x.x.@
    @x...x...x...x...x.x.x.x.x.x.@
    @xxxxxxxxxxxxxxxxx.x.x.x.x.x.@
    @..................x...x...x!@
    @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@