A2: Adventure

Adventure

In this assignment, you will develop a text adventure game (TAG), also known as interactive fiction. The characteristic elements of TAGs include gameplay driven by exploration and puzzle-solving, and a text-based interface in which users type natural-language commands and the game responds with text. The seminal work in this genre is the Colossal Cave Adventure, which you can play online.

You will actually implement not just a single game, but a game engine that could be used to play many adventures. The game engine is an OCaml program that implements the gameplay and user interface. An adventure is a data file that is input by the game engine and describes a particular gaming experience: exploring a cave, hitchhiking on a spaceship, finding the missing pages of a powerful magical book, etc. This factoring of responsibility between the engine and input file is known as data-driven design in games.

This assignment is about the same difficulty as A1. I had originally planned a more difficult assignment. But because of the overlap with the Prelim, I have scaled back the scope. On a similar assignment last fall, students reported working a mean of 11.1 hours and a standard deviation of 4.0 hours. Please get started right away and make steady progress each day. Please track the time that you spend. I will ask you to report it.

Collaboration policy: This assignment is to be completed as an individual. You may have only limited collaboration with other students as described in the syllabus.

What you’ll do: Implement and test a few OCaml modules.

Objectives:

Table of contents:

FAQs

Regarding the List module

Because of the particular focus on lists and trees in this assignment, we’re going to pay unusual attention to how you use (or don’t use) the List module from the standard library. Here are two coding practices to keep in mind from the beginning:

  1. Do not reimplement a list function from the standard library. Use the library’s version.

  2. Do not use any of List.hd, List.tl, or List.nth on this assignment. This is the last assignment on which those will be prohibited. The purpose of that prohibition is guide you toward better solutions. Those functions might seem attractive at first, but you need to get used to coding without them. Note that, by the previous point, you cannot get around the prohibition by reimplementing them yourself under a different name.

Step 1: Get Git Going and Explore the Release Code

Create a new git repo for this assignment. Make sure the repo is private. Add the release code to your repo. Refer back to the instructions in A1 if you need help with that.

Now that we are using modules and creating larger programs, working in utop becomes difficult; your normal mode of interaction with OCaml is necessarily going to shift away from utop and toward VS Code and the command line. Nonetheless we do still provide a simple make target that will open utop with all your code available for use.

Here is a summary of all the Makefile targets:

The latter two targets won’t be used until much later in the assignment; we discuss them below where they become relevant.

Feel free to browse through the release code at this point, but don’t worry about familiarizing yourself with all of it yet. The steps of the assignment, below, will take you through them in a guided order.

Documentation. With so many files now in the code base, it will be helpful to browse HTML documentation instead of source code. The makefile target make docs will use Ocamldoc (similar to Javadoc) to build that for you. Running it will produce two directories of documentation, doc.public and doc.private. Open index.html from either of those directories in your web browser to see the documentation.

The public HTML documentation includes only the names exposed through the interface files. Since those files were provided to you, they have been fully documented already. The private HHTML documentation includes all the names in the implementation files. If a name is documented in both the .mli and .ml files, then in the public HTML documentation it will contain only the comment from the .mli file; whereas in the private HTML documentation, it will contain both the comments from the .ml file and .mli files merged together. That means you can add information to comments for maintainers, but clients won’t see it. That also means you should not copy comments from the .mli file into the .ml file: they are automatically added by Ocamldoc when you generate the HTML documentation, so copying them would lead to the comment showing up twice in the private HTML documentation.

Do this now:

  1. Run make docs. Note that is plural: “docs” not “doc”.
  2. If you’re running on Ugclinux: right-click on doc.public in the Explorer pane of VS Code and download it locally.
  3. Open doc.public/index.html in your local web browser.
  4. Browse to the documentation for the Author module. Read it.
  5. Open author.mli and author.ml. For each comment in the source files, figure out whether it’s showing up in the HTML, and where.

Check your understanding:

Finally:

Step 2: JSON Tutorial

The adventure files that your game engine will input are formatted in JSON, the widely-used JavaScript Object Notation. If you’ve never used JSON before, read the brief overview of it on the JSON webpage.

