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Auto-Grading Racket Code in Gradescope

This repository enables educators to autograde student submissions written in Racket on Gradescope.

At least in its current structure, you will want to make a different copy of this code for each assignment. Since this codebase is very lightweight, I don't anticipate changing that in the near future.

Docker

Gradescope relies on Docker. You don't strictly have to know how to use Docker yourself, but it certainly helps.

If you're not comfortable with Docker, you can just create the test suite (see below), upload files to Gradescope, and leave the building to them, and test on their machines. Be aware this can be quite slow, and if you make a mistake, you'll have to do it all over again.

Assuming you will use Docker locally:

  1. Make a local image for testing:
make base-image
make grader-image a=<tag>

There are two images due to staging. The first simply installs Racket, which is unlikely to change, but the second installs the assignment-specific grader content, which is likely to change quite a bit as you're developing it. We don't have to name both images, but it might make it a bit clearer to navigate (and it's sometimes also useful to go into a pristine base-image to test some things).

  1. When you're ready to test, run make grade a=<tag> s=<dir>, where <dir> is the sub-directory of tests that houses the (mock) student submission, and <tag> is the assignment tag on the grader scripts. See examples below.

Creating a Test Suite

The library currently depends on rackunit. If you're only used to the student language levels, don't worry, it's pretty similar. Stick to the test-* forms (as opposed to the check-* ones). You can read up more in the documentation.

Warning: Use only the test- forms, not the check- forms! The former work well in the presence of run-time errors caused by a check. The latter do not, so one erroneous test can truncate grading.

Create a file named grade-<tag>.rkt. (Leave all the other files alone unless you really know what you're doing.) File names of this format can be automatically managed by Makefile and installed into Docker container and Gradescope setup script using the a=<tag> input.

There are three files to help you create your own grader:

  • a template file, which doesn't itself work: grade-template.rkt

  • three working example files: grade-sq.rkt, grade-two-funs.rkt, and grade-macros.rkt

If you use the template, be sure to edit only the UPPER-CASE parts of it.

You can have only one test suite per file/grading run, but test suites can be nested so this does not impose much of a limitation.

Once you've created your test suite, you will probably want to test it locally first. This requires you to have Docker installed (but not necessarily know much about how to use it). See the instructions above. If you're skipping local testing, you can move on to deployment.

Naming Tests

Note that all the test-* forms take a name for the test. (This is a difference from the student languages.) To name it wisely, it is important to understand how this name is used.

When a test fails (either the answer is incorrect, or it has an error), students get a report of the failure. In the report, they see the name of the test, rather than the actual test or test output. This is because both of those can leak the content of the test. A cunning student can write a bunch of failing tests, and use those to exfiltrate your test suite, which you may have wanted to keep private.

Many course staff thus like to choose names that are instructive to the student for fixing their error, but do not give away the whole test. If, on the other hand, you want students to know the full test, you could just write that in the test. For instance, assuming you are testing a sq function that squares its input, your could write any of these, from least to most informative:

  (test-equal? "" (sq -1) 1)
  (test-equal? "a negative number" (sq -1) 1)
  (test-equal? "-1" (sq -1) 1)

Test suites can also have names, and be nested. For example:

(test-suite
  "Test suite 1"
  (test-equal? "" (sq -1) 1)
  (test-suite
    "Tricky Tests"
    (test-equal? "-1" (sq -1) 1)))

When names suites have names, the names of test suites and their tests are appended hierarchically when reporting test failures and errors to GradeScope. In the above example, if the tricky test fails, then the test will report Test suite 1:Tricky Tests:-1 failed.

Why define-var

You might wonder why you have to use define-var to name the variables you want from the module, especially if the module already exports them. (If you don't use define-var you'll get an unbound identifier error.) There are two reasons:

  1. Some languages, like Beginning Student Language, do not export names. Therefore, the autograder has to “extract” them from the module, and wouldn't know which ones to extract.

