Assignment 1: Shapes

In this assignment, you will build a Haskell program that uses the CodeWorld API to draw colourful shapes on the screen, including lines, polygons, rectangles, circles, and ellipses.

This assignment is worth 10% of your final grade.

Part A: Saturday March 23, 2019, at 9:00am Canberra time sharp
Part B: Monday April 8, 2019, at 9:00am Canberra time sharp
Please note that Daylight Savings ends on Sunday 7 April.

Indicative marks and feedback for Part A will be returned at the end of week 5.

Required Knowledge

If you have finished the week 3 lab, you should be able to complete Part A.

If you have finished the week 4 lab, you should be able to complete the majority of the assignment. Some parts require recursion over lists, which is covered in the week 5 lab.

Overview of Tasks

You will be required to complete different tasks depending on your enrolled course:

  COMP1100 COMP1130
Task 1: Helper Functions 20 marks 10 Marks
Task 2: Rendering Shapes 35 marks 30 Marks
Task 3: Handling Events 30 marks 25 Marks
1130 Extensions   30 Marks
Technical Report 15 marks 25 Marks

Part A of the assignment requires you to complete Task 1. Part B of the assignment requires you to complete all assigned tasks. As you complete each task (or even each function within a task), you should commit and push your work with a sensible commit message.

The purpose of Part A is to give you an opportunity to collect feedback on your code and your progress in the course, and for us to give you an indicative mark for your work so far. This will be returned to you before the census date. Part A will be re-marked alongside your Part B submission, giving you a final mark for the assignment.

Getting Started

Fork the assignment repository and create a project for it in IntelliJ IDEA, following the same steps as in Lab 2. The assignment repository is at

Overview of the Repository

Most of your code will be written to Haskell files in the src/ directory. We are using the model-view-controller pattern to structure this assignment. Each file is called a module, and we use modules to group related code together and separate unrelated code.


The model is a data type that describes the state of the running program. The program will move to new states (new values of type Model) in response to user actions, as defined by the controller.


The view turns the model into something that can be shown on the screen; in this project, that is the CodeWorld Picture type.


The controller considers user input (and other events), along with the current model, and uses that to decide what the new model should be.

Other Files

  • tests/ShapesTest.hs contains some unit tests - simple checks that verify small parts of your program are working correctly. You are not required to write tests for this assignment, but you might find it useful to do so.

  • tests/Testing.hs is a small testing library used by tests/ShapesTest.hs. You are not required to understand it for this assignment.

  • app/Main.hs ties your functions together into the final program that runs. You are not required to understand it.

  • comp1100-assignment1.cabal tells the cabal build tool how to build your assignment. You are not required to understand this file, and we will discuss how to use cabal below.

  • Setup.hs tells cabal that this is a normal package with no unusual build steps. Some complex packages (that we won’t see in this course) need to put more complex code here. You are not required to understand it.

Overview of Cabal

cabal is the build tool for Haskell programs and libraries. It provides several useful commands:

  • cabal build: Compile your assignment.

  • cabal run shapes: Build your assignment (if necessary), and run the shapes program.

  • cabal repl comp1100-assignment1: Run the GHCi interpreter over your project. This gives you the same ghci environment you use in labs, but with the assignment code loaded. (Aside: REPL is the name for interactive sessions like GHCi - it stands for read-eval-print loop. Many modern languages have REPLs.)

  • cabal test: Build and run the tests. Tests will abort on the first failure, or the first call to a function that is still undefined.

You should execute these cabal commands in the top-level directory of your project: ~/comp1100/assignment1 (i.e., the directory you are in when you launch the IntelliJ Terminal tool for your project).

Overview of the Program

You use a web browser to interact with the shapes program that you launched with cabal run shapes. Once you have completed the assignment, it will respond to the following actions:

Action Effect
Esc (key) Clear the canvas
M (key) Display a mystery image
C (key) Cycle colour (of shape to draw)
T (key) Cycle tool (type of shape to draw)
Backspace/Delete (key) Remove the last added shape
D (key) Print the current Model to the terminal (useful for testing)
Click-drag-release (mouse) When not drawing polygons, draw a shape between the click-point and release-point
Click (mouse) When drawing polygons, add a point to the polygon
Spacebar (key) Finish drawing a polygon, adding it to the canvas.