In OCaml, you can use the Yojson library’s Basic module for parsing JSON. That library is large and provides much more functionality than you need for this assignment. So we provide a small tutorial in release-code file json_tutorial.ml.

Do this now: read the tutorial, following along and entering lines in utop to experience it firsthand. Then answer the following questions to check your understanding:

(There’s no need to turn in your answers.)

Caution in case by chance you stumble across the chapter in the book Real World OCaml about JSON: Ignore it. The features used in that chapter are more complicated than you need, and will be more confusing than helpful for this assignment. The ATDgen library and tool at the end of that chapter are not permitted for use on this assignment, because using them would preclude some of the list and tree processing that we want you to learn from this assignment. Note that the Core library used in that book is not supported in this course and will cause your code to fail make check.

Step 3: Load Adventure Files

The gameplay of TAGs is based on an adventurer moving between rooms. Rooms might represent actual rooms, or they might be more abstract—for example, a room might be an interesting location in a forest. Rooms have named exits through which the adventurer may move to other rooms. The human player’s goal is to explore the rooms.

Adventure files are formatted in JSON. We provide a couple example adventure files (lonely_room.json, ho_plaza.json) in the release code. Take a look now to familiarize yourself with them. An adventure file contains these entries:

Note that JSON strings are case sensitive and may contain whitespace. Unfortunately JSON does not support multiline strings. That’s why the descriptions in one of those examples necessarily violate the 80-column limit. Likewise, any test JSON files you create might need to do the same, and that’s okay.

Your task: Implement and test the Adventure compilation unit provided in the release code. Use test-driven development (TDD) as in A1: write a unit test that fails, then write code to make the test pass, then polish the code. Keep doing that until you are convinced that your unit tests are sufficient to demonstrate that your code is correct and complete. All your tests should be in the file test.ml, which is provided in the release code. The make test target will run your test suite from that file.

The Adventure documentation mentions set-like lists. A set-like list is a list in which no element appears more than once, and in which order is irrelevant. So [1; 2; 3] and [3; 2; 1] are both set-like lists and are considered equivalent, but [1; 1; 2; 3] is not a set-like list. The starter code provided in test.ml contains a couple helper functions for tests involving set-like lists. Tip: make sure that, anywhere a function specification says it returns a set-like list, you remove any duplicates that might be in the list. Otherwise, it is not a set-like list and will fail the staff test cases. The function List.sort_uniq is a good way to remove duplicates.

The Adventure documentation also mentions valid JSON adventure representations. A JSON representation of an adventure is valid if and only if:

Note that validity is a precondition, not postcondition, of Adventure.from_json. Recall what you have learned about preconditions: when they do not hold, a function is free to do anything it wants, including setting the computer on fire.

Although it is not technically part of the definition of “valid”, we promise that in our testing of your submission the adventures we use will not be huge. There will be at most on the order of magnitude of 100 rooms, and each room will have at most on the order of magnitude of 100 exits. That means tail recursion generally will not be needed.

Testing and interfaces. You cannot and should not test anything that is not exposed in an interface. That includes the definitions of abstract types as well as helper functions. For example:

For the latter, you might argue that you really do want to test helper functions. The designers of OUnit would disagree with you. Here’s what their manual says:

“Test only what is really exported: on the long term, you have to maintain your test suite. If you test low-level functions, you’ll have a lot of tests to rewrite. You should focus on creating tests for functions for which the behavior shouldn’t change.”

This is the stopping point for a satisfactory solution.

Step 4: Parse Commands

The interface to a TAG is based on the player issuing text commands to a prompt; the game replies with more text and a new prompt, and so on. Thus, the interface is a kind of read-eval-print-loop (REPL), much like utop. For this assignment, commands will be phrases of the form <verb> <object>. Verbs are always a single word, whereas objects might consist of multiple words separated by spaces.

There are only two verbs your engine needs to support:

Commands are case sensitive, as are exit names. So whereas go clock tower would move the player from Ho Plaza to McGraw Tower in the sample adventure file, neither GO clock tower nor go Clock Tower would.

Your task: Implement and test the Command compilation unit. The non-deprecated functions in the standard library String module are perfectly adequate for the work you need to do. Hint: investigate String.split_on_char.

