W1165 Karel Introduction

Within these castle walls be forged Mavens of Computer Science ...

— Merlin, The Coder

CoderMerlin™ Karel

Prerequisites

• W1157 Nested Loops

Background

Richard Pattis

Karel the Robot

Karel Čapek

 Coder Merlin™ Karel , also known as "that little blue bot", is strongly inspired by Richard Pattis' book written during his studies at Stanford. Richard Pattis currently teaches at the University of California, Irvine. Pattis chose the name Karel in honor of the Czech playwright Karel Čapek, who introduced the word robot in his 1921 play R.U.R.

Eric Roberts, Department of Computer Science, Stanford University, September 2005:

In the 1970s, a Stanford graduate student named Rich Pattis decided that it would be easier to teach the fundamentals of programming if students could somehow learn the basic ideas in a simple environment free from the complexities that characterize most programming languages. Drawing inspiration from the success of Seymour Papert’s LOGO project at MIT, Rich designed an introductory programming environment in which students teach a robot to solve simple problems.

 Coder Merlin™ Karel  is a very simple robot living in a very simple world. Programming  Coder Merlin™ Karel  is accomplished by specifying a series of simple instructions.  Coder Merlin™ Karel  initially understands only a very few instructions, but over time you can expand the vocabulary by defining functions and enhancing  Coder Merlin™ Karel 's abilities.

All instructions issued to  Coder Merlin™ Karel  must be very precise and unambiguous, a key requirement of well-written programs. The programs themselves must obey a set of syntactic rules which define which commands (and the ordering of those commands) are acceptable. These rules define a programming language, such as the Swift programming language which we've been using. The details of the subset of instructions which  Coder Merlin™ Karel  understands are very easy to master, enabling us to focus on problem solving.

It is problem solving which is the focus and essence of computer programming.

Introduction (Welcome to my World)

Streets, Avenues, and Corners

Figure 1

 Coder Merlin™ Karel  lives in a city with green avenues running vertically north-south and blue streets running horizontally east-west. The intersection of a street with an avenue is called a corner.  Coder Merlin™ Karel  can only be position on a corner and must point in one of the four cardinal directions.  Coder Merlin™ Karel  always points with his hat, so in Figure 1  Coder Merlin™ Karel  is positioned at (avenue: 1, street: 1) and is facing north. Note that worlds may be of various sizes depending upon the challenge that needs to be solved.

Beepers

Figure 2

Corners may have zero or more beepers present, indicated by a yellow/red rotating beacon. The number in the center of the beacon indicates the number of beepers present on the corner. In Rich Pattis’s book, beepers are "plastic cones which emit a quiet beeping noise."  Coder Merlin™ Karel  can only detect a beeper if it is on the same corner.  Coder Merlin™ Karel  carries with him a hidden beeper bag; the number on the front of  Coder Merlin™ Karel  indicates how many beepers are in the bag (Figure 3).

Figure 3

Walls

Figure 4

Walls may be present between streets and avenues and serve as a barrier to  Coder Merlin™ Karel ; unable to pass through them  Coder Merlin™ Karel  must go around them. Note that the presence of walls is implied around the boundary of the city.

Challenges

 Coder Merlin™ Karel  is integrated into  Merlin Mission Manager  but because it is built on top of IGIS a few additional steps are necessary to complete  Coder Merlin™ Karel  challenges.

Getting Started

 Coder Merlin™ Karel  missions take a similar form to other missions and the usage of the  Merlin Mission Manager  is similar. You'll need to follow the same paradigm for accepting and preparing missions and challenges as that of previous missions and challenges. However, there are a few difference which follow:

File Structure

The challenges require use of a framework (a series of libraries). While most of this is transparent you'll notice several more files in the challenge directory, including:

• ✅ ActionPlan.swift - SHOULD be MODIFIED to develop an effective solution to the challenge.
• 🚫 dylib.manifest - Generally SHOULD NOT be altered. It's responsible for providing a list of required dynamic libraries and their versions.
• ✅ instructions.txt - SHOULD be READ and closely followed before beginning the challenge.
• 🚫 main.swift - Generally SHOULD NOT be altered. It's responsible for using the  Coder Merlin™ Karel  framework to create the world.
• 🚫 make.sh - Generally SHOULD NOT be altered. It contains the instructions necessary to build the project.
• 🚫 Package.swift - Generally SHOULD NOT be altered. It contains a description of the package to be built.
• 🚫 run.sh - Generally SHOULD NOT be altered. It contains the instructions necessary to run the project.
• 🚫 WorldPlanner.swift - Generally SHOULD NOT be altered. It contains a description of the world which  Coder Merlin™ Karel  must navigate along with the goal of an effective solution.

