Toy Robot - The Elixir Version
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Toy Robot - The Elixir Version

About the Book

The Toy Robot exercise was originally developed by Jon Eaves. He explains why he did it in this blog post.

If you're a new Elixir developer who's gone through some basic Elixir tutorials and you're looking for the next thing to build your skills, this book is a great start. It covers the Toy Robot exercise from start to finish, testing with Elixir features such as ExUnit and Doctests along the way.

The Toy Robot exercise is commonly used in interviews as the ways to solve it in any language are not as simple as they first may seem. This book covers one implementation of this exercise in Elixir. It is not intended to be the most perfect implementation of the Toy Robot exercise possible, but instead is my personal take on it.

In this book, I demonstrate best practices for developing a small Mix application that can solve the Toy Robot exercise. Along the way, you'll learn about:

  • How to break a complex problem down into small, approachable chunks
  • When to use Doctests or regular ExUnit tests
  • Tradeoffs between different ways of pattern matching
  • How to work with Elixir processes using GenServer + DynamicSupervisor

The Toy What Now?

The Toy Robot! It's a very common interview exercise given to new programmers. Here's the variant of the problem's description that we use in the book:

The application is a simulation of a toy robot moving on a square tabletop, of dimensions 5 units x 5 units. There are no other obstructions on the table surface. The robot is free to roam around the surface of the table. Any movement that would result in the robot falling from the table is prevented, however further valid movement commands are still allowed.

The application reads a file using a name passed in the command line, the following commands are valid:

  • MOVE
  • LEFT

Here's some rules for these commands:

  • PLACE will put the toy robot on the table in position X,Y and facing NORTH, SOUTH, EAST or WEST.
  • The origin (0,0) is the SOUTH WEST most corner.
  • All commands are ignored until a valid PLACE is made.
  • MOVE will move the toy robot one unit forward in the direction it is currently facing.
  • LEFT and RIGHT rotates the robot 90 degrees in the specified direction without changing the position of the robot.
  • REPORT announces the X,Y and F of the robot.

The file is assumed to have ASCII encoding. It is assumed that the PLACE command has only one space, that is PLACE 1, 2, NORTH is an invalid command. All commands must be in upcase, all lower and mixed case commands will be ignored.


All of that may seem a little bit overwhelming but if you break it down into little chunks and tackle those chunks one-at-a-time (like this book does!) it becomes much easier.

About the Author

Ryan Bigg
Ryan Bigg

Ryan has been writing books and documentation for programming languages for over a decade. These books have sold over 20,000 copies worldwide.

Ryan lives in the lovely coastal town of Warrnambool, Australia.

Table of Contents

    • 1. Introduction
    • 2. Introducing the Toy Robot
      • Simplifying the problem
      • Creating the application
      • The MOVE command
      • Moving west
      • Moving north and south
      • Moving in the right direction
      • Turn left!
      • Turn right!
    • 3. Catching bugs
      • Finding the bugs within
      • Regression testing the turn_right function
      • Regression testing the move function
      • Regression testing the move_west function
      • Regression testing the move_north and move_south functions
      • Jumping back to the manual test
      • Reflections
    • 4. Placing the robot on a table
      • The Table module
      • The missing link
    • 5. Building our simulation
      • Moving a robot, within a simulation
      • Turning a robot, within a simulation
      • Reporting the robot’s position
    • 6. Reading and handling commands
      • The Command Interpreter
      • The Command Runner
      • Piecing it all together
    • 7. Building the CLI
      • Reading commands from a file
      • Verifying the robot’s behaviour
      • Conclusion
  • I The Toy Robot Game
    • 8. A Single, Supervised Player
      • A single player
      • Let it crash!
      • Watching processes with supervisors
      • Giving names to players
      • Conclusion
    • 9. Multiplayer Toy Robot
      • Prelude: a short refactor involving Player and tables
      • Creating a Game
      • Handling an invalid placement (off the board)
      • Handling an invalid placement (square occupied)
      • A refactoring interlude: Breaking Server apart
      • Preventing robots from colliding
      • Preventing robots from respawning in occupied spaces
      • The End
      • Homework

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