Karel Interpreter

A interpreter for programs written in the Apartment List dialect of Karel.

Installation

Add this line to your application's Gemfile:

gem 'karel-interpreter'

Usage

The interpreter takes a single file as an argument and prints out the final state of the Karel grid after executing the program.

Example

# square_dance.krl
move
put
turn_left
move
turn_left
move
put
turn_left
move

% karel square_dance.krl
location: (0, 0)
direction: right
tokens: [{"location"=>"(-1, 0)", "count"=>1}, {"location"=>"(0, 1)", "count"=>1}]
operations: 9

Options

The interpreter optionally takes a Karel configuration file (-i FILE) to initialize tokens on the Karel grid. The file format looks like

tokens: [{"location": "(-1, 0)", "count": 1}, {"location": "(0, 1)", "count": 2}]

The above initialization file will place one token at location (-1, 0) and two tokens at (0, 1)

Example

# initial_tokens.kin
tokens: [{"location": "(-1, 0)", "count": 1}, {"location": "(0, 1)", "count": 2}]

# noop_program.krl
# Do nothing

% karel -i initial_tokens.kin noop_program.krl
location: (0, 0)
direction: up
tokens: [{"location"=>"(-1, 0)", "count"=>1}, {"location"=>"(0, 1)", "count"=>2}]
operations: 0

Output

• Location - location of Karel when the program finishes
• Direction - direction Karel is facing when the program finishes
• Tokens - location and counts of all tokens left on the grid
• Operation - number of built-in commands executed

Karel Language Specification: Apartment List Dialect

Karel is a creature that lives on an infinite two dimensional grid. Using built-in commands as the basic building blocks, rudimentary control flow, and user-defined commands, programs can move Karel around the grid and place and remove tokens from squares on the grid.

The Karel language is case-sensitive and indentation-sensitive. All built-in commands and reserved words for control flow and user-defined commands are lower case. Statements within the body of control flow branches and user-defined commands must be indented two spaces beyond the containing branch/command definition. Only one statement is allowed per line. Blank lines are permitted.

Comments begin with a # and continue to the end of the line. Comments can be placed on their own line or after a statement.

Built-in commands

The following commands are available out of the box.

• move - Move Karel one square forward in the direction she is facing
• turn_left - Turn Karel left (CCW) 90 degrees
• token? - Return true if there is one or more tokens on Karel's current location, otherwise false
• pick - Pick up one token from Karel's current location. If there are no tokens, this command will crash the program.
• put - Put one token down on Karel's current location.

Control flow

Karel responds to if/else branching and while loops. The condition evaluated for these two control flow structures can be negated with a ! (no spaces are allowed between the negation operator and the condition)

Example: a basic if statement

if token?
  pick
end

Example: if/else

if token?
  pick
else
  move
end

Example: negated condition

if !token?
  put
end

Example: nested if

if token?
  pick
  if token?
    move
  end
end

Example: a basic while statement

# This will safely remove all tokens from a square.
while token?
  pick
end

User-defined commands

The Karel language supports user-defined commands. These commands do not take arguments and do not return a value. Commands are defined with the following syntax

def <command-name>
  <statement-1>
  <statement-2>
  ...
end

Example

def turn_around
  turn_left
  turn_left
end

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/[USERNAME]/karel-interpreter. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the Karel::Interpreter project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.