Syntax Highlighting

Syntax highlighting and other Language Services will perform normally in emacs but require additional setup after preparing the challenge. These steps are only required once per challenge.

1. Enter the challenge directory
2. Generate a new emacs configuration file

john-williams@codermerlin:~/Merlin...$ dylibEmacs

1. Open ActionPlan.swift in emacs
2. A message will appear at the bottom of emacs. When warned about "The local variables list in /home/john-williams/Merlin/...contains values that may not be safe (*)" press ! to apply the local variable list and permanently mark the values as safe
3. A message will appear at the bottom of emacs:

ActionPlan.swift is not part of any project. Select action:



i==>Import project root ~/Merlin/.../.

I==>Import project by selecting root directory interactively.

d==>Do not ask again for the current project by adding ~/Merlin/.../ to lsp-session-folders-blacklist.

D==>Do not ask again for the current project by selecting ignore path interactively.

n=>Do nothing: ask again when opening other files from the current project.

Press I (capital 'I') and select the directory for this challenge in which Package.swift is located.

Building

Like most challenges, the project can be built by issuing a build command on the console:

john-williams@codermerlin:~/Merlin...$ build

Alternatively, one can use F5 b from within emacs.

Running

Like most challenges, the project can be run by issuing a run command on the console:

john-williams@codermerlin:~/Merlin...$ run

Alternatively, one can use F5 r from within emacs.

Helpful Hint

In order to terminate a running program within emacs:

1. Switch to the window in which the program is running using CONTROL-x o, repeating as many times as is necessary. (The window will be titled "*Async Shell Command*".)
2. Terminate the running program with CONTROL-c CONTROL-c

Viewing

Unlike most challenges, very little information will be displayed to the console. Rather, a browser needs to be opened to view the output.

Helpful Hint

Each user has their own, dedicated URL for viewing Igis programs. For example, John Williams' URL is: https://www.codermerlin.com/igis/john-williams/

Be sure to change your URL in accordance with your user name. Of course, you'll only be able to view your program while it is running.

Simulation Control

Controlling  Coder Merlin™ Karel  occurs through programming of the Action Plan (described below). However, there are a few keypresses that may be helpful while running the simulation:

Key Sequence	Purpose
Space	Toggles between the current and goal views
f	Causes the animation to run faster
s	Causes the animation to run slower

The Goal

Each challenge has a target goal state which may be viewed by toggling to the goal view. The goal view appears with a gold background. The first step of solving any  Coder Merlin™ Karel  challenge begins by running an empty Action Plan which will show the initial state and then toggling to the goal state. The objective is to formulate an action plan to alter the world from the initial state to the goal state.

Initial (Current) View

Goal View

In the above case, we can see that the initial view has  Coder Merlin™ Karel  at (avenue: 1, street: 1) with one beeper (indicated as a "1" on his body). After achieving the goal,  Coder Merlin™ Karel  is now located at (avenue: 3, street: 3) after having dropped his beeper at (avenue: 2, street: 3) along the way.

Action Plan

The Action Plan is located in the file ActionPlan.swift. Initially, the file is nearly empty:

import MerlinKarel

class ActionPlan: KarelExecutor {

    // Declare functions here:

    override func run() {
        // Insert your code here:
    }

}

• Line 1 informs the Swift compiler that we'll be relying on functionality imported from the MerlinKarel library. This line should not be changed.
• Line 3 begins the definition of a new class descending from a KarelExecutor. We'll learn more about classes later. This line should not be changed.
• Line 7 begins the definition of a required function called run. You'll insert your code within the body of this function. Any other functions can be inserted below line 5.

When shipped from the factory,  Coder Merlin™ Karel  understands the following instructions: