Prof. Stephen Chong
Fall 2019

Course information

This course introduces students to the design and implementation of compilers for programming languages. Specifically, students will learn how to systematically translate modern, high-level, programming languages into efficient, executable machine code.

The course introduces a number of important concepts, such as parsing and program analysis that are useful in many other contexts beyond compilers, such as software engineering and security. Perhaps the most useful outcome of the course is that students will deeply understand the capabilities and limitations of modern compilers, and how they can be used most effectively. This knowledge is important for aspiring language designers and implementors, but also for debugging and optimizing just about any application.

This course is extremely programming intensive, as most of the understanding comes from constructing (small) compilers.

Recommended Prerequisites

Computer Science 51 or Computer Science 61. (Ideally, both CS51 and CS61.) Students should be comfortable programming in OCaml. Note that this is a programming-intensive course: some weeks may require tens of hours of programming.

You can try the self assessment to help figure out whether you are adequately prepared. Also, the first homework assignment (released in the first week) is designed to help get you up to speed in programming in OCaml.

Time and Place

Tuesdays and Thursdays, 12:00pm-1:15pm, room Maxwell Dworkin G125.

Course staff

See below for Office Hours.

All questions and issues related to assignments, course content, etc., should be posted to Piazza. Questions related to grades, special consideration, etc. can be sent directly to Prof. Chong. In general, sending email to individual course staff will delay a response. Note that course staff may take up to 48 hours to respond.

Homeworks, exams, and grading

There will be a final exam on Thursday December 19, 9am-12pm (location TBD). Extension school students will take the same exam, during a 24-hour window starting Thursday December 19 9am EST. There are no midterm exams. There will be ~6 projects during the semester. The exact number and timing will be determined later.

Your grade will be determined by a weighted average of your scores on projects, the final exam, and class participation. The percentage breakdown (roughly and subject to change) is 75% projects (including ~1% for the Embedded EthiCS homework), 20% final exam, and 5% participation (which includes attendance and participation in class and office hours, and contributing to online discussion).

Extension school

CS 153 is offered through the Extension School as CSCI E-153. Information specific to Extension School students can be found on the Extension School page.


The textbook "Modern Compiler Implementaton in ML" by Andrew W. Appel is recommended but not required. In most cases, the class materials should suffice. The instructor will provide the lecture slides after the lecture (or before if they are ready in time).

Embedded EthiCS Module

Ethical reasoning is an essential skill for today's computer scientists. At Harvard Computer Science, we teach ethical reasoning by integrating ethics modules into courses throughout our curriculum. These modules are part of the Embedded EthiCS program, a collaborative effort by Computer Science and the Philosophy Department. The module for this course will focus on an ethical issue raised by the technical material for the course, and will be developed by a graduate or postdoctoral fellow in Philosophy working in collaboration with course faculty and postdoctoral fellows in Computer Science. Among other things, Embedded EthiCS modules teach students to:

  1. Identify and anticipate ethical and social issues in their work.
  2. Think clearly about those issues, both alone and collaboratively.
  3. Communicate their understanding of those issues effectively.
  4. Design systems that take into account ethical and social concerns.

CS153 will have an Embedded EthiCS module. The date for the module will be announced shortly; all students are expected to attend. The module will consist of one or two class sessions, together with an assignment that will constitute part of your grade for the course. You can learn more about Embedded EthiCS at the program's website:


See here for more information.

Office hours

Office hours are managed via a Google calendar, shown here:


There will be ~6 projects in this course, mostly involve implementing parts of a compiler. Implementation will be in OCaml, and we assume that students are comfortable with programming in OCaml. See the schedule for the timing of the projects.

Projects are done individually: you must write all your own code. Do not share code with others (including letting others look at your code), do not accept code from others, and do not look for code online. However, you are encouraged to talk about the project with others, to share ideas and thoughts. All students should respect the Harvard academic integrity policy and the course collaboration policy below.

There may be opportunity for extra credit by augmenting your compilers (or the language we're compiling) with new features or optimizations. However, a word of warning: the extra credit only applies if the rest of the compiler is working.

Plan ahead and get the work done. We will not accept submissions that do not type-check and compile. See below for the late-minute and extension policy.

Diversity and Inclusion*

I would like to create a learning environment that supports a diversity of thoughts, perspectives and experiences, and honors your identities (including race, gender, class, sexuality, religion, ability, etc.) To help accomplish this:

If you ever are struggling and just need someone to talk to, feel free to stop by office hours, or to reach out to me and we can arrange a private meeting.

Inclusive Learning and Accessibility

Your success in this class is important to me. We will all need accommodations because we all learn differently. If there are aspects of this course that prevent you from learning or exclude you, please let me know as soon as possible. Together we'll develop strategies to meet both your needs and the requirements of the course.

I encourage you to visit the Accessible Education Office to determine how you could improve your learning as well. If you need official accommodations, you have a right to have these met. There are also a range of resources on campus. The Bureau of Study Counsel provides many resources, including academic counseling and peer tutors. (Extension School students, please see this page for more information.)

If you have a letter from the Accessible Education Office, please try to get that to me earlier rather than later. Concretely, please try to give it to me by the end of the second week of classes.

Mental Health

If you experience significant stress or worry, changes in mood, or problems eating or sleeping this semester, whether because of CS15 or other courses or factors, please do not hesitate to reach out immediately, at any hour, to any of the course staff to discuss. Everyone can benefit from support during challenging times. Not only are we happy to listen and make accommodations with deadlines as needed, we can also refer you to additional support structures on campus, including, but not limited to:

Financial Aid

We do not require that students purchase any books, hardware, or software. While not required, having one's own laptop is helpful. Students without their own laptops are encouraged to reach out at the start of the course to discuss possibilities.

Late minutes, Penalties, and Extensions

Each student has 14,400 "late minutes" (which is the number of minutes in 10 days) which can be applied to any of the projects. A late minute extends the due date/time by 1 minute. At most 4,320 late minutes (= 3 days) can be used on any single project. (This allows us to grade your projects and return them to you within a reasonable period.)

Late minutes are intended to help you manage your time effectively. They are not meant to be a substitute for starting projects early.

Late minutes are not meant to be used for health issues (including mental health issues), family emergencies or other extenuating circumstances. In those situations, please contact Prof Chong, or have your senior tutor do so.

Course Expectations

I try to be explicit about the expectations I have of students enrolled in this course. If you have any questions or concerns about what is expected of you, please get in touch!

Collaboration Policy

Discussion and the exchange of ideas are essential to doing academic work. For projects in this course, you are encouraged to consult with your classmates as you work on projects. However, after discussions with peers, make sure that you can work through the problem yourself and ensure that any answers you submit for evaluation are the result only of your efforts. In addition, you must cite any books, articles, websites, lectures, etc that have helped you with your work using appropriate citation practices. Similarly, you must list the names of students with whom you have collaborated on problems.

Do not pass solutions to problem sets nor accept them from another student. For programming projects, this means do not share code. Do not post course materials (including problem sets, solutions, exams, etc.) to websites (including public GitHub repositories, and similar) or course-content archives. Also, it is never okay to look up solutions to homework problems in this class, i.e., don't look on the web for solutions.

You can definitely engage in "high level" discussions with your peers, for example, about the problem statement. You can definitely engage in "low level" discussions with your peers, for example, about OCaml syntax and libraries, compiler error messages, etc. "Mid level" discussions require discretion and should be limited in this course.

If you are ever in doubt, ask the course staff to clarify what is and isn't appropriate.

Some text for this course's policies is based on material by Monica Linden, Neuroscience, Brown University and David Malan, Computer Science, Harvard University.