Homework #4

Due: Thu, Mar 1, 10:59pm

How to Submit

  • Submit one solution per team (each team can have 1–3 members), through TEACH. Put the names and ONID IDs of all team members as a comment at the top of the file.

  • Your submission should consist of a file named KarelSemantics.hs.

  • Your Haskell file must compile without errors in GHCi. Put all non-working parts of your solution in comments! If your file does not compile, the TA will not evaluate it.

  • If you can’t solve a problem, you can get partial credit by describing in comments what you tried and where you got stuck.

  • Late submissions will not be accepted. Do not send solutions by email.


The Karel language is a simple imperative programming language for directing an on-screen robot. The robot can move around a grid, pick up and place “beepers”, and test aspects of its environment to determine what to do next. The syntax of the language is based on Pascal. You can read the documentation for an open source implementation of Karel, if you’re interested.

In this assignment, you’ll be implementing an interpreter for a slightly modified version of Karel. This version is feature-complete with the real Karel, but the syntax has been updated to be a bit more concise and orthogonal.

A detailed description of the Karel language is provided at the link below. You should read this page, and the provided support files thoroughly.

Template and Supporting Files

The following zip file contains several files. Note that you should only change the template file, KarelSemantics.hs, since this is the only file you will submit. If you change the supporting files, we will not be able to run your code.

The support code is well documented, so you should start by reading the code and trying out the functions in GHCi. If you have any questions about this code, please post them to Piazza. Here is a brief overview:

  • KarelSyntax.hs: Defines the abstract syntax of Karel.

  • KarelState.hs: Karel’s program state is rather complicated compared to the other languages we’ve defined. This file defines types for representing this state and also several helper functions that perform all of the basic operations you will need to write the semantics.

  • KarelExamples.hs: Provides some simple worlds and Karel programs to use in testing.

  • KarelTests.hs: A test suite for evaluating your semantics, and for helping you understand exactly how it should behave.

  • KarelSemantics.hs: The template you will be completing. The types of the required functions are provided, along with a couple example cases to get you started.


The overall task is to define an interpreter for Karel. An interpreter in Haskell corresponds very closely to a denotational semantics. Aside from some theoretical differences, another difference is that when writing an interpreter we care about pragmatic issues like providing useful error messages.

I have broken this large task down into several smaller tasks, ordered roughly by difficulty. Hopefully this helps you get started and make progress.

  1. Define the evaluation functions for Test. A test is essentially a query over the state of the world and the state of the robot, which can either be true or false. Therefore, the semantic domain is: World -> Robot -> Bool.

  2. Define the evaluation function for the first five constructs of Stmt (two of these are provided already). These statements manipulate the state of the world and/or robot. The ultimate result of evaluating a statement is a value of type Result. This is one of (1) an OK value that contains the updated state of the world and robot, (2) a Done value that should only be produced by the Shutdown statement, or (3) an Error value that contains an error message. Note that to get the tests to pass, you’ll have to reverse engineer the error messages by reading the doctests.

  3. Extend the stmt evaluation function to handle blocks. Once you get to this point, you should be able to pass the first section of tests.

  4. Extend the stmt function to handle conditional statements (If). Now you can hopefully pass the conditional tests.

  5. Extend the stmt function to handle macro calls. The lookup function in the Haskell Prelude is very handy for this! Now you can hopefully pass the macro tests.

  6. Finally, extend the stmt function to handle the looping constructs. Now you should be able to pass all of the tests!

Back to course home page