Task 1: Helper Functions (COMP1100: 20 marks; COMP1130: 10 marks)

The easiest way to solve a large problem is often to break it apart into smaller, easier problems. Programming is the same. In this task you will write some helper functions that will make future tasks easier. You can test your implementations by running cabal test.

The functions you need to write for this task are:

  • toolLabel in src/View.hs. This function should return instructions for the user on how to use each Tool, according to the following table:
Tool Label
LineTool "Line... click-drag-release"
PolygonTool "Polygon... click 3 or more times then spacebar"
RectangleTool "Rectangle... click-drag-release"
CircleTool "Circle... click-drag-release"
EllipseTool "Ellipse... click-drag-release"

Note: At the time this assignment is released, the course will have only briefly covered lists. You do not need to manipulate lists to write toolLabel; you can use a blank pattern (_) to ignore them.

  • nextColour in src/Controller.hs. This function should return the next colour in our set of ColourNames:
Argument Result
Black Red
Red Orange
Orange Yellow
Yellow Green
Green Blue
Blue Violet
Violet Black
  • nextTool in src/Controller.hs. This function implements tool-switching, but should not change Tool if the user is halfway through an operation:

    • If the tool is not holding a point (that is, a non-PolygonTool tool holding Nothing or a PolygonTool holding the empty list []), select the next tool in the following sequence: Line -> Polygon -> Rectangle -> Circle -> Ellipse -> Line -> ....

    • If there is a Point stored in the given tool (because it’s holding a Just value or the list in PolygonTool is non-empty), return the argument unchanged.

    • If this is unclear, study the nextToolTests in test/ShapesTest.hs.

Note: At the time this assignment is released, the course will have only briefly covered lists. You can write the PolygonTool case for nextTool without using list recursion. Use [] to match an empty list. In a subsequent case, give the entire list a name like points to match any nonempty list. Alternatively, you might use the Prelude function null, which returns True if a list is empty and False if it is not.

Part A ends here.

Submitting Part A

Your submission for Part A should include implementations of toolLabel, nextColour and nextTool that compile without warnings and pass the tests run by cabal test. You are welcome to continue working on Part B of your assignment and committing and pushing changes, so long as the code continues to compile and the tests continue to pass.

Part B begins…

Task 2: Rendering Shapes (COMP1100: 35 marks; COMP1130: 30 marks)

In src/View.hs, modelToPicture converts your Model type into a CodeWorld Picture, so that it can be displayed on the screen. It currently does not work, because colourShapesToPicture is undefined. In this task you will fill in that missing piece, building up a function to convert the [ColourShape] from your Model into a Picture. You can test these functions individually by using cabal repl comp1100-assignment1, using drawingOf to show small pictures on the screen. You can also test everything as a whole by launching the program with cabal run shapes and pressing the M key to show the mystery image. The functions you need to write for this task are all in src/View.hs:

  • colourNameToColour: This function turns your ColourName type from the model into a CodeWorld Colour. You should check the CodeWorld documentation for information on colours.

  • shapeToPicture: This function turns your Shape type into a CodeWorld Picture. You will need to consider the constructors for Shape individually, and work out the best way to turn each one into a Picture. Here are some hints to help you along:

    • CodeWorld has no function to draw a single line segment. It does have a function to draw a line made of multiple segments - polyline.

    • Polygons, rectangles, circles and ellipses should be drawn as solid (filled) Pictures.

    • Most of CodeWorld’s functions draw individual shapes centred on the origin - (0, 0). You will need to figure out how to slide (translate) the generated Picture so it winds up where it’s supposed to go. Drawing diagrams will help. The abs function will also help - it computes the absolute value of its argument (i.e., abs x == x if x > 0, and abs x == negate x otherwise).

    • Ellipses are specified as two opposite coordinates for a bounding box that wraps the ellipse. CodeWorld has no primitive that draws an ellipse, but an ellipse is a stretched circle. Does CodeWorld have a function to stretch (scale) a Picture?

    • You do not have to completely implement this function before moving onto others. If you have written code to render Rectangles but nothing else, you can create a drawing out of Rectangles only, and check that your other functions work, before returning to finish shapeToPicture.

