version: 1
name: Bind notation
description: |
  Learn about command return types and how to bind the results.
objectives:
  - goal:
      - |
        Every command returns a value.  However, some simple commands, like
        `move`, do not have any meaningful
        value to return.  Swarm has a special type, `unit`, with only one value,
        called `()`. Since there is only one possible value of type
        `unit`, returning it does not convey any information.
        Thus, the type of `move` is `cmd unit`.
      - |
        Other commands do return a nontrivial value after executing.
        For example, `grab` has type
        `cmd text`, and returns the name of the
        grabbed entity as a text value. Try it:
      - |
        move; grab
      - |
        To use the result of a command later, you need bind notation, which
        consists of a variable name and a leftwards-pointing arrow
        before the command.  For example:
      - |
        move; t <- grab; place t
      - |
        In the above example, the result returned by `grab` is assigned
        to the variable name `t`, which can then be used later.
        This could be useful, for example, if you do not care what you
        grabbed and just want to move it to another cell, or if you
        are not sure of the name of the thing being grabbed.
      - Once you are done experimenting, do `place "Win"` to finish this challenge.
        (Note, you might need to `grab` the entity you are standing on
        or move to an empty cell first, since each cell can only
        contain at most one entity.)
    condition: |
      try {
        w <- as base {has "Win"};
        return (not w);
      } { return false }
solution: |
  place "Win"
entities:
  - name: Win
    display:
      attr: device
      char: 'W'
    description:
      - Do `place "Win"` once you are done with this challenge.
      - You might need to `grab` the entity you are standing on
        or move to an empty cell first.
    properties: [known, portable]
robots:
  - name: base
    dir: [1,0]
    devices:
      - treads
      - grabber
      - logger
      - compass
      - dictionary
    inventory:
      - [1, Win]
      - [0, tree]
world:
  default: [blank]
  palette:
    '>': [grass, null, base]
    'T': [grass, tree]
    '┌': [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: |
    ┌───┐
    │>TT│
    └───┘