Step 5: State Transitions

As the player progresses through an adventurer, some information does not change: the rooms, their exits, and so forth. But other information does change: the player’s current room, and the set of rooms the player has visited. In this assignment we’ll keep track of the latter kind of information as part of the game state. In an imperative language, the game state would be a mutable variable that is changed by functions that implement the game. But in a functional language, the game state must instead be an immutable value. Which leads to the question: how to represent changes?

Looking back at A1’s step function (which you might or might not have implemented), we can spot an answer: functions can take in an old state and return a new state. That’s exactly the solution we’ll use in this assignment. In particular, when the player attempts to move the adventurer from one room to another, the function that implements that movement will take in the current state of the game, and return a new state in which the adventurer has moved. Or perhaps the movement will turn out to be impossible, in which case the state will not change.

Your task: Implement and test the State compilation unit. Note carefully that the State.go function’s specification does not permit it to print, which is intended to guide you toward an idiomatic and functional implementation.

This is the stopping point for a good solution. If you want to stop here, that’s perfectly fine.

Step 6: Interface

At last, it’s time to build the user interface and make the game playable. The requirements for the interface are relatively minimal:

We leave the rest of the design of the user interface up to your own creativity. In grading, we will not be strictly comparing your user interface’s text output against expected output, so you have freedom in designing the interface.

The Makefile contains a new target, make play, that will build your game engine and launch the interface.

Your task: Implement the Main compilation unit. Your user interface must be implemented entirely within main.ml. It may not be implemented in state.ml. As the specification of State.go says, that function may not have any side effects, especially not printing.

All the console I/O functions you need are in the Stdlib module. The output functions under the heading “Output functions on standard output” are sufficient for this assignment. The read_line function is what you should use for input. You’re welcome to investigate the Printf and Scanf modules, but they are overkill for this assignment. You will likely find the String.concat function useful in manipulating object phrases.

The Main compilation unit is the only part of this assignment for which you are not required to write unit tests. Instead, you should interactively playtest your interface. You would likely find it helpful to recruit a friend to play your game and observe what confuses them about the interface. It’s fine for that friend to be another 3110 student as long as you adhere to the “Limited Collaboration” policy, but you might find a non-programmer to be a better test subject.

Step 7: New Adventure

Create your own adventure by constructing your own JSON file. It must not be based on any sample files we have already given you. We encourage you to create an interesting and creative adventure! But your grade won’t be based on that. Instead, we simply require that it have at least five rooms. If you are stuck for ideas, consider trying to model your dorm at Cornell, or the set of Friends or The Office. Name your file new.json so that the grader can quickly find it.

Rubric

Testing Rubric. The graders will continue to assess these aspects of your test suite:

Your test suite additionally needs to satisfy these requirements:

It would be wise to unit test much more extensively than that, of course, but you will get full credit on testing for doing the above.

As usual:

Code Quality Rubric. The graders will assess the following aspects of your source code:

Excellent Scope Rubric. The graders will attempt to play your game by checking the following:

The new adventure will be worth about 1 out of the 5 points. You won’t get that point if your game interface is not implemented.

Submission

Record your name and NetID in author.mli, and set the hours_worked variable at the end of author.ml.

Run make zip to construct the ZIP file you need to submit on CMS. Our autograder needs to be able to find the files you submit inside that ZIP without any human assistance, so:

→ DO NOT use your operating system’s graphical file browser to construct the ZIP file. ←

Use only the make zip command we provide. Mal-constructed ZIP files will receive a zero from the autograder. If CMS says your ZIP file is too large, it’s probably because you did not use make zip to construct it; the file size limit in CMS is plenty large for properly constructed ZIP files.

For the excellent scope: as long as your new adventure file is named with a .json extension and is located in the same directory as the rest of your source code, it should automatically be included in your ZIP file. But double check just to be sure. We’re not going to accept any regrade requests or restore any points if it’s missing from your submission.

Ensure that your solution passes make finalcheck. Submit your adventure.zip on CMS. Double-check that the MD5 sum is what you expected. Re-download your submission from CMS and double-check before the deadline that the contents of the ZIP are what you intended.

Congratulations! You’ve had an Adventure!

Acknowledgement: Adapted from Prof. John Estell (Ohio Northern University).