  2. Simply accepting all the names from a module may be dangerous: a malicious student could export names that override the autograder's functionality (since this code is public, after all), thereby giving themselves they grade they want, not the one they deserve (unless it's a course on malicious behavior, in which case, they've earned whatever they award).

define-var lets you carefully limit which names the student provided you actually end up with. It will first look for the exported name and, only if it isn't found, extract from the module. This avoids the slight nuisance of having to return a student's assignment for having forgotten to export a name. (If this behavior isn't desired, talk to me and I can help you edit the source.)

Why mirror-macro

Macros are not values. Therefore, we can't simply extract the macro as a value from the student's module. Instead, the library sets up a mirror of that macro in the testing module. Note that it currently has the following consequences (some are weaknesses, others aren't as clear):

  • All evaluation happens in the student's module. Thus, all references to names are resolved in that module. This means a local helper function cannot be referenced directly.

  • Because of the way mirror-macro is written, the mirroring step takes place each time a test uses a macro. This does not seem to be prohibitively costly, but it is at least annoying. If this proves to be a performance problem, let me know.

  • Because this code goes into the student's module, it uses the namespace local to that module, not the names exported. Practically speaking, this means it disregards rename-out in the student's module. Since we don't expect students will be using this feature if they are still at the level where this library makes sense, this should not be much of a problem.

Deploying to Gradescope

Run make zip a=<tag> to generate the Zip file that you upload to Gradescope. If you have broken your grader into multiple files, be sure to edit the Makefile to add those other files to the archive as well. (And don't forget to add them to the repository, too!)

Following Gradescope's instructions (see below), upload the Zip.

Gradescope will build a Docker image for you.

When it's ready, you can upload sample submissions and see the output from the autograder.

Examples

The directory tests/sq/ contains mock submissions of a sq function that squares its argument, and grade-sq.rkt a test suite for it. So install that test suite, then check the several mock submissions:

make grader-image a=sq
make a=sq s=sq/s1
make a=sq s=sq/s2
...

where s1 is the first student submission, s2 is the second, etc. Focus on the JSON output at the end of these runs. See tests/sq/README.txt to understand how the submissions differ.

The default Makefile target (grade) will automatically try to rebuild the grader-image when necessary, so running make grader-image is probably not necessary.

The directory tests/two-funs/ illustrates that we can test more than one thing from a program; grade-two-funs.rkt is its test suite:

make a=two-funs s=two-funs/s1

The directory tests/macros/ illustrates that we can also test for macros; grade-macros.rkt is its test suite:

make a=two-funs s=macros/s1

(In this directory, student programs are purposely called student-code.rkt to show that you can choose whatever names you want; they don't have to be code.rkt.)

Scoring

Gradescope demands that every submission be given a numeric score: either one in aggregrate or one per test. I find this system quite frustrating; it imposes a numerics-driven pedagogy that I don't embrace on many assignments. Therefore, for now the score given for the assignment is simple: the number of passing tests divided by the total number of tests. That's it. Someday I may look into adding weights for tests, etc., but I'm really not excited about any of it. Either way, please give your students instructions on how to interpret the numbers.

Open Issues

See https://github.com/shriram/gradescope-racket/issues/.

Is This Current?

This code has been tested with Gradescope in late May 23, 2020, using Racket 7.7. Since Gradescope's APIs are in some flux, you may encounter errors with newer versions; contact me and I'll try to help. In addition, Gradescope intends to make it easier to deploy autograders using Docker; when that happens, it would be nice to upgrade this system.

This code does not have any non-vanilla-Racket dependencies.

Gradescope Specs

Gradescope's "specs" are currently at:

https://gradescope-autograders.readthedocs.io/en/latest/getting_started/

https://gradescope-autograders.readthedocs.io/en/latest/specs/

https://gradescope-autograders.readthedocs.io/en/latest/manual_docker/

https://gradescope-autograders.readthedocs.io/en/latest/git_pull/

Acknowledgments

Thanks to Matthew Flatt, Alex Harsanyi, David Storrs, Alexis King, Matthias Felleisen, Joe Politz, and James Tompkin.