  • colourShapeToPicture: This function should render the Shape and colour it using the Colour that corresponds to the given ColourName.

  • colourShapesToPicture: This function should turn every ColourShape in a list into a single Picture. You will need to recurse over the input list. If you have not yet completed Lab 5, you may want to work on other parts of the assignment and come back to this.

Task 3: Handling Events (COMP1100: 30 marks; COMP1130: 25 marks)

It is now time to tackle handleEvent in src/Controller.hs. CodeWorld calls this function whenever something interesting happens (like a key press, a pointer press, or a pointer release). This function is called with two arguments:

  • The Event that just happened, and
  • The current Model at the time the Event happened.

handleEvent then returns a new Model for the program to use moving forward.

(Aside: Elm is a functional programming language that uses a similar pattern to build front-end web applications that are compiled to JavaScript.)

Let’s trace a simple interaction. If the user wants to draw a red line by clicking on the screen at coordinates and releasing the mouse at coordinates . starting at a blank canvas, the state would transition as follows, starting with the initial model:

  1. Model [] (LineTool Nothing) Black

  2. The user presses “C” to change the colour from red to black:

    Model [] (LineTool Nothing) Red

  3. The user presses the mouse button at changing the state to

    Model [] (LineTool (Just (1.0,1.0))) Red

  4. The user releases the mouse button at changing the state to

    Model [(Red,Line (1.0,1.0) (2.0,2.0))] (LineTool Nothing) Red

Note that the Tool and the ColourName do not reset to the default values after a shape has been drawn. However, the Maybe Point inside the tool revert to Nothing.

Task 3.1: Handling Mouse Input

CodeWorld provides a few different event constructors for mouse input, but the ones we’re interested in are PointerPress for when the user clicks, and PointerRelease for when the user releases the mouse button.

When a PointerPress event arrives, you will need to store it in the current Tool. For everything except PolygonTool, you will store it in the Maybe Point argument. For PolygonTool, you will add it to the list of vertices.

When a PointerRelease event arrives, we can ignore it for PolygonTool because we’ll finish polygons using the spacebar in Task 3.2. For everything else, a PointerRelease means the end of a click-drag-release action, so you should construct the appropriate shape and add it to the [Shape] in the Model. You should also remove the starting point from the current Tool, so that future shapes draw properly too.

Task 3.2: Handling Key Presses

To handle keyboard input, CodeWorld provides a KeyPress event. This case is already present in the assignment skeleton, because we have implemented some keyboard functionality already. In the “Overview of the Program” section, we listed the full set of keyboard commands that your program will respond to. You need to implement the missing functionality for these keys:

Key Effect
C Cycle colour (of shape to draw)
T Cycle tool (type of shape to draw)
Backspace/Delete Remove the last added shape
Spacebar Finish drawing a polygon, adding it to the canvas.

If you have made it this far, you should not need to write a lot of code to implement these. A few hints:

  • Think back to Task 1.
  • Backspace/Delete with no shapes drawn should not crash the program.

Task 4: Extensions (COMP1130 Only: 30 marks)

COMP1100 students are welcome to attempt extensions, but will not attract marks for completing them.

COMP1130 students will need to complete two of the five extensions in the pool below. If you attempt more than two extensions, you must indicate in your report which ones should be marked. Otherwise, we will mark the first extensions that you have attempted, according to the order they appear in this document.

If you change the behaviour of tool or colour switching, you may want to adjust the tests in tests/ShapesTest.hs so they continue to pass.

Note: Extension functionalities must be executed through the CodeWorld API just like the rest of the assignment. Interactions should be received through the handleEvent function, which you will need to modify. If you need to add fields to the Model constructor, please do so after the existing ones: data Model = Model [ColourShape] Tool ColourName YourField1 YourField2 ....

Task 4.1

Allow the user to delete multiple shapes using the mouse. The user should enter some kind of “mouse delete mode”, and then delete multiple shapes at once by pressing the mouse button and dragging a rectangle around them. When the user releases the mouse, any shape that is wholly within the rectangle should be deleted.

Task 4.2

Add a free-hand drawing tool. When using this tool, moving the mouse with the button pressed should draw short lines or curves, as if a pen is being dragged across the screen. If the button is released, the program should stop drawing.

Task 4.3

Add the ability to select and delete a shape using the keyboard. How you select shapes, and how you enter and leave “keyboard delete” mode, is all up to you.

Task 4.4

Preview shapes as the user draws them. If the user has started to draw a shape, then you should show the shape currently being drawn using a hollow (not solid) rectangle, polygon, circle or ellipse. Lines will draw the same way as usual.

For polygons, you won’t be able to show a preview until two or more points have been saved.

Task 4.5

Allow the user to delete all shapes that have something in common. For example, all lines, all orange shapes, or all yellow rectangles. Whether you consider colour, shape, both, or something else as your measure of similarity is up to you. How the user selects what similarities to search for is also up to you.

Do not try to claim that “everything is similar to everything else”. That would collapse this problem to “erase the entire canvas”, which is too easy.

Technical Report (COMP1100: 15 marks; COMP1130: 25 marks)

You should write a concise technical report explaining your design choices in implementing your program. The maximum word count is 1000 for COMP1100 students. COMP1130 students have a maximum word count of 1500, so they have space to discuss their extensions. This is a limit, not a quota; concise presentation is a virtue. If you are in COMP1130 and can detail your program and extensions in 1000 words, it is better to submit the 1000 word report than to waste your time padding it to 1500.

Once again: These are not required word counts. They are the maximum number of words that your marker will read. If you can do it in fewer words without compromising the presentation, please do so.

Your report must be in PDF format, located at the root of your assignment repository on GitLab and named Report.pdf. Otherwise, it may not be marked.

The report must have a title page with the following items:

  • Your name
  • Your laboratory time and tutor
  • Your university ID

Content and Structure

Your audience is the tutors and lecturers, who are proficient at programming and understand most concepts. Therefore you should not, for example, waste words describing the syntax of Haskell or how recursion works. After reading your technical report, the reader should thoroughly understand what problem your program is trying to solve, the reasons behind major design choices in it, as well as how it was tested. Your report should give a broad overview of your program, but focus on the specifics of what you did and why.

Remember that the tutors have access to the above assignment specification, and if your report only contains details from it then you will only receive minimal marks. Below is an potential outline for the structure of your report and some things you might discuss in it.


If you wish to do so you can write an introduction. In it, give:

  • A brief overview of your program:

    • how it works; and
    • what it is designed to do.
  • If you have changed the way the controls work, or added something that may make your program behave unexpectedly then it would be worth making a note of it here.

Writing an introduction may be useful for those who have written more complicated programs (i.e. COMP1130).


Talk about why you structured the program the way you did. Below are some questions you could answer:

  • Program design
    • Describe what each relevant function does conceptually. (i.e. how does it get you closer to solving the problems outlined in this assignment spec?)
    • How do these functions piece together to make the finished program? Why did you design and implement it this way?
    • What major design choices did you make regarding the functions that you’ve written, and the overall structure of your program?
  • Assumptions
    • Describe assumptions you have made about how a user might use the program, and how this has influenced your design decisions.
  • Testing
    • How did you test individual functions?
      • Be specific about this - the tutors know that you have tested your program, but they want to know how.
      • Describe the tests that prove individual functions on their own behave as expected (i.e. testing a function with different inputs and doing a calculation by hand to check that the outputs are correct).
    • How did you test the entire program? What tests did you perform to show that the program behaves as expected in all (even unexpected) cases?
      • Again, be specific - did you check that you can draw shapes from left to right? What about right to left? An ellipse is defined by the two opposite corners of a bounding box - did you check that it fits perfectly within a rectangle drawn from the same two input points?
      • Have you checked edge cases (this is a computer science term that refers to unexpected or unlikely inputs that may cause a program to crash or behave in strange ways)? It is not likely that someone would try to change the tool halfway through drawing a shape, but it is essential that it has behaviour defined for that scenario. Describe similar tests that you have done to ensure the program can handle all inputs.
  • Inspiration / external content
    • What resources did you use when writing your program (e.g., published algorithms)?
    • If you have used resources such as a webpage describing an algorithm, be sure to cite it properly at the end of your report in a ‘References’ section. References do not count to the maximum word limit.


