version: 1
name: Lambda
description: |
  Learn how to define functions.
objectives:
  - goal:
      - Your goal in this challenge is to send a robot to grab the
        flower in the lower right (you don't need to bring it back).
      - |
        The path looks complex, but if you study it, you will see that it
        has a lot of structure.  In particular, there are many parts of
        the path that repeat four times; it seems like it could be really useful to have
        a function to repeat a command four times.
      - |
        To write a function, you use lambda syntax: in general, `\x. blah` is the
        function which takes an input (locally called `x`) and returns
        `blah` as its output (`blah` can of course refer to `x`). For example:
      - |
        def x4 : cmd unit -> cmd unit = \c. c; c; c; c end
      - That is, `x4` is defined as the function which takes a command, called `c`,
        as input, and returns the command
        `c; c; c; c` which consists of executing `c` four times.
    condition: |
      try {
        teleport self (32,-16);
        b <- ishere "flower";
        return (not b)
      } { return false }
solution: |
  def x4 = \c. c; c; c; c end;
  def m2 = move; move end;
  def m4 = x4 move end;
  def tR = turn right end;
  def tL = turn left end;
  def s = m4; tL; m2; tL; m4; tR; m2; tR end;
  build {
    require "treads"; require "lambda";  // #540
    turn right;
    x4 (x4 s; m4; m2; tR; x4 (x4 move); tL; m2);
    grab
  }
robots:
  - name: base
    dir: [0,1]
    display:
      char: Ω
      attr: robot
    devices:
      - logger
      - 3D printer
      - dictionary
    inventory:
      - [10, logger]
      - [10, compass]
      - [10, treads]
      - [10, solar panel]
      - [10, grabber]
      - [10, lambda]
      - [0, boulder]
      - [0, flower]
world:
  default: [blank]
  palette:
    'Ω': [grass, null, base]
    '.': [grass]
    '*': [grass, flower]
    '@': [grass, boulder]
    '┌': [stone, upper left corner]
    '┐': [stone, upper right corner]
    '└': [stone, lower left corner]
    '┘': [stone, lower right corner]
    '─': [stone, horizontal wall]
    '│': [stone, vertical wall]
  upperleft: [-1,1]
  map: |
    ┌─────────────────────────────────┐
    │.......@.......@.......@.......@@│
    │.@@@@@.@.@@@@@.@.@@@@@.@.@@@@@.@@│
    │.....@.@.....@.@.....@.@.....@.@@│
    │@@@@.@.@@@@@.@.@@@@@.@.@@@@@.@.@@│
    │.....@.@.....@.@.....@.@.....@.@@│
    │.@@@@@.@.@@@@@.@.@@@@@.@.@@@@@.@@│
    │.....@.@.....@.@.....@.@.....@.@@│
    │@@@@.@.@@@@@.@.@@@@@.@.@@@@@.@.@@│
    │.....@.@.....@.@.....@.@.....@.@@│
    │.@@@@@.@.@@@@@.@.@@@@@.@.@@@@@.@@│
    │.....@.@.....@.@.....@.@.....@.@@│
    │@@@@.@.@@@@@.@.@@@@@.@.@@@@@.@.@@│
    │.....@.@.....@.@.....@.@.....@.@@│
    │.@@@@@.@.@@@@@.@.@@@@@.@.@@@@@.@@│
    │.....@.@.....@.@.....@.@.....@.@@│
    │@@@@.@.@@@@@.@.@@@@@.@.@@@@@.@.@@│
    │Ω....@.......@.......@.......@..*│
    └─────────────────────────────────┘