Sarah “Slim” Lim

Computer Science

Northwestern University


slim [at] (this domain) [dot] com

Photograph taken in the Smoky Mountains
Sk8er Boi was a pretty good song


I am a researcher, technologist, and designer living in San Francisco. I currently work at Khan Academy, a not-for-profit organization with the mission of providing a free, world-class education to anyone, anywhere. Previously, I was part of the Early Product Development group at Khan; Human Experience and Design at Microsoft Research Cambridge; and Design, Technology, and Research at Northwestern University.

My research interests span programming languages, human factors, learning sciences, and personal computing. I’m interested in programming languages as user interfaces and tools for thought: in short, how language design affects the way people think about and write programs. For instance, I ask questions about the usability of static and gradual type systems, the role of functional programming within computing education, and how provable guarantees might alleviate cognitive load while writing software.

I graduated with a BA in Computer Science from Northwestern University, where I researched CSS inspection with Haoqi Zhang and Nell O’Rourke and peer grading algorithms with Jason Hartline. My studies were generously supported by scholarships from Google, Microsoft, Palantir, Box, and Quip, among others.

I really like browser engines, type systems, public transit, WebAssembly, Rust, crossword puzzles, classical music, policy debate, document preparation, cognitive disability advocacy, the Nintendo Switch, hiking, and critical theory.

Most people call me Slim; you should feel free to do the same.


Apr 2019

New CSS inspection features inspired by Ply are coming soon to a Firefox release near you!

Mar 2019

I’m attending this year’s Oregon Programming Languages Summer School. Be my friend in Eugene please

Oct 2018

Our paper Ply: A Visual Web Inspector for Learning from Professional Webpages received an Honorable Mention Award at UIST 2018! I presented this work in Berlin (see the recording and my slides).

Sep 2018

Watch me plug the growth mindset in glorious 1080p! Thanks to Weinberg College for the video profile.

I’m excited to be attending PLMW at ICFP, along with Strange Loop and RacketCon.

Jul 2018

Started an internship at MSR Cambridge. Here, arugula is “rocket,” which is kind of fun.

Jun 2018

I graduated! I was honored to be named Outstanding Senior in Computer Science.

May 2017

Spoke at the Northwestern Big Ideas Forum, “How We Learn About Learning,” with professors Nell O’Rourke and David Uttal, and fellow undergrad Gabby Ashenafi.

Ply wins the CHI 2017 Student Research Competition! Northwestern Engineering has a nice write-up about the whole thing.

Apr 2017

Received a Microsoft Tuition Scholarship for 2017-18.

Jan 2017

Ply: Visual Regression Pruning for Web Design Source Inspection is accepted to the CHI 2017 SRC.


Recent escapades in research, development, and coursework.

Ply: Visual Web Inspection

Delta Lab


CSS is syntactically straightforward, but has a steep learning curve and complicated semantics. Inspecting the source of existing webpages can help illustrate concepts, but such webpages are typically too complex to serve as useful learning materials. Drawing inspiration from prior research in both software engineering and the learning sciences, we present a new web inspection tool and set of techniques for pruning irrelevant CSS and identifying implicit dependencies between properties. Supervised by Haoqi Zhang and Nell O’Rourke. Honorable Mention Paper at UIST 2018, Berlin.

Evaluating peer graders

Northwestern University


Most of the literature on peer grading focuses on inferring a true grade from a set of noisy reports. We study a different problem: inferring the skill and effort of reviewers, from the same reports. Supervised by Jason Hartline.

Tracing WebAssembly function calls

EECS 396: Systems Programming in Rust


With Meg Grasse and mentorship from Nick Fitzgerald and Jim Blandy, we developed a proof-of-concept tool for instrumenting WebAssembly binaries written in Rust to log function calls at runtime.

Classroom exercise reports

Khan Academy


I developed new exercise reports to help teachers visualize student progress and work through problems in the classroom. Mentored by John Resig during my internship at Khan Academy.

Visual regression pruning

Delta Lab


We introduce a visual significance heuristic for removing irrelevant CSS source code during web design reverse-engineering tasks. CHI 2017 Student Research Competition Winner, Denver, Colorado.

Guiding Web Inspection with Tutorial Keyword Frequency

Delta Lab


In order to bridge the gap between web design tutorials and real-world examples, we extend a web inspector to highlight CSS properties frequently mentioned across a given set of tutorials. Google Scholars’ Retreat 2016, Mountain View, California.

SVG Charting Library



An opinionated Ember.js addon to replace Highcharts with native SVG and DOM APIs. Released addon as a company-wide multiproduct. I worked on this project during my internship at LinkedIn, under the mentorship of Cody Coats and Michail Yasonik.

Predicting the Popularity of User-Generated Discussion Questions

EECS 349: Machine Learning


Using Python with the Reddit API and NLTK library, we collect information about AskReddit posts over a two-week period to analyze what makes a question popular. Alternating decision trees achieve 72.9819 accuracy with 10-fold cross-validation, an improvement over the ZeroR baseline of 51.0708. Features related to the language of the question, time and day of posting, and initial commenting behavior prove most informative. With Sameer Srivastava, Jennie Werner, and Aiqi Liu.


Northwestern Debate Institute


End-to-end Google Apps Script-based pipeline for publishing practice debate comments to individual students’ feedback pages. Previously, instructors needed to manually edit the feedback pages for all four students in order to provide feedback from practice rounds. Deployed at the 2015 Northwestern Debate Institute and subsequently adopted for the entire program in 2016.


I was a teaching assistant every quarter beginning my sophomore year, sometimes for two courses at once. Terms marked with an asterisk (*) denote a head or sole teaching assistant role.