Discuss the reasoning behind your decisions, rather than what the decisions were. You can reflect on not only the decisions you made, but the process through which you developed the final program:

  • Did you encounter any conceptual or technical issues?
    • If you solved them, describe the relevant details of what happened and how you overcame them.
    • Sometimes limitations on time or technical skills can limit how much of the assignment can be completed. If you ran into a problem that you could not solve, then your report is the perfect place to describe them. Try to include details such as:
      • theories as to what caused the problem;
      • suggestions of things that might have fixed it; and
      • discussion about what you did try, and the results of these attempts.
  • What would you have done differently if you were to do it again

    • What changes to the design and structure you would make if you wrote the program again from scratch?
  • Are parts of the program confusing for the reader? You can explain them in the report (in this situation you should also make use of comments in your code).

  • If you collaborated with others, what was the nature of the collaboration? (Note that you are only allowed to collaborate by sharing ideas, not code.)
    • Collaborating is any discussion or work done together on planning or writing your assignment.
  • Other info
    • You may like to briefly discuss details of events which were relevant to your process of design - strange or interesting things that you noticed and fixed along the way.

This is a list of suggestions, not requirements. You should only discuss items from this list if you have something interesting to write.

Things to avoid in a technical report

  • Line by line explanations of large portions of code. (If you want to include a specific line of code, be sure to format as described in the “Format” section below.)
  • Pictures of code or IntelliJ.
  • Content that is not your own, unless cited.
  • Grammatical errors or misspellings. Proof-read it before submission.
  • Informal language - a technical report is a professional document, and as such should avoid things such as:
    • Unnecessary abbreviations (atm, btw, ps, and so on), emojis, and emoticons; and
    • Stories / recounts of events not relevant to the development of the program.
  • Irrelevant diagrams, graphs, and charts. Unnecessary elements will distract from the important content. Keep it succinct and focused.

If you need additional help with report writing, the academic skills writing centre has a peer writing service and writing coaches.


You are not required to follow any specific style guide (such as APA or Harvard). However, here are some tips which will make your report more pleasant to read, and make more sense to someone with a computer science background.

  • Colours should be kept minimal. If you need to use colour, make sure it is absolutely necessary.
  • If you are using graphics, make sure they are vector graphics (that stay sharp even as the reader zooms in on them).
  • Any code, including type/function/module names or file names, that appears in your document should have a monospaced font (such as Consolas, Courier New, Lucida Console, or Monaco)
  • Other text should be set in serif fonts (popular choices are Times, Palatino, Sabon, Minion, or Caslon).
  • When available, automatic ligatures should be activated.
  • Do not use underscore to highlight your text.
  • Text should be at least 1.5 spaced.


Do not post your code publicly, either on Piazza or via other forums. Posts on Piazza trigger emails to all students, so if by mistake you post your code publicly, others will have access to your code and you may be held responsible for plagiarism.

Once again, and we cannot stress this enough: do not post your code publicly . If you need help with your code, post it privately to the instructors.

When brainstorming with your friends, do not share code. There might be pressure from your friends, but this is for both your and their benefit. Anything that smells of plagiarism will be investigated and there may be serious consequences.

Sharing ideas and sketches is perfectly fine, but sharing should stop at ideas.

Course staff will not look at assignment code unless it is posted privately in piazza.

Course staff will typically give assistance by asking questions, directing you to relevant exercises from the labs, or definitions and examples from the lectures.

Before the assignment is due, course staff will not give individual tips on writing functions for the assignment or how your code can be improved. We will help you get unstuck by asking questions and pointing you to relevant lecture and lab material. You will receive feedback on you work when marks are released.

Submission Checklist

Once you have finished your assignment, and preferably 24 hours prior to the deadline, you should make sure that:

  • You have fully read and understand the entire assignment specification.
  • Your work has been pushed to GitLab.
  • Your program compiles and runs, including the cabal test test suite.
  • Your program works on the lab machines - if the program does not work on the lab machines, it might fail tests used by the instructors.
  • You have proof-read and spell-checked your report.
  • The report is in PDF format, located at the root of your project on GitLab (i.e. the main directory, not a subdirectory) and named Report.pdf. That capital R is important - Linux uses a case-sensitive file system. Otherwise, it may not be marked.

Updated:  23 Mar 2019/ Responsible Officer:  Director, RSCS/ Page Contact:  Course Convenor